Estuaries: Where Rivers Meet the Sea

What is an EstuaryAn Estuary is where freshwater from rivers merges with the ocean and is bordered by extensive wetlands (mudflats or saltmarshes)

Characteristics of Estuaries

Salinity varies spatially and over the course of a day as the tides move in and out

Nutrients from the rivers enrich estuarine waters making them one of the most biologically productive environments on earth

Salt marsh grasses, algae, and phytoplankton are the major producers

Many organisms are found in the muddy bottom

Estuaries rank among the environments most affected by humans

Functions and Values of EstuariesProvide open space and recreationProvide natural flood controlPurify the waterProduce oxygenAre outdoor laboratories for scientists and

educatorsProvide sediment traps and erosion controlServe as nurseries for marine fishesProvide homes for endangered speciesProvide habitat for migrating, wintering and

breeding birdsServe as repositories for native plants and

animals

Types of Estuaries – Four basic groups based on their origins

Drowned River Valley or Coastal Plain – Fig. 12.1 – formed as a result of the sea invading lowlands and river mouths. This is the most common type of estuary.

Types of EstuariesBar-built estuary – Fig. 12.1 – formed when a barrier

island or sand bar separates a section of the coast where freshwater enters. (Pictured below is an aerial view of Cape Hatteras, North Carolina.)

Types of EstuariesTectonic

Estuary – formed as a result from sinking land due to movements of the earth’s crust. San Francisco Bay formed in this way.

Types of EstuariesFjords – Fig. 12.2 – formed in a deep valley

created by a retreating glacier. (Kenai Fjord National Park, Alaska)

Physical Characteristics of Estuaries – unique because of mixing of salt and

freshwaterSalinity fluctuates as the tides move

seawater in and out of an estuary.Organisms are subjected to these

changes every day and therefore have adapted to these conditions.

Other factors that influence salinity include; the shape of the estuary, the bottom, wind and evaporation, and freshwater runoff

Salt Wedge – a layer of denser, saltier seawater that flows along the bottom

Physical Characteristic - Salinity

Salt Wedge

Sand and other coarse material settle out first near the river mouth

Finer particles settle out into “main” body of water, therefore most estuaries are soft mud (silt & clay) which is very dense and rich in organic material

Decomposition by bacteria depletes oxygen in the substrate and produces hydrogen sulfide (rotten egg smell)

Tidal flow keeps water in estuaries oxygen rich

Physical Characteristic - Substrate

Close up of Estuary Sand

Physical CharacteristicsTemperature – fluctuates widely because of tides,

organisms that are exposed at low tides also face seasonal temperature changes

Turbidity – estuaries usually have large amounts of suspended sediments which can reduce water clarity

Maintaining the proper salt and water balance is essential for all organisms.

Organisms that live in estuaries must cope with rapid changes in salinity.

Most estuarine organisms are marine species that have evolved to tolerate low salinities.

Coping with Salinity

Euryhaline species can tolerate a wide range of salinities.

Stenohaline species can only tolerate a narrow range of salinities (yellow fin tuna)

Osmoconformers (molluscs and worms) – osmotic balance is maintained by changing with the salinity of the water

Osmoregulators keep internal salt balances constant despite salinity levels of the water in which they live

Osmoconformer vs Osmoregulator

Osmoconformer vs Osmoregulator

Coping with SalinityMarine fish – have a lower salt concentration

than seawater, therefore tend to loose water; adaptations to maintain homeostasis include (Fig. 4.14)Drinks seawaterExcretes excess salt through gillsExcretes small volumes of concentrated salty

urine

Coping with SalinityAdaptations of other

marine organismsSea turtles/sea

gulls/sea lions have glands near the eyes that excrete “salty” tears (Fig. 4-15)

Mangroves/spartina grass – excrete excess salt through leaves

Salt crystals on spartina grass

Coping with Salinity – Pickleweed accumulates large amounts of water to dilute the salts they take up.

Most animals burrow or live in permanent tubes beneath the sediment

Inhabitants tend to be stationary or slow moving

Salinity fluctuations are less drastic than in the water column

Organisms overcome the depletion of oxygen by:Pumping oxygen rich water into their burrowsSome have blood that contains hemoglobin, an

oxygen carrying molecule

Adapting to the Mud

Adapting to the Mud

Different types of communities are associated with various estuaries

Open Water community includes those organisms that come and go with the tide (plankton, fishes, jellies).

Other types of communities are permanent parts of the ecosystem, these include:MudflatsSalt MarshesMangrove ForestsSea-grassesOyster Reefs

Estuarine Communities

Organisms in this type of community include phytoplankton, zooplankton, and marine fish that are flushed in and out by the tides.

Many types of marine fish and shrimp use estuaries as nurseries for their young.

Some fish migrate through estuaries.Anadromous – marine fish that live their

lives in the sea but return to freshwater to breed (salmon)

Catadromous – freshwater fish that migrate to the sea to breed (freshwater eels)

Open Water Community

Open Water CommunityZooplankton

Open Water CommunityPhytoplankton

Open Water CommunityAnadromous

Open Water CommunityCatadromous

Low tides expose organisms to desiccation, fluctuating temperatures and salinity, and predation.

Primary produces consist of diatoms, bacteria, and some algae

Consumers are typically burrowing deposit and suspension feeders that feed on detritus

Mudflat Communities

Mudflat Communities

Mudflat Communities

Mudflat Communities

Mudflat Communities

Mudflat Communities

Mudflat Communities

Mudflat Communities

Mudflat Communities

Salt Marsh CommunitiesSalt marshes are grassy areas that

extend along the shores of estuaries and sheltered coasts in temperate and subpolar regions.

Salt marshes are subject to the same fluctuations in salinity, temperature, and tidal changes that affect mudflats.

Pronounced zonation of vegetation in salt marsh communities is a result of fluctuations in salinity, height relative to the tide, and rate of evaporation

Cordgrass (Spartinia alterniflora) is the most common plant found where the marsh meets the mudflat. Helps to stabilize the bottom.

Salt Marsh CommunitiesZonation depends on several factors: two

important ones are salinity and nutrient availability.

Salinity changes depending on these conditions:a. frequency of tidal inundation

b. rainfall c. tidal creeks and drainaged. soil texture e. vegetation f. depth of water tableg. freshwater inflow

Nutrient availability varies considerably, especially the supply of usable nitrogen and phosphorous. Often there is not enough oxygen present in marsh soils to combine into useful nitrates and phosphates.

Salt Marsh Communities

Salt Marsh Communities

Salt Marsh Communities

Mangrove Forest CommunitiesMangrove forests are formed by

mangroves, tropical and subtropical trees and shrubs adapted to inundation by seawater.

Grow on protected coasts where muddy sediments accumulate.

The red mangrove is found along the coast of Florida. Can be identified by its prop roots.

Mangrove Forest Communities

Mangrove Forest Communities

Mangrove Forest Communities

Mangrove Forest Communities

Coral growing on amangrove root.

Mangrove Forest CommunitiesSnapper living among the mangrove roots.

Mangrove Forest CommunitiesCattle egret and nest in mangrove.

Sea Grass CommunitiesRoots help to stabilize the sediment and provide shelter to organisms

Florida lobster(yummy)

Oyster Reef Communities

Oyster Reef CommunitiesFriends of Hunting Island help resort oyster reefs.