Climate Vulnerability || Vulnerability of Estuaries to Climate Change

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4.22 Vulnerability of Estuaries to Climate ChangeW Kimmerer, San Francisco State University, Tiburon, CA, USAMJ Weaver, Berkeley, CA, USA

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4.22.1 Introduction 2724.22.2 What Is an Estuary? 2724.22.2.1 Physical Responses of Estuaries to External Influences 2744.22.2.1.1 Tides and Flow 2744.22.2.1.2 Freshwater, Salinity, and Mixing 2744.22.2.1.3 Other Riverine Inputs: Sediment 2754.22.2.1.4 Low-Flow Estuaries 2754.22.2.1.5 Ocean Conditions 2754.22.2.1.6 Vegetated Beds 2754.22.2.2 Biological Responses of Estuaries to Physical Conditions 2754.22.2.3 Human Influences 2764.22.2.4 Ecosystem Functions and Services 2774.22.2.5 Estuaries as Islands 2774.22.3 Long-Term Change 2774.22.3.1 Climate Change 2774.22.3.1.1 Temperature Increase 2774.22.3.1.2 Sea-Level Rise 2784.22.3.1.3 Acidification 2794.22.3.1.4 Storms and Precipitation 2804.22.3.1.5 Wind and Upwelling 2804.22.3.2 Direct Human Effects 2804.22.4 Modes of Estuarine Response 2814.22.4.1 Mitigation and Adaptation 2814.22.4.2 Interactions 2824.22.4.2.1 Precipitation, Freshwater Runoff, and Salinity (Figure 2) 2824.22.4.2.2 Sediment Supply, Transport, and Loss (Figure 3) 2834.22.4.2.3 Vegetated Beds (Figure 4) 2834.22.4.2.4 Eutrophication (Figure 5) 2834.22.4.2.5 Species Shifts (Figure 6) 2844.22.4.3 Catastrophes 2864.22.4.4 Application to Particular Estuaries 2864.22.4.4.1 San Francisco Estuary 2864.22.4.4.2 Chesapeake Bay 2874.22.4.4.3 Murray–Darling System, Australia 2884.22.4.4.4 Summary of Case Studies 2884.22.5 Implications for Ecosystem Functions and Services 288References 289

Glossary
Autotrophic organisms Photosynthesize (orchemosynthesize) to produce organic carbon, releasingoxygen; in an autotrophic estuary, oxygen production fromphotosynthesis and chemosynthesis exceeds consumptionby respiration.Barrage Is an artificial obstruction in a river designed toincrease its depth or to divert its flow.Benthic organisms (benthos) Live on or in the bottomsediments.Diadromous fishHave some life stages in freshwater andsome in saltwater. This includes anadromous species such
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as salmon which spawn in freshwater but spend muchof their lives in saltwater, and catadromous species suchas blue crabs that spawn in estuaries and rear in theocean.Epibenthic organisms Live on or near the bottom but maymove up into the overlying water.Epiphytic organisms Are algae and other small organismsthat live on plant stems and leaves.Eutrophication An increase in the rate of organic mattersupply to an ecosystem, often to the point of undesirableconsequences such as hypoxia.

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272 Vulnerability of Estuaries to Climate Change

Heterotrophic organisms Oxidize organic matter toobtain energy. An estuary that is net heterotrophic hasa negative oxygen balance, consuming oxygen andorganic matter and releasing CO2.Hypoxia Condition of low oxygen concentration (usually<2 mg l�1) in natural waters.Loading The rate at which a substance is brought intoa water body, e.g., from a river.Pelagic organisms Live in the open water, dissociated fromthe bottom or the shores.

Phenology The timing of natural events such asmetamorphosis or migration.Plankton Pelagic organisms with weak swimming ability.Most are small. This includes jellyfish and larval forms ofmost fish and benthic organisms.Nekton Pelagic organisms with strong swimming ability,including nonlarval fish.Residence time The average time that a molecule of wateror a particle remains in a water body.Stratification A vertical density gradient in which low-density water overlies higher-density water.

4.22.1 Introduction

As transition zones, estuaries are subject to influences fromocean, atmosphere, and land. Coastal and estuarine ecosystemsare particularly sensitive to aspects of climate change, includingrising sea level, increasing air temperature, and changingpatterns of freshwater runoff (Goberville et al. 2010).

In places where human populations congregate, estuariesprovide numerous valuable ecosystem services (Table 1). Estu-aries play an essential role in buffering against storms, floods,and drought and in storing and processing nutrients, sediments,contaminants, and other materials. They provide habitat forcountless species, including nursery grounds for some marineand anadromous species and feeding grounds for resident andmigratory birds, and they support substantial commercial andrecreational fisheries for both finfish and shellfish. Estuaries alsoprovide opportunities for recreation and cultural activities. Thevalue of estuaries as public resources is acknowledged by theestablishment of reserves such as the US National EstuarineResearch Reserve system, and governmental and nongovern-mental bodies tasked with stewardship of estuaries.

Estuaries have been severely altered by human activities.Many cities and most ports and harbors are on estuaries.Shorelines have been realigned, and marshes and shoals filledand converted to hard structures such as airports, roads, andresidential and industrial areas. The watersheds of estuariessupply water to farms and cities, altering seasonal patterns andquantities of freshwater flow. Rivers and estuaries serve asdumping grounds for waste. Contaminants, including heavymetals, organic toxins, nutrients, and pathogens, are dischargedinto estuaries. Some estuarine functions have been transformedby introduced species. As a result, many estuaries today wouldbe unrecognizable to an observer of a century ago.

Three themes run through this analysis. First, climate is notthe only driver of long-term change. The typical timescale forconsidering climate effects is up to the middle or the end of thetwenty-first century. Over that timescale impacts of humanactivities in estuaries and their watersheds will intensify. Thusany effect of climate, including changes in regional and localclimate, on estuaries can be understood only in the context ofregional and local human activities including adaptation toclimate effects.

The second theme is the heterogeneity among estuaries. Theword ‘estuary’ is used to describe a diversity of water bodies

ranging from coastal wetlands to inland seas, and from fjordsto river deltas. They include the mouths of large rivers andisolated basins with no freshwater flow. Some are remote andnearly pristine, whereas others have been altered intensively forhuman use. Comprehending the scope of long-term change inestuaries as a general class of ecosystems requires carefulattention to the unique characteristics of each estuary, and tothe ways that climate and local influences will be modified bythose characteristics (Canuel et al. 2012).

The third theme is the complex interactions among driversand mechanisms of long-term change. Responses of estuarineecosystems are unlikely to be related simply to the proximateinfluences of climate such as increased temperature.

We start with a discussion of the structure and ecosystemfunctions of estuaries and how they respond to physical,chemical, and biological influences (see Section 4.22.2), beforeturning to the important drivers of long-term change (seeSection 4.22.3). We then examine the complex responses ofestuaries to long-term change and provide a few specificexamples by reviewing recent literature for three disparateestuaries (see Section 4.22.4). In Section 4.22.5, we summarizethe implications for ecosystems services and management.

4.22.2 What Is an Estuary?

Pritchard (1967) defined an estuary as “a semi-enclosed coastalbody of water which has a free connection with the open seaand within which seawater is measurably diluted with fresh-water derived from land drainage.” This definition, based solelyon the physical and geographic features of estuaries, has provedtoo narrow. For example, many water bodies lacking freshwaterrunoff behave like estuaries (Potter et al. 2011). Moreover, theregional setting and bathymetry of estuaries may be asimportant as the degree of dilution by freshwater in deter-mining biological features such as species assemblages(Kimmerer 1991).

Several broader definitions and classifications have beendeveloped (e.g., Engle et al. 2007; Dürr et al. 2011), althoughno attempt to define such a heterogeneous collection will suitall needs (Elliott and McLusky 2002). For the purposes of thischapter, an estuary is defined as a body of water that: (1) iscoastal; (2) is geographically distinguished from the opencoast; (3) is permanently or intermittently connected to the

Table 1 Ecosystem services for estuaries based on Costanza et al. (1997). Threats are trends in climate or human activities that negatively affect the provision of that ecosystem service in some estuaries.Mechanisms refer to figures and text in Section 4.22.4.2 (increased temperature, habitat alteration, and increased water demand and nutrient loading are threats to all services but are not listed here)

Service Ecosystem functions Examples

Examples of threats

MechanismsClimate Human

Disturbanceregulation

Capacitance, damping,integrity ofecosystem responseto environmentalfluctuations

Storm protection, floodcontrol, drought recovery;habitat response mainlycontrolled by vegetationstructure

Sea-level riseExtreme storms, windReduced precipitation

Altered sedimentloading

Introduced plantspecies

Sediment (Figure 3)Vegetated beds(Figure 4)

Nutrient cycling Storage, cycling,processing,acquisition ofnutrients

Nitrogen fixation, nutrientand other elementalcycles

Sea-level riseReduced precipitation

Increased waterdemand

Sediment (Figure 3)Vegetated beds (Figure 4)Eutrophication (Figure 5)

Food production Portion of primaryproductionextractable as food

Production of fish, shellfish,and game by hunting,gathering, fishing, oraquaculture

Reduced precipitationOcean acidification

Introduced species Runoff (Figure 2)Eutrophication (Figure 5)Species (Figure 6)

Raw materials Portion of primaryproductionextractable asraw materials

Production of fuel andother raw materials byharvestor aquaculture

Sea-level riseStorms, windReduced precipitation

Overharvest Runoff (Figure 2)Eutrophication (Figure 5)

Biologicalcontrol

Trophic-dynamicregulations ofpopulations

Keystone predator control ofprey species, reduction ofherbivory by top predators

Sea-level riseReduced precipitation

Introduced speciesAltered sediment

loadingIncreased water

demandOverharvest

Runoff (Figure 2)Eutrophication (Figure 5)Species (Figure 6)

Habitat/refugia

Habitat for resident andtransient populations

Nurseries, habitat formigratory species,regional habitats forlocally harvestedspecies, oroverwintering grounds

Sea-level riseStorms, windReduced precipitation

Introduced speciesAltered sedimentloading

Runoff (Figure 2)Eutrophication (Figure 5)Species (Figure 6)

Recreation Providingopportunities forrecreational activities

Ecotourism, sport fishing,and other outdoorrecreational activities

Introduced speciesOverharvest

Eutrophication (Figure 5)Species (Figure 6)

Cultural Providing materials andopportunities fornoncommercial uses

Esthetic, artistic, educational,spiritual, and scientificvalues of ecosystems

Introduced speciesOverharvest

Eutrophication (Figure 5)Species (Figure 6)

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open sea; and, has at least one of the following characteristics:(a) seawater mixing with freshwater from a river or ground-water; (b) elevated salinity caused by evaporation; and (c)when the mouth is open, tidal exchange with the ocean.

This definition includes water bodies that range in size fromtiny (w1 km2) salt marshes on arid coasts of western NorthAmerica, Australia, and Africa to the Baltic Sea (375 000 km2).Although larger, river-dominated estuaries are permanentlyopen to the sea, a vast number of small estuaries dotting thearid coasts of the world are seasonally or infrequently open tothe sea and can be hypersaline (Largier et al. 1997). Freshwaterflows into estuaries range from none at all in desert estuariessuch as Shark Bay, Western Australia (Smith and Atkinson1984) to the massive flows of the St. Lawrence, Mekong, andother large rivers. Tides can be minuscule and depend mainlyon wind and atmospheric pressure, or they can be driven toa height of many meters by the gravitational effects of the sunand moon. Exchange with the ocean may be mediated mainlyby tides or by density-driven circulation as in fjords (Cokeletand Stewart 1985).

Estuaries are often characterized by their high physical energyin the form of tidal currents, wind waves, and freshwater inputs(Nixon 1988). Freshwater inputs and tidal flow also strongly linkestuaries to their watersheds, adjacent lands, and the coastalocean. This linkage transports sediment and dissolved substancessuch as nutrients and contaminants as well as organisms. Estu-aries share some characteristics with each of the linked systems,while retaining some unique characteristics of their own (Elliottand McLusky 2002), many of these related to the interactions offresh and saltwater in a shallow tidal environment.

The physical complexity of estuarine systems leads toa diversity of habitat types ranging from terrestrial with anaquatic influence (salt and freshwater marshes) to tidallydominated pelagic and benthic habitats. The resulting mosaiccan support a high diversity of organisms. In addition, estuariesare often highly productive ecosystems because of high inputsof physical energy from atmosphere, ocean, and land (Nixon1988). These features often mean that estuarine responses toexternal influences can be quite strong and complex.

Knowledge of estuaries is based on a rather small subset ofthe world’s estuaries, principally those in more developedcountries. This is because research focuses on larger estuariesand those where human impacts have resulted in conflict overresources and services. For example, a search in the Web ofScience reveals more than 1000 articles with San Francisco Bayin the title and only 52 with Humboldt Bay, a small estuary ina largely rural area of northern California. This disparity is dueto their relative size and the struggle over water, habitat, andother resources in the watershed of San Francisco Bay, whichhas led to a need for better understanding. This selection biascolors the conclusions of this chapter.

4.22.2.1 Physical Responses of Estuariesto External Influences

The physical configuration and dynamics of an estuary areinfluenced directly by factors that mediate responses of estu-aries to climate change. This discussion focuses mainly on theestuaries with appreciable freshwater flow and tidal currents.Low-flow estuaries are discussed in Section 4.22.2.1.4.

Location is the primary underlying factor that determinesthe characteristics of an estuary. Latitude constrains regionalclimate, including storms, precipitation, temperature, and icecover. Topography of the watershed influences precipitationand runoff. Regional geology and runoff influence the shapeand bathymetry of an estuary and the loading, chemicalcomposition, and size distribution of land-derived sediments(Canuel et al. 2012). Orientation of the coast determinesregional weather patterns and conditions in the adjacent ocean;for example, the temperate west coasts of all major continentsare upwelling regions with moderate ocean temperatures,persistent sea breezes, coastal fog, and dry summer conditionson land. Geographic location (e.g., latitude, continent) deter-mines regional climate and therefore limits the species thatcould potentially colonize an estuary.

4.22.2.1.1 Tides and FlowPhysical conditions in estuaries respond to the interplay amongbathymetry and flows driven by tides, freshwater input, anddensity differences between fresh water and saltwater. Tidalheight at the mouths of estuaries is driven by a combination ofastronomical effects (gravitational pull of the moon and sun)and meteorological effects (atmospheric pressure gradients andwind setup). Astronomical tides in the open ocean can rangefrom a few centimeters (much of the Arctic Ocean) to severalmeters (northeast Atlantic Ocean), but these tides can be greatlyamplified or suppressed by interactions with the coast. Thedimensions and shape of estuaries can also amplify tidal energythrough resonance (e.g., the semidiurnal lunar component inthe Bay of Fundy, Garrett 1972) or attenuate it through narrowor shallow choke points (Fernandes et al. 2004).

Tidal flows transport salt, other dissolved and particulatematter, and even plankton and small nekton. Although theprincipal motion of tidal currents is oscillatory, several mech-anisms cause substances and organisms to mix between theocean and the estuary, and between regions within the estuary.Because of tidal mixing, any property that has a higher (lower)concentration within the estuary than in the coastal ocean willbe mixed out of (into) the estuary. This includes any neutrallybuoyant, passively moving particles or organisms in the water.However, sediments and other negatively buoyant particles canbe trapped in estuaries because they sink. The interaction oftidal flows with sediment deposition, erosion, and transportresults in complex geomorphic features such as offshore sandbars, mudflats, and deep channels.

4.22.2.1.2 Freshwater, Salinity, and MixingIn estuaries with substantial river input, freshwater flow isa dominant control on physical conditions. However, estu-aries respond differently to variations in river flow, dependingon bathymetry, tidal currents, and wind. River input sets upseaward net flow that carries sediment and other materialsand organisms out of the estuary. Freshwater mixes withocean water, producing a horizontal salinity gradient. Salinewater penetrates some distance into the estuary except whereextremely high flowmoves the salinity gradient into the ocean(e.g., Amazon River plume; Lentz and Limeburner 1995).

Overall, the rate at which estuarine water is mixed or dis-charged into the ocean determines the minimum rates needed

Vulnerability of Estuaries to Climate Change 275

for physical, chemical, and biological processes to be sustained.An estuary with a very high proportional flushing rate and shortresidence time because of high river flow or strong tides willexport much of the river-derived material to the ocean andbehave physically as either an extension of the river (if flow isstrong) or the ocean (if flow is negligible). Conversely, estuarieswith long residence times will retain river-derived materialsand chemical and biological processes will be much moreunder endogenous control.

The salinity gradient produces a tendency for stratification,with low-salinity water at the surface and denser, high-salinitywater at the bottom. Although much of the world’s ocean isstratified by temperature, salinity has a much stronger influ-ence than temperature on stratification in most estuaries. Theextent and frequency of stratification is a key feature of estua-rine physics and is used to classify estuaries because of itsimportant role in mixing and transporting substances andorganisms (Hansen and Rattray 1966). Stratification isenhanced by high river flow and is suppressed by strong tidalcurrents and wind, which mix the water vertically. Deep estu-aries and deep channels in estuaries tend to stratify more thando shallow estuaries and shoals. For example, fjords areusually permanently stratified (Cokelet and Stewart 1985),whereas shallow lagoons are usually well mixed vertically (e.g.,Newton and Mudge 2003).

The degree of vertical mixing or stratification determines theextent of the horizontal salinity gradient and its response tochanges in freshwater flow (Hansen and Rattray 1965, 1966;Monismith et al. 2002). In a shallow, narrow, well-mixedestuary (e.g., Hudson River estuary), the salinity gradientmoves along the channel in response to both tides and fresh-water flow. In deeper regions or deeper estuaries, stratificationlimits the extent of this movement (e.g., Delaware Bay).

Stratification produces a physical barrier to vertical mixing,isolating the surface from the bottom waters and allowinggravitational circulation, whereby the ebb currents are strongerat the surface and flood currents at the bottom. This is a keymechanism for landward transport of sediment and organismsthat sink into the bottom water, concentrating them at thelandward end of deep channels or the landward limit of saltpenetration (Postma and Kalle 1955). The paradoxical result isthat increasing freshwater flow reduces residence time for waterbut can increase that of sediment and some organisms.

4.22.2.1.3 Other Riverine Inputs: SedimentRiver flow brings more than water to an estuary: It carriesnutrients, sediment, organic matter, organisms, and in manyestuaries, contaminants. Sediment concentration typicallyincreases sharply with increasing river flow. Loading, theproduct of concentration and flow, is therefore much higherduring floods than at other times. Loading of organic matterfrom land can be an important component of the estuarinecarbon budget and is also tied to runoff (Canuel et al. 2012).

Sediment loading to estuaries nourishes geomorphicfeatures such as sand bars, marshes, and mudflats, and alsocontrols the supply of suspended sediment that turns manyestuaries turbid. Sediment within an estuary is alternatelydeposited and resuspended by wind-driven and tidal currents,and mixing with and net flow into the ocean remove sus-pended sediment.

4.22.2.1.4 Low-Flow EstuariesMany, and perhaps most, estuaries worldwide have little or noriverine input for at least part of the year, including those inarid regions (e.g., Western Australia, southern Africa, southernCalifornia, and Mexico; Largier et al. 1997). In the absence offreshwater flow, tides, wind, and density-driven flow because ofthe salinity gradient mix estuarine waters with the ocean. Theseestuaries can fluctuate markedly between states of low salinityduring wet periods and hypersalinity during dry periods(Largier et al. 1997), especially if their mouths close during lowrunoff (Potter et al. 2011).

Estuaries without appreciable river flow may still havea salinity gradient due to groundwater input or net precipita-tion or evaporation (Largier et al. 1997). These gradients areoften sufficient to induce stratification or two-layer circulation,although stratification caused by temperature differences canbe important in the absence of a strong salinity gradient.

4.22.2.1.5 Ocean ConditionsConditions in the coastal ocean can have a strong influence onestuaries through changes in both water level and composition.Along coasts without substantial tidal energy, circulationbetween estuaries and the ocean is driven by wind, river input,and density gradients. Depending on the orientation andtopography around the estuary, coastal wind patterns caninfluence vertical mixing, horizontal transport, and wind waves(Geyer 1997; Feng and Li 2010). Storms can inundate rivermouths and coastal lakes with saltwater, rearrange sedimentsincluding sand bars and barrier islands, and provide a signifi-cant source of sediment to marshes (Liu 2004; Turner et al.2006). Estuaries adjacent to upwelling zones can gain much oftheir productivity from the import of oceanic nutrients andorganic matter (Smith and Hollibaugh 1997).

4.22.2.1.6 Vegetated BedsMost estuarine classifications rely on physical characteristicssuch as bathymetry, freshwater flow, and stratification (e.g.,Hansen and Rattray 1966; Elliott and McLusky 2002; Humeet al. 2007). However, estuaries can be shaped by biologicalfactors, particularly the development of reefs and vegetatedbeds. Reefs formed by corals, oysters, and other calcifyingorganisms can alter the shape of an estuary. Mangrove forests,marsh vegetation, and seagrasses alter flow patterns (Nepf andVivoni 2000) and thereby trap and consolidate sediment. Thismodifies and stabilizes bathymetry, producing habitat fora variety of species and supporting or concentrating ecosystemprocesses (Waycott et al. 2009; Polidoro et al. 2010).Mangroves and marsh plants, occupying the land margins ofestuaries, provide a valuable buffer against storms and can‘migrate’ up or down the land gradient through die-back andgrowth as conditions change (Simas et al. 2001).

4.22.2.2 Biological Responses of Estuariesto Physical Conditions

Many estuaries support high biological productivity owing totheir physical dynamics, shallow depths, and high inputs ofnutrients (Nixon 1988). Biological interactions in estuariescan be strong (e.g., Nichols et al. 1990), though sometimes


difficult to detect because of the physical variability.Temperature has a direct influence on biota, but mostmechanisms by which climate and other external drivers affectestuarine biota are indirect, including residence time, thelocation of the salinity gradient, stratification, and transportof nutrients, other substances, and organisms from the riverand the ocean.

Estuarine habitats can be divided into shallow vegetatedbeds including marshes, seagrass beds, and mangrove forests,and open-water habitats. Which habitat predominates dependson physical attributes of the estuary such as depth, residencetime, sediment supply, tidal range, and freshwater flow.Although there are considerable interaction and movementbetween these habitats, vegetated beds are essentially terrestrial(although wet), whereas open-water habitats are truly aquatic.In vegetated habitats, space is a resource worth competing over,whereas in open-water habitats resources such as food andnutrients are ephemeral. The key organisms in vegetated beds(i.e., vascular plants that provide the structure) are long-livedand many have limited dispersal capabilities. In open waters,the key producers are microbes with lifetimes of hours to days,and dispersal is rapid even for most benthic organisms whichhave planktonic larvae. This discussion excludes coral reefs,which have characteristics of both of the preceding classes.

Estuarine species are most abundant over a particular rangeof salinity and less abundant where salinity deviates from thatrange. Plankton and nekton live in a moving frame of referenceand therefore move with the salinity gradient. Benthic organ-isms and marsh vegetation may take weeks, months, or evenyears to adjust to a changed salinity distribution by dying backwhere conditions are unfavorable and colonizing favorableareas.

An estuary with short residence time caused by strong tidalmixing or high freshwater flow may support small planktonspecies only if they have very rapid population growth (Rogers1940), although many estuarine animals have swimmingpatterns that overcome these losses (Cronin and Forward1979). At intermediate rates of mixing, the abundance ofphytoplankton may be directly related to residence time(Jassby et al. 2002). Increasing freshwater flow may stimulatephytoplankton production through increased nutrient loading(Nixon 1988; Mallin et al. 1993) or through enhanced strati-fication that traps phytoplankton in well-lit surface waters(Cloern 1984). By these mechanisms, phytoplankton produc-tion can either increase or decrease with freshwater flow, andthese responses can occur in the same estuary. For example, inthe San Francisco Estuary (SFE) phytoplankton biomass variesinversely with flow through residence time in the freshwaterregion (Jassby et al. 2002), positively with flow through strat-ification in South San Francisco Bay (Cloern 1984), and isunresponsive to flow in the low-salinity region of the northernestuary (Kimmerer et al. 2012).

Stratification not only stimulates phytoplankton growth, italso traps settling phytoplankton and other organic matter inthe deep waters of an estuary, where microbial respiration canresult in hypoxia, limiting the suitability of these areas formany organisms (Diaz 2001; Conley et al. 2009). Whenphytoplankton biomass is high and the estuary is eutrophicbecause of the combination of nutrient loading and stratifica-tion, the resulting turbidity can inhibit the growth of plants

such as seagrasses through light limitation (Short and Neckles1999). In some estuaries suspended sediments limit penetra-tion of light into the water and thereby limit production ofphytoplankton and rooted plants even when nutrient concen-trations are high (Cole et al. 1992; Goosen et al. 1999).

The abundance of planktonic animals and small nektonmay vary with freshwater flow because of changes in phyto-plankton biomass. Abundance may increase with flow becausestronger stratification causes higher retention of species thatswim into deeper water or migrate in synchrony with tidalcurrents (Cronin and Forward 1979). Up-estuary transport ofyoung crabs and shrimp could be enhanced, resulting in highersurvival, when high freshwater flow causes increased landwardflow of seawater near the bottom (Schlacher and Wooldridge1994; Kimmerer 2002). Abundance may also vary with resi-dence time as for phytoplankton. Transport of the young ofanadromous species such as striped bass and salmon from theriver to the estuary may be enhanced by high river flow (Stevensand Miller 1983).

4.22.2.3 Human Influences

Most of the world’s coastal cities are on estuaries, and manyestuaries have been heavily modified to support human activi-ties (e.g., Nichols et al. 1986; Lotze 2010). Marshes (Day et al.2008) and mangrove forests (Polidoro et al. 2010) have beendestroyed for development, resulting in extensive losses ofestuarine habitat as well as increasing risk of shoreline erosion.Shorelines have been modified further by the installation ofdocks, jetties, and hard barriers such as levees, seawalls, and tidalgates (Nienhuis and Smaal 1994). These physical alterationstogether with channel dredging and dredge disposal amend theresponse of an estuary to tides, freshwater flow, and storms.

Land use in the watershed can have substantial impacts onan estuary. Extensive agriculture and urban developmentdemand freshwater, cause erosion, and increase peak flowsduring storms. Diversion of freshwater from the watershed andimpoundment of water behind dams reduce the rate anddampen the seasonal pattern of freshwater flow into estuaries(Poff et al. 2007). In addition, rivers and estuaries can bestarved of their sediment supply by trapping behind dams(Schoellhamer 2011).

Some of the freshwater diverted from watersheds and estu-aries is lost to evapotranspiration and some returns as dischargefrom farms, wastewater treatment plants, and industrial facili-ties (or returns elsewhere as precipitation). Eutrophication fromnutrients and organic matter in point-source (wastewatertreatment plants) and non-point source (agricultural and urbanrunoff) discharge is the predominant cause of environmentaldegradation in many estuaries (Cloern 2001).

These human influences have numerous repercussions forestuarine biota, many of which are discussed in Section4.22.4.2. In extreme cases, dams and diversions can starveestuaries of freshwater, resulting in a complete reorganizationof ecosystems (e.g., Colorado River estuary; Kowalewski et al.2000). Similarly, estuarine ecosystems can be severely degradedby extensive hypoxia, resulting in the loss of valuable species(Diaz 2001). The global movement of species through shipping(in ballast water or cargo or attached to ships) and otheractivities is homogenizing biota across oceans (Ruiz et al. 1999).


4.22.2.4 Ecosystem Functions and Services

A variety of ecosystem functions and services can be attributed toestuaries (Table 1). We take an inclusive view of ecosystemservices beyond those that have amarket value (Norgaard 2010).

Estuaries serve as corridors linking land to sea. They supplywater, nutrients, and organic matter to the coastal ocean andsupply sediment, including most of the sand that forms bea-ches in temperate zones. Estuaries and in particular theirvegetated beds act as a filter, dampening variation in riverineand oceanic sources of materials and removing or buryingcontaminants (Klamer et al. 1994; Dürr et al. 2011).

Important nutrient cycling processes in estuaries caninclude nitrogen fixation (Howarth et al. 1988), denitrification(Seitzinger 1988), uptake, remineralization, and sequestrationin plants and sediments. Some estuaries are net autotrophic inthat they consume inorganic nutrients and CO2, producingorganic matter that is either buried or exported to the coastalocean (Kemp et al. 1997); some are net heterotrophic (Smithand Hollibaugh 1997); whereas others are close to balance(Smith and Hollibaugh 2006).

Storage of materials within an estuary generally increaseswith residence time of the water and with the strength oftrapping of sediments by estuarine currents. Growth of organ-isms resident in an estuary results in storage of some of thismaterial in their biomass, with turnover times that can bemuch longer than that of the water. Sediments form anotherpool for nutrients, contaminants, and other substances with aneven longer turnover time than that of most organisms. Sedi-ments may become buried, sequestering these materials, orthey may be exported to the coastal ocean during periods oftidal or storm-driven erosion.

Estuaries provide habitat for a variety of organisms, inparticular nursery habitat for many marine and anadromousspecies. Most estuaries support potentially large populations ofmollusks, crustaceans, and finfish, and many also supportmarine mammals and large populations of birds. Anadromousfish such as salmon and sturgeon use estuaries as rearinghabitat and for passage to and from spawning grounds.

4.22.2.5 Estuaries as Islands

Some of the most severe effects of global warming haveoccurred on mountaintops where low-lying areas forma barrier to species’ range extension and thereby isolatespecies’ populations (Parmesan 2006). Mountaintops in thiscase are effectively islands with limited dispersal for thesespecies. Estuaries may function as islands in that the coastalocean may be inhospitable to some species because of highsalinity, long distances between estuaries, or adverse currentpatterns. Genetic divergence among populations of copepodsin estuaries along the Atlantic coast of North America (Caudilland Bucklin 2004; Milligan et al. 2011) and among pop-ulations of gobies in California estuaries (Earl et al. 2010)suggests limited dispersal among estuaries for these organ-isms. Other case studies indicate high connectivity, such asbetween estuarine populations of copepods in California andMexico (Trujillo Ortiz et al. 1995) and amphipods in NewZealand (Stevens and Hogg 2004).

Resident species in isolated estuaries may be vulnerable tolocal extirpation. For example, populations of several

diadromous fishes in the North Atlantic basin, including Atlanticsalmon, have been extirpated from estuaries through a combi-nation of climate and local anthropogenic effects (Limburg andWaldman 2009). No examples have been reported to date ofextinction of estuarine species caused by climate change.

4.22.3 Long-Term Change

This section discusses direct effects of climate change and ofother, mostly anthropogenic long-term changes. For eachmechanism of long-term change, we assess the likely magni-tude of the effects on estuaries to provide a means ofcomparing among these effects and assessing their relativeimportance. Although direct effects of both climate and humanintervention have been observed and are forecasted to grow,indirect effects and interactions among these effects may bemore important at the level of individual ecosystems, as dis-cussed in Section 4.22.4 Modes of Estuarine Response.

4.22.3.1 Climate Change

Of the many ways that climate Change may influence estuaries,three are consistently present inmodel forecasts and observed inhistorical records and are likely to affect most estuaries (Table 2,Figure 1). Global mean air and ocean temperatures are pre-dicted to continue rising, althoughwith considerable variability,and some areas will even cool (e.g., upwelling zones). Sea-levelrise is also global, but heterogeneous and modified locally bytides and weather. Ocean acidification is pervasive, althoughseveral processes may offset its effect in estuaries.

Other modes of global climate change are either morespatially and temporally sporadic than the preceding three, ortheir historical and forecasted trends are less certain (IPCC2007). In particular, precipitation, storm intensity, and windspeeds will increase in some areas but not in others.

4.22.3.1.1 Temperature IncreaseTemperature exerts a fundamental control on biochemical ratesin ecosystems and on physiological rates in most marine andestuarine organisms. Biogeochemical rates such as nutrientcycling generally increase with temperature. Solubility ofoxygen in water decreases as temperature rises, so that lessmicrobial respiration is required to reach a hypoxic condition.At the same time, respiration increases with increasingtemperature at a higher rate than does photosynthesis, so thatmicrobial processes will consume more oxygen (Lopez-Urrutiaet al. 2006). The overall effect is a trend toward net hetero-trophy and an increased incidence of hypoxia in estuaries.Warmingmay also contribute to the formation of harmful algalblooms, which appear to be increasing in water bodiesworldwide (Paerl and Huisman 2008).

Rates of processes such as respiration and consumptionincrease with temperature up to a point beyond which they falloff rapidly toward the upper lethal temperature (Pörtner 2002).The setpoints and temperature sensitivities vary among thephysiological processes and with species and acclimatizationhistory. Below the peak, rising temperature causes positiveresponses of consumption, nutrient uptake and release,reproduction, growth, and development, while at the sametime mortality rates increase as predators, pathogens, and

Table 2 Projected long-term changes in estuaries relative to current conditions. Quantitative projections are in the middle of the range; see Section4.22.3 for sources. Causes in bold are those with supporting evidence and consistent trend in many estuaries. Projections listed as "Regional" refer toincreases in some areas and decreases in others; those listed as "Variable" have trends with less statistical support than, for example, temperature.Consequences exclude interactions among causes and effects (see Section 4.22.4.2 Interactions).

Cause Projection to 2100 Potential consequences for some estuaries

Change in phenology, range of estuarine speciesTemperature rise 3 �C Species introductions and extirpations

Reduced survival, reproduction, and growth of desirable species

Sea level rise 1�2 m

Reduced growth rate of submerged vegetationIncreased erosion of shorelines, mudflats, and marshesIncreased inundation of shoreIncreased salt penetration, species changesIncreased strength of tidal currents

Acidification +0.3 pH units Impaired calcification of bivalvesTotal precipitation Regional Possible increase in extreme floods and droughtsTiming of runoff Regional, earlier Shift to earlier runoff peak where dominated by snowmeltWind speed Variable Increased erosion with increased wind speed; increased upwellingStorm Intensity Variable Increased erosion, flooding

Catastrophic changeBreaches of levees or barrier islands due to storms may alter physical configurationShifts to alternative stable ecosystem states

Dams in watershed Increases in lessdeveloped areas

Reduction in total freshwater flow, alteration of seasonal hydrograph

Dredging/armoring Regional increase Increase in erosional power in unarmored areas, loss of sediment for marshesPopulation growth Regional increase Increased demand for ecosystem services; increased urbanizationWater demand Regional increase Decreased inflow and increased salinity

Sediment loadingRegional decrease Increased water clarity, erosion

Increased loading with storm intensity, decreased water clarity, depositionHabitat alteration Regional increase Elimination of vegetated areas, degradation of othersNutrient & organic loading More eutrophication Hypoxic areas, harmful algal bloomsHarvest Regional increase Alteration of trophic structureSpecies introductions Difficult to predict; depends on what species and where


parasites increase their metabolic rates. Thus, the vital rates ofbirth and death for any population vary nonlinearly withtemperature, and no two populations respond the same way.Sharp changes in relative abundance are likely because of thesedifferential responses to temperature among the species in anestuarine assemblage.

Temperature also affects organisms directly through theirphenology, and this effect could be significant for estuarinespecies, although there are few reports of phenological shiftsfrom estuaries and coastal environments. Photoperiod has anequally strong influence on phenology, and as spring warmingcomes earlier and fall cooling later, various organisms mayreceive conflicting signals from temperature, season, andsunlight, and reproductive success may decrease (Lawrence andSoame 2004). For example, a delay in reproduction of autumn-spawning limpets because of higher temperature resulted inrecruitment failures (Moore et al. 2011).

Increases in temperature at one location may result inextirpations but favor populations of species with higherthermal maxima. At a larger scale the net result is polewardmovement in the ranges of species that can cross the inter-vening ocean. Such range shifts have been observed in open-ocean phytoplankton, zooplankton, and fish (Mantua et al.1997; Reid et al. 1998; Beaugrand and Reid 2003; Perry et al.2005). Range shifts in invertebrates of rocky shores in centralCalifornia have been linked to increasing temperature (Sagarin

et al. 1999). High-latitude estuaries and coasts will lose icecover and the intense spring bloom of ice algae will give way toa more diffuse bloom of phytoplankton, cutting off much ofthe food supply to the benthos (Grebmeier et al. 2006).

Case studies in estuaries show some evidence of range shiftsin response to warming. A northward shift in distributions ofpredatory marine species in response to decadal-scale climatefluctuations may have induced a trophic cascade in the SFE(Cloern et al. 2007). Encroachment of tropical species of fishinto a South African estuary apparently was driven by increasesin temperature (James et al. 2008). A similar pattern wasobserved in seagrass meadows of the northern Gulf of Mexico(Fodrie et al. 2010). However, no shifts were seen in inverte-brates on rocky shores of eastern Australia over 50 years despitemeasurable warming (Poloczanska et al. 2011). A meta-analysis showed no clear evidence of range shifts in marine,estuarine, and freshwater fishes in Australia, although theauthors of that study acknowledged thedifficulty of determiningranges of such highly mobile species (Booth et al. 2011). Thepaucity of reports of range shifts in estuaries may reflect thepaucity of studies or an actual lower rate of range contraction orexpansion in estuaries compared to other ecosystems.

4.22.3.1.2 Sea-Level RiseGlobal sea-level rise has been estimated to accelerate and toreach 1–2m by 2100 (Vermeer and Rahmstorf 2009).

Figure 1 Schematic of significant long-term influences on a generic estuary, represented here by the Chesapeake Bay. Each box is a potentialinfluence on estuarine functions or the provision of estuarine services. Rectangular boxes on the left-hand side are climate effects, ovals on the right-handside are human activities, and rounded rectangles at the top are human adaptations to climate, and other long-term, changes. Green borders indicatea positive trend in some regions. Gray borders indicate a positive or negative trend. Black arrows indicate causative relationships between influences andecosystem responses that may be positive or negative. Table 2 gives additional details about mechanisms underlying some of these relationships.


However, locally the rise is amplified or offset by erosion,accretion, or vertical movements of the earth’s crust (Peltier1999). For example, the Mississippi Delta is losing land areabecause of sea-level rise coupled with a reduction in sedimentsupply in the Mississippi River caused by dams. Rising relativesea level will have numerous direct effects and many indirecteffects on estuaries. The most obvious effect would be inun-dation of lands that are now dry and conversion of intertidalhabitats to subtidal. For example, the area inundated by the100-year flood in San Francisco Bay at a 50-cm higher sea levelwould include about 35 000 ha of nearshore lands, of whichabout half is developed and half is grassland (Knowles 2010).

The consequences of sea-level rise are difficult to predict ina given estuary. First, high tides are likely to increase more thanmean sea level through frictional effects, and this effect wouldbe exacerbated by any increase in tidal resonance, storm surges,and freshwater floods (Cayan et al. 2008). Second, increasingsea level will alter sediment transport through erosion, depo-sition, and changes in circulation patterns, resulting in a changein bathymetry that is difficult to predict without accuratesediment-transport models. Third, trapping of sediment by

vegetated areas such as marshes and seagrass beds and upslopemigration of marshes and mangrove forests will alter theoutcome of rising sea level. Finally, human populations willnot remain passive to rising sea level, but will armor shorelineswith levees and seawalls.

4.22.3.1.3 AcidificationOcean water resists changes in pH through a buffering systemin which dissolved CO2 is in chemical equilibrium withhydrogen, bicarbonate, and carbonate ions. Since the begin-ning of the industrial age about 40% of the net release of CO2

into the atmosphere has dissolved in the ocean (IPCC 2007).This increase in dissolved CO2 has shifted the equilibrium tomake the ocean more acidic by 0.1 pH, and projections are fora further reduction by 2100 of w0.3 pH units (Caldeira andWickett 2003), a 2.5-fold increase in acidity.

The decrease in ocean pH will cause dissolution of, andincreased metabolic cost of producing, hard parts of calcareousorganisms including corals and bivalves. The most prominenteffect is the reduction in calcification by corals, which can slowtheir growth and kill reefs (Orr et al. 2005). Oyster, clam, and


mussel beds and several groups of open-ocean plankton arealso likely to be affected severely and rather unpredictably.For example, two species of oyster that were exposed experi-mentally to different levels of atmospheric CO2 had verydifferent responses in growth and shell formation (Miller et al.2009).

Research since the IPCC report (2007) reveals that a widevariety of organisms may suffer negative physiological effectsfrom acidification (Fabry et al. 2008), although effects on non-calcifying organisms may not be severe (Hendriks et al. 2010).Ecological effects of acidification have scarcely been addressed.

The pH along coasts and in estuaries is subject to influencesnot prominent in the open ocean, notably inputs of waste-water, freshwater, sediment, acids, and bases, as well as localbiological processes, particularly microbial production andrespiration, that alter total CO2 concentrations. Upwelled wateris initially supersaturated with dissolved CO2 and low in pH,although pH increases with time at the surface (Fassbenderet al. 2011). Estuaries generally are sources of CO2 to theatmosphere because of a combination of net heterotrophy,eutrophication, and discharges from wastewater treatmentplants (Caffrey 2004; Borges et al. 2006; Feely et al. 2010; Caiet al. 2011).

4.22.3.1.4 Storms and PrecipitationThe rising heat content of the oceans adds moisture and energyto the atmosphere and will increase the frequency or strength ofstorms (Knutson et al. 2010), but regional weather responsesare rarely known. Nevertheless, future precipitation and thefrequency and intensity of storms are less well constrained inglobal climate forecasts than are temperature, sea level, andacidification (Nijssen et al. 2001; Trenberth et al. 2003).Regional projections of annual precipitation using downscaledclimate models differ even in the direction of the trend. Theresponse of precipitation to ocean-scale climate forcing such asthe El Niño-Southern Oscillation (ENSO) is informative: ElNiño patterns bring wet conditions to northwestern NorthAmerica and dry conditions to southern California and westernMexico, and central California can be either wet or dry (Cayanet al. 1999). Where much of the runoff comes from melting ofsnow or glaciers, the warming trend will lead to earlier runoffpeaks. This is both observed and forecasted for wide areas ofwestern North America where much of the annual runoff isfrom snow melting in high mountains (Aguado et al. 1992).

Violent storms can have a wide variety of physical effects onestuaries that depend on orientation of the coast, elevation,and vulnerability of coastal features to overtopping or scour.Infrequent, powerful storms may be important in supplyingsediment to coastal marshes and beaches (Turner et al. 2006).Even in the absence of geomorphic effects, storms can causeflooding and temporary transformation of estuaries intofreshwater systems, or coastal lakes into saline systems (Paerlet al. 2001; Liu 2004).

4.22.3.1.5 Wind and UpwellingAtmospheric pressure gradients responsible for wind are likelyto intensify with increasing temperature gradients and stormintensity. The direct influence of wind on estuarine dynamicsdepends on water depth and wind direction (see Section4.22.2.1.5 Ocean Conditions.). On the West coasts of

continents an increase in wind strength is likely to result in anincreased frequency and intensity of upwelling (Snyder et al.2003), potentially increasing the loading of nutrients andorganic matter to estuaries.

4.22.3.2 Direct Human Effects

Human population is likely to continue growing, although ata declining rate (United Nations 2004). At least for some timein some countries this growth will be accompanied byincreasing standards of living resulting in an increase in effi-ciency of agriculture and a shift in the economic basis fromagriculture to industry. More people will live in cities andaround estuaries. Increasing urbanization, together withincreasing demand for water, land, and food, will increasedemand for estuarine services and have negative impacts onestuaries (Figure 1, Table 2).

Globally, forecast shortages of water are more likely to arisefrom increasing demand as human populations grow thanfrom decreasing precipitation (Vörösmarty et al. 2000).Current shortages in arid regions will be exacerbated. Majorriver systems that form or cross international borders includethe Amazon, Nile, Mekong, Brahmaputra, and Tigris-Euphrates, many of which have substantial dams in place orunder construction. Although minimum flows are legallymandated in many rivers and estuaries (e.g., SFE; Kimmerer2002), such mandates are difficult to maintain where riverscross borders, as in the Colorado River estuary (Kowalewskiet al. 2000).

The increasing conversion of land to support the growinghuman population will result in more destruction of marshesand mangrove forests for farming, urban development, andbiofuel (Polidoro et al. 2010). Some shifts in land use, such asintensification of agriculture, and limits on water supply arelikely to lead to further increases in use of fertilizer and agri-cultural chemicals (Tilman et al. 2001). The resulting increasein nutrient loading to estuaries will worsen eutrophication(Nixon 1995; Tilman et al. 2001).

Sediment inputs to estuaries likely will be reduced in mostareas of the world. Dams trap sediments, causing erosion inrivers downstream (Kondolf 1997) and limiting sedimentsupplies for maintaining estuarine marshes (Blum and Roberts2009), with consequent erosion in estuaries as seen in the Nileand Mississippi Deltas. The reduction in sediment supplies toriver-dominated estuaries will cause turbid waters to clear(Schoellhamer 2011), contributing to algal blooms andexacerbating any tendency toward eutrophication.

The human demand for food will increase fishing pressure,even with more comprehensive management and regulation offisheries. Already overfishing has caused reorganization ofoceanic and coastal ecosystems through removal of top pred-ators (Scheffer et al. 2005), and additional effects are beingseen as fishing effort shifts to smaller fish at lower trophic levels(Pauly et al. 1998). A worldwide increase in abundance ofjellyfish in oceanic and coastal waters may be caused byremoval of competition by fish, along with warming (Purcell2005; Purcell et al. 2007). Jellyfish contribute little to fisheries.

Shoreline modification will accelerate, leading to installa-tion of more hard edges and barriers (Bulleri and Chapman2010). Armoring shores with seawalls and leveesmay accelerate


as part of a strategy of adaptation to rising sea level, particularlyin low-lying areas (Nicholls and Cazenave 2010). Armoredshores can lead to increased and focused wave and tidal energy,causing erosion along unprotected shores. Restoration ofmarshes also alters shorelines, although it is generally done toimprove or enhance estuarine ecosystems (Callaway 2005).

The increase in global population and trade has caused anincrease in worldwide shipping and that trend is expected tocontinue. Besides the requirement for expansion of port facil-ities, shipping brings an unavoidable risk of accidents and oilspills that damage estuaries, especially marshes and otherintertidal areas.

Global shipping also transports species between estuaries(Carlton and Geller 1993; Ruiz et al. 1999, 2000). Althoughregulations that require treatment of ships’ ballast water may beeffective in reducing this transport, the increase in shipping,particularly into ports that lack regulations or enforcement, willallow more transport of organisms. Certain introduced specieshave altered the physical structure of estuaries, notably plantsthat create dense beds (e.g., cordgrass; Callaway and Josselyn1992; Crooks 2002; Levin et al. 2006; Silliman and Bertness2004). Some bivalves, including the zebra mussel (Caraco et al.1997) and the clams Corbicula fluminea (Phelps 1994) andCorbula amurensis (Nichols et al. 1990), have had strongimpacts on water-column organisms through predation andcompetition. Other taxonomic groups have had big impacts(Purcell et al. 2007; Lehtiniemi and Gorokhova 2008), and thenative faunas of some estuaries have been overwhelmed byintroduced species (e.g., SFE, Cohen and Carlton 1998).

A catastrophic change in the physical configuration of anestuary has a low probability of occurring in any one year, butover a timescale of many decades it can become likely. Forexample, by definition a 100-year flood has a 1% probability ofoccurrence in any one year and is likely to occur at least once inthe next 70 years. Catastrophes that can be anticipated includegeomorphic rearrangement by large storms (Morton and Sal-lenger 2003), earthquakes (Mount and Twiss 2005), andtsunamis (MacInnes et al. 2009). These changes may or maynot be reversible through intensive human intervention such asby repairing breaches in levees or seawalls.

4.22.4 Modes of Estuarine Response

The responses to long-term change are likely to differ greatlyamong estuaries in different regions. Some regions will see anincrease in precipitation and others a decline, some regions willbe harder-hit by strong storms than others, and rivers in somemountainous watersheds will see a shift to more runoff inwinter and less in spring. The rate of sea-level rise will dependon local or regional vertical movements of the earth’s crust.Estuaries at high latitudes will have a longer ice-free season.Estuaries in upwelling zones will receive a higher input ofnutrients as upwelling strengthens.

The unique physical and geographic configuration of eachestuary also influences its response to long-term change. Forexample, estuaries formed from river-derived sediment such asdeltas and bar-built estuaries are particularly vulnerable tochanges in freshwater flow, impoundment of sediments, sea-level rise, and alteration of longshore sediment transport by

ocean waves. The ability of an estuarine species to extend itsrange depends on the distance to adjacent estuaries, outflowpatterns, and ocean currents, as well as its ability to surviveoceanic conditions.

The relative importance of climate and other anthropogeniceffects will depend on the extent of human use of an estuary.Where estuaries or their watersheds have been extensivelyaltered for human use, trends in human demand for freshwater,food, and land will be a key determinant of long-term change.In addition, human adaptation to climate change will entailalterations that also influence ecosystem function.

Moreover, changes that are expected within any oneestuary will be concurrent, and their effects will interact (seeSection 4.22.4.2. Interactions). These interactions are likely toplay out in different ways in different estuaries.

Some likely differences among the responses of estuaries toclimate change can be inferred from their heterogeneousresponses to seasonal (somewhat predictable) and interannual(unpredictable) variation in inputs. For example, estuariesshowed widely varying seasonal and interannual patterns ofphytoplankton production, probably reflecting differentcontrols on productivity (Cloern and Jassby 2008, 2010).Likewise, estuarine organisms can respond to freshwater flowby a wide variety of mechanisms that can even be of oppositesign (see Section 4.22.2.1.2 Freshwater, Salinity, and Mixing;Kimmerer 2002; Gillanders and Kingsford 2002).

Estuarine scientists usually study only one or a few estu-aries, and comparisons among estuaries are valuable butinfrequent (e.g., Boynton et al. 1982; Nixon 1988; Seitzinger1988; Emmett et al. 2000; Cloern and Jassby 2008, 2010;Gillanders et al. 2011). This emphasis on particular estuariesreflects the heterogeneity among estuaries, their differentresponses to climate and other influences, and the differencesin research topics. For example, eutrophication is arguably theprocess of greatest management concern in many estuariesin Europe and eastern North America, and therefore receivesa lot of research attention. Eutrophication is much lessprevalent, and therefore less often studied, in estuaries inwestern North America. The heterogeneity among estuariesand among the topics studied limits the scope of generalstatements on the effects of climate change on estuarineecosystems.

4.22.4.1 Mitigation and Adaptation

Mitigation activities that affect estuaries directly includeincreases in the generation of renewable energy by dams andtidal and wave generators or by production of biofuel throughharvesting or aquaculture. Each of these can exact penalties tothe functioning of estuarine ecosystems. Dams alter hydro-graphs, and tidal and wave generators can kill estuarineorganisms (Aprahamian et al. 2010). The use of large-scalecoastal aquaculture to raise food or biofuels requires a largearea with attendant loss of habitat and potential foreutrophication.

Adaptation in this context refers to actions to reducedeleterious effects of climate change at a local or regional scale(Scheraga and Grambsch 1998; Nillesen and van Ierland2006). This may include abandoning areas that becomeunlivable because of changing climate, which is already

Figure 2 Schematic of the physical and biological responses of an estuary to changes in ‘precipitation and freshwater runoff,’ as described in Section4.22.2.1 Physical Responses of Estuaries to External Influences. Boxes as in Figure 1 plus blue trapezoids are physical responses to external influencesincluding the ‘salinity’ gradient. Green trapezoids are biological responses, perhaps mediated by physical responses. Border colors as in Figure 1. Blackarrows as in Figure 1. Green arrows indicate a positive effect as the influence increases. Red indicates a negative effect, which refers to the underlinedquantities in the center boxes. Numbers in boxes link responses to other figures where processes are detailed.


occurring in the Mekong Delta and elsewhere (de Sherbininet al. 2011). Less extreme measures include armoring toprotect property against erosion caused by rising sea level andmore powerful storms. In agricultural watersheds, adaptationto higher temperature and increased drought frequency mayrequire increasing irrigation flows, further increasing demandfor water, unless improvements in irrigation practices offsetthis increase. Desalination to offset loss of freshwater flowrequires power and the disposal of brine.

Many adaptations will entail changes in policy and exacer-bate conflict over dwindling resources, particularly whereestuaries and their rivers cross international borders. Policychanges could include abandonment or reduction of environ-mental protection in favor of protecting human life andproperty, resulting in devastating alterations to estuaries.

4.22.4.2 Interactions

In general, the anticipated long-term changes in climate andhuman influences are clear (see Section 4.22.3); the processesthat control estuarine responses to those influences are under-stood in broad terms (see Section 4.22.2). However, these effectswill not occur in isolation. The network of interactions amongclimate variables, other long-term trends, and physical andbiological responses are complex and our summary of interac-tions is necessarily simple and incomplete. Each subsection

proceeds generally from the immediate effects of climate forcing,to the effects of human activities, and then to the interactions.

4.22.4.2.1 Precipitation, Freshwater Runoff,and Salinity (Figure 2)Climate and human activities influence the temporal patternand quantity of runoff into estuaries, with numerous ramifi-cations for ecosystem function. Annual precipitation, timing ofrunoff peaks, and variability because of seasonal patterns andstorms, all of which can have climate-based trends, determinethe water available for runoff. Rising temperature will increaseevapotranspiration, which increases uptake of water by plantsfrom groundwater, reducing flow into estuaries (Nijssen et al.2001).

Increasing demand for water, increased evapotranspiration,and seasonal shifts and unreliability in precipitation willstimulate construction of dams to capture more water for timesof low flow. Although dams dampen seasonal variability infreshwater flow, greater variability in precipitation andconstruction of impervious surfaces and levees will increaseshort-term variability by hours to days. Local diversions ofwater from rivers and groundwater pools will further depletethe water available to flow into estuaries, especially duringdroughts.

Changes in runoff will be accompanied by changes inloading of materials such as sediments and nutrients. However,


these loadings in general are more influenced by regional orlocal human activities such as dams and agriculture than bychanges in flow patterns.

Changes in freshwater flow into estuaries have numerousconsequences, and numerous causal pathways to those conse-quences (see Section 4.22.2.1.2.). The influence of freshwaterflow on salinity patterns in an estuary depends on bathymetry,sea level, tides, and wind speed and direction. Rising sea levelwould increase salinity in the absence of other changes but inmost estuaries it probably has a minor influence on long-termchange compared with alterations in flow and bathymetry.

The physical changes associated with increasing freshwaterflow induce a cascade of biotic responses. In tidal freshwaterregions flow directly influences distributions and residencetime of organisms. In more saline areas the influence of flow ismediated by salinity. Shifts in the salinity gradient alter thedistributions of benthic and marsh biota through salinitytolerance and species interactions such as predation andcompetition. While pelagic species are less affected by themovement of the salinity gradient because they move with thewater (Laprise and Dodson 1993), high flows can flush pelagicorganisms from an estuary and diminish their estuarinepopulations.

Variation in freshwater flow into estuaries affects phyto-plankton production (see Section 4.22.2.2), which is thepredominant source of available organic carbon for mostestuarine ecosystems. In many river-dominated estuaries,freshwater flow stimulates primary production. Fish and largeinvertebrates may grow and reproduce faster if high flowstimulates primary production, increasing production up thefoodweb. Alternatively, estuarine animals may respond tochanges in physical habitat; for example, through retentionmechanisms or changes in the overlap of favorable salinity withfavorable bathymetry (Kimmerer et al. 2009).

4.22.4.2.2 Sediment Supply, Transport, and Loss (Figure 3)River flow controls much of the loading of sediment into anestuary (see Section 4.22.2.1.3.). Sediment loading mayincrease with an increase in peak river flows, or under dryconditions with an increase in the frequency of fires thatexpose land to greater erosion. Dredging, mining, andconstruction of hard or impervious surfaces in the watershedcan increase sediment loading. In many of the world’s rivers,though, the predominant trend is likely to be a decrease insediment loading because of the construction of dams thattrap sediments.

Within an estuary the short-term balance between erosionand deposition is controlled by the strength of tidal and river-derived currents and wind waves and how these interactwith bathymetry. Sea-level rise, an increase in wind speed orpeak storm intensity, armoring as a defense against localerosion, and dredging will shift this short-term balance towarderosion.

The long-term balance between supply and loss to the oceanand to burial within the estuary depends on the short-termbalance, which governs howmuch of the sediment is suspendedin the water (Schoellhamer 2011). The long-term balance alsodepends on residence time of suspended sediment in theestuary. Sediment can be trapped by two-layer estuarine circu-lation, which usually increases with freshwater flow and with

increasing water depth (see Section 4.22.2.1.2.). Estuaries thatare open only seasonally may remain open for longer periods asthe sand supply dwindles and peak wave height increases,although decreasing river flow may shorten the open period.

Turbidity caused by suspended sediment can limit growthrates of plants, including phytoplankton and seagrass (Goosenet al. 1999). Turbidity also interferes with visual detection ofprey by predators, creating a refuge for the prey (De Robertiset al. 2003). Increasing water depth caused by tidal scouring ofchannels, in some places accelerated by dredging, can changetidal circulation, salt balance, and the movement and retentionof planktonic and epibenthic organisms (Hansen and Rattray1965; Monismith et al. 2002).

4.22.4.2.3 Vegetated Beds (Figure 4)Marshes, mangrove forests, and seagrass beds exist ina dynamic equilibrium of sediment erosion and deposition.Long-term changes in factors that affect sediment budgets (seeSection 4.22.4.2.2.) are essential to the development andstability of vegetated beds. Conversely, vegetated beds reducecurrent speed and wave height, capturing sediment and resist-ing erosion.

Accretion of marshes is faster where sediment supply isadequate and the tidal range is large enough to transport thesediment quickly (Simas et al. 2001). If the sediment supply isinadequate for marsh elevation to keep up with rising sealevel, marshes may ‘move’ up-slope as upland is replaced bymarsh and low marsh by open water. However, this will belimited in many estuaries by the slope of the land and theextent of hardened shoreline (Osterkamp et al. 2001; Chustet al. 2011).

Vegetated beds are vulnerable to other effects. Forexample, seagrasses may be affected simultaneouslyby increased CO2 and temperature, which enhance growth;increased sea level, which decreases light availability;increased strength of tidal currents; and increased salinitypenetration, which limits the range of seagrass beds throughphysiological tolerance (Short and Neckles 1999). However,local anthropogenic impacts such as habitat destruction,nutrient loading, and sediment discharge may cause a muchgreater loss of submerged and marsh vegetation than climatechange. Nutrient loading can stimulate phytoplanktonblooms which reduce light penetration and degrade vegetatedbeds. Direct human impacts have devastated seagrass beds insome areas (Orth et al. 2006), and a recent estimate of globalrate of loss was an astonishing 7% per year (Waycott et al.2009). Similar rates of loss apply to mangrove forests, withthe principal cause being development (Polidoro et al. 2010).In both cases, valuable ecosystem services such as nutrientcycling and shoreline protection are lost.

4.22.4.2.4 Eutrophication (Figure 5)Eutrophication in estuaries is usually caused by the loading ofnutrients or organic matter from land (Ryther and Dunstan1971; Cloern 2001). Organic loading stimulates estuarineeutrophication directly, and nutrient loading stimulateseutrophication by enhancing phytoplankton growth. Althoughthe cause of eutrophication is straightforward, reducing it hasproved difficult, and several long-term trends suggest that thisproblem will get worse (Diaz 2001). In developed countries

Figure 3 Schematic of the effects of changes in ‘sediment supply, transport, and loss’ on sediment deposition and erosion within an estuary, asdescribed in Section 4.22.4.2.2 Sediment Supply, Transport, and Loss. Boxes, arrows, and numbers are as in Figures 1 and 2. Green is positive for erosionand negative for deposition. Red is negative for erosion and positive for deposition.


nutrient reductions achieved through improved treatment havebeen be offset by intensification of loading from non-pointsources such as farms (Kemp et al. 2005).

Long residence time allows for production and decompo-sition of organic matter, and persistent stratification increaseslocal residence time for sinking particles (see Section4.22.2.1.2. Freshwater, Salinity, and Mixing), whereas turbiditycaused by suspended sediment inhibits primary production.Therefore, either increases or decreases in freshwater flow couldexacerbate eutrophication in estuaries, as could a reduction inturbidity because of decreases in suspended sediment.

Hypoxia is a persistent problem in many estuaries andcoastal regions (Diaz and Rosenberg 1995; Diaz 2001; Conleyet al. 2009), particularly in places in which eutrophicationcombines with restricted circulation such as in stratified, deepbasins. Few benthic organisms can survive extended periods ofhypoxia, and the hypoxic region is excluded as productivehabitat (Breitburg 1992; Diaz and Rosenberg 1995). Increasing

temperature raises microbial metabolic rates and can increasethe frequency or extent of hypoxia.

The frequency of harmful algal blooms may be increasingworldwide, partly owing to high nutrient concentrations andhigh temperature (Anderson et al. 2002; Paerl and Huisman2008). These blooms can reduce production of fish and othervalued estuarine biota.

Increases in contaminant loading are also likely; forexample, from urban runoff and intensified agriculturalproduction. We do not consider contaminants further becausetheir effects are often site-specific and sporadic.

4.22.4.2.5 Species Shifts (Figure 6)Changes in range or phenology caused by regional warming,competition with introduced species, or other trends canincrease or decrease the abundance of a species in an estuaryand allow for colonization from other estuaries. To colonize anestuary, a founding population must survive transport from

Figure 4 Schematic relating climate, and other long-term, changes to the stability of ‘vegetated beds’ in estuaries, such as marshes, mangrove forests,and seagrass beds, as described in Section 4.22.4.2.3. Boxes, arrows, and numbers are as in Figures 1–3 plus red borders indicate a likely negative trend.

Figure 5 Schematic relating climate, and other long-term, changes to ‘eutrophication’ in estuaries, as described in Section 4.22.4.2.4 Eutrophication.Boxes, arrows, and numbers are as in Figures 1–3.


one estuary to another, so colonization may be more frequentwhere estuaries are large, close, and hydrologically linked, as inthe southeast United States. However, colonizers may be nearthe limit of their thermal tolerance. Species introduced artifi-cially from another estuary of similar climate (e.g., Cordell etal. 2008) are more likely to find favorable conditions andquickly integrate into their new environment than naturalcolonizers (Cordell et al. 2008). Introductions are likely toremain the major mechanism for colonization in many

estuaries, and are also likely to accelerate colonizations thatwould eventually occur through range shifts.

Successful colonization depends on environmental condi-tions such as suitable temperature, salinity, sediment type, flowconditions, and residence time. Interactions with local speciesare equally important (e.g., food supply, predators, shelterfrom predation, harvest). These physical and biological attri-butes are difficult to assess even in well-studied situationsbecause of the myriad and often subtle species interactions

Figure 6 Schematic relating climate, and other long-term, changes to shifts in ‘species composition’ within estuaries, as described in Section 4.22.4.2.5.Boxes, arrows, and numbers are as in Figures 1–4. Except for the ‘species interactions’ box, this diagram refers to a single species in a single estuary.


such as concurrent shifts in range and phenology of predatorand prey. For example, relative changes in phenology betweena copepod and its gelatinous predator in Narragansett Bay,apparently caused by warming, have intensified the predator–prey relationship, resulting in seasonal extirpation of thecopepod (Costello et al. 2006). Intertidal mussels on a rockyshore could withstand increasing temperature only in theabsence of predators (Harley 2011). Thus, the ‘species inter-action’ box in Figure 6 hides a tangle of interactions.

The potential also exists for long-distance effects of changesin species abundance. For example, some ducks and shorebirdsmigrate thousands of kilometers between summer and winterhabitats. Some of these species are important components ofestuarine ecosystems (e.g., Poulton et al. 2002), and changes intheir abundance caused by events in their alternate habitatwould affect those ecosystems.

4.22.4.3 Catastrophes

Although catastrophic events such as earthquakes and floodsare inherently unpredictable, often their cumulative proba-bility over decades can be calculated. Mount and Twiss (2005)estimated a two-thirds probability by 2050 of multiple failuresamong weakened levees in the California Delta, cutting offa water supply for about 23 million residents and about half ofCalifornia’s agriculture. This scenario is reminiscent of Hurri-canes Katrina and Rita in the Gulf coast (Day et al. 2007) andthe recent tsunamis in Indonesia (2010) and Japan (2011). Theimpact of catastrophes obviously depends on the particularcharacteristics of the estuary.

Catastrophic change may also occur when systems arepushed from one stable state into another. A slight change insalinity in a Danish fjord, caused by a decision to open a sluicegate to increase exchange with the ocean, transformed theestuary from a turbid state with high phytoplankton biomass toa clear state with numerous benthic filter-feeders (Petersen et al.2008). Although examples of such apparently reversible state

changes are uncommon, similar but irreversible state changescan accompany changes such as the spread of invasive species(see Section 4.22.4.2.5. Species Shifts (Figure 6)).

4.22.4.4 Application to Particular Estuaries

Here we describe anticipated responses to long-term change ofthree disparate estuaries. The San Francisco, Chesapeake, andMurray–Darling estuaries are large and intensely studied, withrecent published reports on observed and predicted long-termchange. Each has prominent intergovernmental agenciesresponsible for stewardship (e.g., the California Delta Stew-ardship Council, the Chesapeake Bay Program, and theMurray–Darling Basin Authority).

4.22.4.4.1 San Francisco EstuaryThe major concerns for the SFE focus on the tidal freshwaterregion, a key element of California’s water supply system, andthe conflict between water supply and freshwater flow into theestuary. Impediments to solving this conflict in the long terminclude the historical loss of most of the fringing marshes todevelopment, the prevalence of nonnative invasive species, thevariable flow regime with a trend to earlier spring runoff peaks,and the seismic vulnerability of levees protecting subsidedfarmlands.

The SFE, which includes San Francisco Bay and the Cal-ifornia Delta, is one of the largest and most-studied estuaries inNorth America. The SFE is a heavily urbanized estuary witha strongly agricultural watershed. Most of the freshwater for theestuary originates as snowmelt in the Sierra Nevada and flowsthrough the Delta of the Sacramento and San Joaquin Rivers.The climate is Mediterranean, with a long summer dry seasonand a winter wet season with up to ninefold variation in annualrunoff. Much of the annual flow is trapped behind dams for usethrough the dry season, and the lower reaches of the watershedare laced with canals and diversions. The intensive use of the


watershed, coupled with interannual variability in seasonalwater supply, is the main reason for the intense conflict overwater in California.

A recent synthesis of information from an interdisciplinarymodeling effort projected the effects of climate change on theSFE and its watershed through the twenty-first century (Cloernet al. 2011). Forecasts were based on two IPCC scenarios, oneof moderate warming (PCM-B1) and the other of rapidwarming and a decrease in precipitation (GFDL-A2), comparedto a baseline period of 1970–1999. Cloern et al. (2011) useda series of models to estimate regional air and water tempera-ture, precipitation, runoff, operation of the water storage anddelivery system, sea level, and estuarine salinity and tempera-ture. The forecasted changes were placed in the context ofa system that has been radically altered by human activities.

Results for the A2 scenario showed a 0.4 �C/decade increasein regional air temperature, a 28-mm/decade decrease inprecipitation, and a 12-cm/decade increase in sea level (seeCloern et al. 2011 for confidence limits). Estimated runoff ifthere were no dams or diversions (‘unimpaired’ runoff)decreased by 2%/decade, and the percent of runoff fromsnowmelt decreased by 1%/decade. Temperature in the Deltaincreased by 0.3 �C/decade and estuary-wide mean salinity by0.5/decade (practical salinity scale). The cooler, wetter B1scenario forecasted similar patterns but with smaller magni-tude for most variables, and the changes in precipitation andrunoff were not distinguishable from zero.

Increases in extremes were also predicted, including highwater level leading to flooding, and high temperature in theDelta, imposing thermal stress on the endangered delta smeltHypomesus transpacificus. The frequency of floods in the lowerwatershed will decrease, to the detriment of estuarine fishesthat spawn or rear on floodplains such as the native Sacra-mento splittail Pogonichthys macrolepidotus.

Broader conclusions were that the Delta would becomewarmer, clearer, and more saline. These trends would reducethe sustainability of delta smelt, which is sensitive to hightemperature, avoids high salinity, and is most abundant inturbid water (Feyrer et al. 2011).

Restoration activities are being planned and conducted withlittle consideration of anticipated changes in hydrology,temperature, and sediment supply, which may limit the feasi-bility of achieving restoration goals. Depletion of groundwaterin the SFE’s watershed (Famiglietti et al. 2011) indicates thatCalifornia’s water budget is oversubscribed. Changes inprecipitation and seasonal runoff, together with the increasedfrequency of extreme flows, will complicate the water supplypicture and aggravate the ongoing conflicts over water.

Cloern et al. (2011) explicitly left out consideration ofhuman activities that might change the outcomes of the modelscenarios, as well as potential impacts on marshes and on theseaward, more saline reaches of the estuary. California’s watersupply system could adapt to much of the change in flowtiming and quantity while maintaining at least some flows forenvironmental conservation (Medellin-Azuara et al. 2008). Thestrong seasonal pattern in precipitation is the primary driver ofseasonal and decadal variability in flow, whereas dams anddiversions have mainly affected long-term trends (Enright andCulberson 2010). However, substantial alterations in thestructure and operation of California’s water supply system are

being planned, notably a massive water diversion system toalleviate problems with pumping water from the tidal fresh-water part of the system (Lund et al. 2007). The effect onestuarine salinity will depend on design of this system and itsoperating rules. Multiple seismically driven levee failures(Mount and Twiss 2005) would cause a large, immediate influxof saline water. Forecasts of salinity distributions based onclimate change alone seem incomplete.

Most of the environmental problems of the SFE have beeneither caused by, or solutions are impeded by, introducedspecies (Nichols et al. 1990; Brown and Michniuk 2007; Lundet al. 2007). Introduction of additional, influential species suchas the quagga mussel are likely, further compounding theuncertainty of climate projections.

A projection of extent and frequency of inundation of landshowed large areas at risk, and predicted that the 100-year floodof year 2000 would be a 1-year event by mid-century (Knowles2010). A review by Parker et al. (2011) addressed effects ofclimate change on tidal marshes, emphasizing sea-level rise aswell as temperature and salinity. Rising sea level will cause lossof marsh area because the present-day sediment supply isinadequate to support accretion of marshes (Knowles 2010)and hard structures limit upland development of manymarshes. Rising temperature and salinity are forecasted to causephysiological stress and reduce diversity of marshes (Parkeret al. 2011). This review does not consider the likely decrease insediment supply caused by trapping behind dams (Schoell-hamer 2011) or in flooded islands (Mount and Twiss 2005).

4.22.4.4.2 Chesapeake BayMuch of the concern for management in the Chesapeake Bayhas focused on eutrophication and its influence on or interac-tion with hypoxia, loss of seagrass, and loss of oyster reefs(Breitburg 1992; Kemp et al. 2005). Non-point source inputs ofnutrients have slowed the reduction of nutrient loading ach-ieved through improved wastewater treatment (Kemp et al.2005).

A recent review by Najjar et al. (2010) summarized the likelyinteractive impacts of end-of-century increases in temperature(2–6°C), sea level (0.7–1.6m), and precipitation on livingresources, particularly eelgrass, fish, and shellfish. The generalprojection is for a continued trajectory toward a plankton-dominated, eutrophic system. Precipitation in the basin isexpected to increase, particularly in winter–spring, with anincrease in the frequency of high-rainfall events. The change inseasonal pattern of precipitation, together with increasingevapotranspiration in the watershed due to higher temperature,will sharpen the seasonal difference between high winter flowsand low summer flows. Sea-level rise will increase salinitypenetration into the Bay and increase the tidal range, resulting instronger vertical mixing. In the brackish northern part of the Baythis tendency will be offset by higher spring flows, likelyresulting in stronger and more persistent spring–summer strati-fication. The combination of stronger tidal currents and greatersediment loading owing to erosion in the watershed and extremewinter flows will likely result inmore suspended sediment in thewater column, although the interactions among factors affectingsediment make quantitative predictions difficult.

Loading of nutrients and organic matter from nonpointsystems will increase. Increased temperature and nutrient


loading will cause spring diatom blooms to begin earlier andproduce more biomass, allowing an earlier start to summerhypoxia. This tendency will be strengthened and extended bythe increased strength of stratification. In addition, becauserespiration increases more steeply than photosynthesis withincreasing temperature, the estuary may shift toward netheterotrophy and possibly lower foodweb efficiency and anincrease in blooms of harmful algae.

Growth rates of vascular plants including submergedaquatic vegetation (SAV) and marsh vegetation will likelyincrease with increasing CO2. Yet the geographic coverage ofeelgrass and other SAV will likely continue to decline withincreasing turbidity because of high phytoplankton biomass. Inaddition, eelgrass, at the southern limit of its range in theChesapeake Bay, will likely suffer more overgrowth byepiphytes and diebacks due to warming. An increase in theduration of low-flow periods during droughts could increasewater clarity and possibly benefit SAV. However, wetlands maybe unable to keep up with the rate of sea-level rise throughaccretion of sediments.

Harvested species of fish and shellfish are subject to severallong-term influences. In particular, habitat for fish and someshellfish will shrink as the extent of hypoxia grows (Breitburg1992). Loss of oysters to overharvesting and disease may haveshifted the entire system into a more eutrophic stable state(Kemp et al. 2005). Similarly, loss of seagrasses owing mainlyto shading by phytoplankton has resulted in an alternativestable state, reversible only through a reduction of phyto-plankton blooms (Kemp et al. 2005). Nonlinear interactionswithin this complex ecosystem, and the likelihood of rangeextensions into the Chesapeake Bay by potentially influentialspecies, preclude confident predictions about futuretrajectories.

4.22.4.4.3 Murray–Darling System, AustraliaThe Murray–Darling river–estuary system illustrates the effectsof long-term change in arid areas (Kingsford et al. 2011).Apparently most of this trend is human-driven. A severedrought in southeastern Australia from 1997 through mid-2010 was followed by floods in 2010–11. Total precipitation inthe basin is not expected to decrease with climate change (Sunet al. 2011), and similar droughts have occurred in the past(Roderick 2011). What is unprecedented is the low flow in theriver; because of water withdrawals for irrigation, almost nofreshwater flowed into the estuary during the last 4 years of thedrought (Webster 2010).

The estuary comprises two broad, shallow lakes and a long,narrow lagoon (the Coorong) behind a barrier beach. Althoughnot urbanized, it is a popular recreational site and an importanthabitat for migratory waterbirds. Barrages installed to guideriver flow to the mouth also restrict flows among the basins.The barrier island and river mouth are in a continual state offlux, exacerbated by the barrages (Bourman et al. 2000).

Toward the end of the drought the southern Coorong wasintensely hypersaline because of high evaporation and sluggishexchange. The salinity of greater than 100 mg/l exceeded thetolerance of nearly all estuarine and marine organisms(Webster 2010). Lowered water levels exposed toxic sulfatesoils and the Australian government spent wA$1B on resto-ration without addressing the shortfall in flow (Kingsford et al.

2011). During subsequent floods the salinity has declined andvalued species of fish and birds have returned to the Coorong.

Responses to the environmental problems in the Coorongwere constrained by the near-total lack of freshwater above theminimum demanded for irrigation. Reallocation of the watersupply is being considered (Roderick 2011; Kingsford et al.2011), as is a pipe to connect the Coorong directly to the ocean(Webster 2010).

Thus, the Murray–Darling system illustrates how humanuses of water can lead to severe degradation in an environ-mentally stressed estuary. This example also shows how localissues direct research; the emphasis in the relevant reportsfound on this system was on flows and barrages, with littlemention of potential effects of warming, ocean acidification, orsea-level rise.

4.22.4.4.4 Summary of Case StudiesWhat is striking when comparing these three estuaries is thedifference among the long-term processes dominant in eachestuary: water supply and introduced species in the SFE,eutrophication in the Chesapeake Bay, and farming in an aridregion in the Murray–Darling system. These estuaries are atabout the same latitude, have temperate climates, and are indeveloped countries. The difference in emphasis is not causedby lack of information; in particular, the Chesapeake Bay andthe SFE are perhaps the world’s most-studied estuaries after theBaltic Sea. Rather, it reflects the prudent consideration of localconditions when researching and managing the effects of long-term change.

‘Climate change’ is a hot topic. Much less attention has beengiven to other long-term changes with similar magnitude andtimescales (see Section 4.22.3.2 Direct Human Effects). Allthree studies presented here, particularly the Murray–Darling,emphasize the point made by Vörösmarty et al. (2000) morethan a decade ago that local and regional human activity candominate the trajectory of long-term change.

4.22.5 Implications for Ecosystem Functionsand Services

Predicting the future of estuarine functions and services isdifficult. First, projections of the state of natural–humansystems over decades and longer are uncertain. Second,unforeseen changes in policy, technology, economics, andsociety will affect how estuarine services are used andmanaged.Third, each estuary is unique, as is its response to change.Fourth, our understanding of climate, other human influences,and ecosystem functions will improve along with efforts tomitigate and adapt.

Despite the uncertainty, some long-term influences havepredictable trends with known risks that are likely to occur inmany estuaries (see Sections 4.22.3.1 and 4.22.3.2, Table 2).These influences include temperature, sea level, ocean acidifica-tion, human population growth, habitat alteration, and intro-duced species. Rising temperature affects estuarine functionsthrough physiological and phenomenological changes and rangeshifts, and through effects on river flow. Rising sea level increaseserosion and degrades vegetated beds, shorelines, and structures.Ocean acidification reduces growth rates of oysters and


mussels. Population growth increases the demand for water andother estuarine services. Increased demand for water reducesfreshwater flow into estuaries. Habitat alterations eliminatevegetated beds and thereby diminish estuarine services such asdisturbance regulation and food production. Introduced speciesalter community composition and biotic interactions but withless predictable consequences. All of these proximate impacts arelikely to occur or are occurring in many estuaries, but outcomesfor a particular estuary are less certain.

We have identified and diagrammed five mechanisms (seeSection 4.22.4.2, Table 1, Figures 2–6) by which long-terminfluences amend estuarine functions and thereby degradeestuarine services in many estuaries: changes in freshwaterflow, supply and loss of sediment, alteration of vegetatedbeds, eutrophication, and shifts in species. Each of thesemechanisms applies to many estuaries, and the mechanismscan be used to deduce the likely long-term trajectory of anindividual estuary.

Globally the supply of water falls short of human demand,and in most estuaries freshwater inflow will likely decrease. Thejoint effects of increased demand and reduced supply due toreduced precipitation and warming in the watershed(4.22.4.2.1) will exacerbate shortages and increase conflict inmany if not most estuaries. Loss of inflow can lead to thecollapse of functions and the complete loss of services,although the Murray–Darling system showed remarkableresilience following a decade-long lack of flow (see Section4.22.4.2.).

The amount of available sediment in estuaries will declineglobally as a result of sea-level rise, erosion, and damming andreduction in flood flows. The resulting reorganization ofbathymetry and loss of shoreline and marsh habitats willimpair estuarine services, particularly those provided bymarshes and vegetated beds (Table 1). Reductions in turbiditywill increase risk of eutrophication and alter predator–preyrelationships. A decline in turbidity over the past three decadesin the SFE is associated with declines in the endangered deltasmelt (see Section 4.22.4.4.1.).

Habitat alteration for urban, agricultural, and other usesalso eliminates ecosystem services provided by vegetated beds.Globally these habitats continue to decline. The interaction ofhabitat alteration with increased erosion resulting from sea-level rise, loss of sediment, and reductions in freshwater inflowleads to a continuing loss of these estuarine services. Theselosses are particularly apparent in south Asia, where removal ofmangrove forests for aquaculture has resulted in losses ofservices such as shoreline protection and nursery habitats(Polidoro et al. 2010).

In eutrophic estuaries, ecosystem services are alreadyunder threat because of habitat degradation through shadingof seagrass beds and development of harmful algal bloomsand hypoxia, with attendant loss of benthic assemblages andhabitat for higher trophic levels (see Section 4.22.4.2.4.).The principal long-term drivers of eutrophication arenutrient and organic loading, but stratification and thereforefreshwater runoff are important factors, and temperature,habitat alteration, and turbidity contribute. Eutrophicationcan impair all of the ecosystem services listed in Table 1. Forexample, the loss of seagrass beds in the Chesapeake Bay,owing partly to eutrophication, eliminated physical habitat

and a significant food resource for many harvested bay andcoastal species.

Species distributions globally are moving poleward inresponse to warming, and flora and fauna are becominghomogenized through introductions. Species gains and losseswill likewise continue in most estuaries through some combi-nation of range shifts and introductions (see Section4.22.4.2.5.). Overharvest may accelerate changes throughalteration of trophic structure and removal of predators andcompetitors of potential colonizers. Further extirpations andeven extinctions are likely.

General patterns and mechanisms of estuarine responses tochanges in climate and other influences, such as those dis-cussed in the preceding and in Section 4.22.4, can be used toestablish a suite of tools (Figures 2–6) and concepts thatresearchers, managers, and policy makers can draw on incharacterizing the attributes, mechanisms, and likely responsesof a specific estuary and in designing management actions thatwill be effective in the long term. Widespread or general trendsand mechanisms such as those summarized herein can be usedas a starting point, provided the existing state of the estuary andthe current stresses it faces are understood, and the parallelroles of climate and local human actions are considered.

The site-specific character of many of the responses to long-term change could be used to advantage. Local action will beneeded to address the consequences of the global failure to act.Many governments and populations around estuaries haveestablished sophisticated arrangements for stewardship andadaptation. These arrangements have led to increases not onlyin protection and restoration, but also in monitoring andresearch to understand these valued estuarine services, as wellas public knowledge and understanding of the repercussions oflong-term changes. These trends will help in understanding,mitigating, and adapting locally to the anticipated global lossof estuarine services.

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