J. Great Lakes Res. 18(4):552-570Internat. Assoc. Great Lakes Res., 1992

COMBINING ECOSYSTEM AND LANDSCAPE APPROACHESTO GREAT LAKES WETLANDS

William J. Mitsch l

School of Natural Resources and Environmental Science ProgramThe Ohio State University

Columbus, Ohio 43210-1085

ABSTRACT. Great Lakes wetlands can be studied systematically and for their importance in thelandscape at different scales. A systems-oriented study of the wetlands oj southwestern Lake Eriealong Ohio's shoreline has been organized at two levels ojhierarchy. At the ecosystem level, a specificwetland model was developed from data collected on hydrology, nutrient budgets, and aquaticmetabolism. The model, calibrated from the field data, was used to predict phosphorus retentionrates and to compare them with results from empirical models and field studies. At the landscapescale, synoptic surveys of diked (hydrologically isolated with impoundments) and undiked (naturalhydrology) wetlands for hydroperiods, water quality, sediment chemistry, and vegetation biomassand species illustrated several differences between these two types of wetlands. Combining datacollected from these surveys with data collected from remote platforms will lead to the developmentof spatial dynamic models for the shoreline to deal with landscape-level questions on the manage­ment of these shoreline wetlands.INDEX WORDS: Ecological model, hierarchy, Lake Erie, phosphorus, Sandusky Bay, spatialmodel, water quality, wetland vegetation, wetland sediments, freshwater marshes.

INTRODUCTION

Coastal wetlands on the shoreline of the Lauren­tian Great Lakes are unique in the world of wet­lands. The wetlands and their associated rivershave been described as parts of estuaries by some(Herdendorf 1987) and rejected as estuaries by oth­ers (Schubel and Pritchard 1990). In presettlementtimes, the vegetation of these coastal wetlands ex­panded and retreated with changing water levelsyet always remained as a buffer between the up­lands and the lakes. As shorelines were stabilizedand the land was drained for agriculture and urbandevelopment, these wetlands were mostly de­stroyed or significantly altered; their buffering ca­pacity was diminished or lost altogether. Many ofthe wetlands that have not been drained for shore­line development have survived because they havebeen impounded as wildlife habitat, thereby isolat­ing them from the Great Lakes and watershedsthat formerly nourished them.

IThe author acknowledges the significant contributions of his gradu­ate students, particularly Brian Reeder and Doreen Robb, and thecollaboration by Mary Roush, Oi-Chul Yi, and Oreg McNelly.

552

Few if any comprehensive systems-level studieshave been carried out on these Great Lakes coastalwetlands (Prince and D'Itri 1985). This is particu­larly apparent when comparing Great Lakes re­search with the abundant "ecosystems" literatureavailable on estuarine and coastal salt marshes(e.g., Nixon and Oviatt 1973; Nixon et al. 1976;Hopkinson and Day 1977; Kremer and Nixon1978; Pomeroy and Wiegert 1981; Wiegert et al.1981; Costanza et al. 1988, 1990). This paper sum­marizes highlights of an ongoing hierarchical studyof a region of wetlands on the southern shore onLake Erie in Ohio. Models of wetlands are beingused to guide field research efforts, to identifygaps in data, to investigate ecosystem behavior,and ultimately to aid in wetland management bypredicting systems-level behavior of coastal wet­lands under different management conditions. Ourapproach uses two levels of hierarchy of modelsand field measurements, ranging from an ecosys­tem level approach that emphasizes ecosystem pro­cesses in individual wetlands to landscape ap­proaches that cover large geographical areas.

ECOSYSTEM AND LANDSCAPE APPROACHES TO WETLANDS 553

Loadings from the Upland Watershed

Coastal Great Lakes wetlands are influenced byrunoff from upstream watersheds which are oftendominated by agricultural use and are thereforesignificant sources of nutrients, pesticides, andsediments to both these wetlands and to Lake Erie(Yaksish et ai. 1982, Baker 1988). For example, the69 km2 primarily agricultural watershed that drainsinto Old Woman Creek wetland on Lake Erie con­tains relatively high levels of nutrients (Heath1987, Klarer 1988, Klarer and Millie 1989) and wasestimated to discharge 0.5 to 1.0 kg P ha-1y-l to thewetland (Mitsch et ai. 1989a). Many counties in

AN OVERALL MODEL

In any effort to understand and model an ecosys­tem or region, an understanding of the forcingfunctions or driving variables of that ecosystem isparamount. Great Lakes wetlands have severalforcing functions that, collectively, make themunique ecological systems to study and manage(Fig. 1). These include: 1) varying hydrologic, nu­trient, and toxic loading from upstream water­sheds; 2) shifting shoreline sediments, moved dur­ing storm events, which can dramatically changethe hydrologic, chemical, and biological connec­tions between the wetlands and the Great Lakes; 3)water level fluctuations of the Great Lakes, whichvary both seasonally and annually; 4) periodicseiches or "wind tides"; and 5) artificial impound­ment construction and water level manipulation byhumans. These forcing functions have one thing incommon - they greatly influence the exchange ofgeologic and biological materials from upstreamwatersheds to and from the Great Lakes.

Great LakesCoastal

Wetlands

Diking,fo4---IManagement

Ohio in the western Lake Erie basin have dischargerates of 0.5 to 2.5 kg P ha-1y-l to the lake (Johnsonet ai. 1978, Novotny 1986). A study group of GreatLake's pollution called PLUARG (Pollution fromLand Use Activities Reference Group) presents arange of 0.1 to 9.1 kg P ha-1y-l (IJC 1980).

Shifting Shoreline Sediments

For most of the year, the general direction of flowthrough open coastal wetlands is from the uplandwatershed, through the wetland, and to the GreatLakes. However, the difference in elevation be­tween a coastal wetland and the lake itself can varywith storm events and short-term Great Lake fluc­tuations. The difference between the two levels canbe exacerbated when the mouth of the stream be­tween the wetland and the lake is closed due toshifting shoreline sediments, a rather frequent evenin some wetlands (Mitsch and Reeder 1992). Thewetland can remain closed for several months, af­ter which the combination of high water levels inthe wetland and a sudden storm event once againopens the wetland to the Great Lakes.

Lake Water Fluctuations

Wetlands along the Great Lakes are heavily influ­enced by water level fluctuations of the GreatLakes. For example, water levels for Lake Eriewhich have been recorded over the past 130 yearsnear Cleveland, Ohio (Fig. 2) show a difference ofalmost 150 cm between low and high water levels inthat period. This amplitude is significant enoughto dramatically affect shoreline processes, particu­larly those in wetlands. The time between high andlow water levels (approximately 10 to 15 years re­cently) is long enough to affect the structure andfunction of coastal wetlands. In presettlementtimes, high water levels would send the wetlands"inland" (Fig. 3a) while wetlands would extend"lakeward" during low water levels (Fig. 3b), oftenleaving the wetlands of the Great Lakes in a stateof disequilibrium with its water level. At a givenlocation, the wetland will vary from a system dom­inated by emergent vegetation (during shallow wa­ter times) to one that is a planktonic or floating­leaved aquatic system (during high water level).

FIG. 1. Conceptual model showing major jorcingjunctions that affect Great Lakes coastal wetlands.

Seiches

Shorter period water level oscillations due to windaction, called seiches, frequently occur on the GreatLakes. The coastal wetlands along the lakes are sub­ject to water and chemical exchanges from seiches

554

573.50

572.50

571.50

570.50feet

(aboveMSL)

569.50

568.50

567.50

W. J. MITSCH

Lake Erie Water Levels· Cleveland

175

174.5

174

m(aboveMSL)

173.5

173

1860 1870 1880 1890 1900 1910 1920

year

1930 1940 1950 1960 1970 1980

FIG. 2. Water level in elevation above sea level ofLake Erie from 1860 to 1988. Data are from station at Cleveland,Ohio, and are courtesy of K. Baker, Ohio Department of Natural Resources.

in much the same way that coastal salt marshes aresubjected to tides, although these seiches are not asperiodic as semi-diurnal coastal tides. Sager et at.(1985), for example, measured 269 seiche events in 1year on lower Green Bay on Lake Michigan with amean amplitude of 19.3 cm and a mean period of9.9 hours. Herdendorf (1987) reported that longitu­dinal seiches occurred in Lake Erie approximately40% of the time and most often at 12-14 hour peri­ods. The importance of seiches to the nutrient bud­gets and biotic communities of Great Lakes wet­lands is not well known although it may beanalogous to the role of tides in salt water wetlands,transporting nutrients and detritus into and out ofthe wetland. Impounded wetlands are isolated fromexchanges due to these seiches.

Artificial Dikes and Water Level Management

The fluctuating, less predictable water level of theGreat Lakes has led to a common practice in marsh

management along the lakes' shorelines of con­structing artificial impoundments (dikes or levees)and maintaining artificial water levels with pumpsand gates. Levees are constructed around wetlandsand pumps or flap gates are installed to keep waterlevels independent of those of the lakes. Duringlow water times or periods of drought, pumps areused to keep water in these managed wetlands (Fig.3c). Because many of the coastal wetlands alongthe Great Lakes are managed for waterfowl, dikedwetlands are a common type of wetland there, es­pecially along southwestern Lake Erie.

A HIERARCHICAL STUDY OF WETLANDSOF WESTERN LAKE ERIE

Our wetland study area centers on the wetlands inthe region of Sandusky Bay on the southwesternshore of Lake Erie in northern Ohio. The area isapproximately 60 km wide and is bounded on theeast by Old Woman Creek wetland, approximately

ECOSYSTEM AND LANDSCAPE APPROACHES TO WETLANDS

HIGH LAKE LEVEL

FIG. 3. Dynamics of wetlands of coastal GreatLakes during a) high lake levels when wetland veg­etation moves landward, b) low lake levels whenwetland vegetation moves lakeward, and c)impounded or diked wetland where water level iscontrolled by pumping and levees.

555

5 km east of Huron, Ohio, and on the west by themouths of Sandusky River and Muddy Creek asthey enter Sandusky Bay southwest of Port Clinton,Ohio. We have chosen initially to look at the LakeErie wetlands in this region on two scales of hierar­chy (Fig. 4). Intensive studies and measurements ofecosystem productivity, nutrient cycling, and pa­leolimnology have been carried out at Old WomanCreek wetland. A landscape assessment of hydrope­riods, water quality, sediments, and productivity,which includes several other marshes along LakeErie, is included in the study to determine if theintensive studies at Old Woman Creek can be gener­alized to coastal wetlands along Lake Erie. Detailsof these studies are contained in Mitsch et al.(1989a); Mitsch (1989); Robb (1989); Reeder (1990);Mitsch and Reeder (1991, 1992); and Robb andMitsch (1992).

ECOSYSTEM SCALE

Our primary site for ecosystem-level studies was atOld Woman Creek National Estuarine Research Re­serve and State Nature Preserve, a natural LakeErie coastal wetland in Erie County, Ohio. Its hy­drology is determined both by Lake Erie water lev­els and runoff from a 69 km2 watershed. The wet­land itself is 56 ha in size and extends about 1 kmsouth of the Lake Erie shoreline (Fig. 4). Depthsmay extend to 3 m in the inlet stream channel, butfor the major portion of its area it is usually lessthan 0.5 m deep. The wetland has an outlet to LakeErie that is often open but which can be closed forextended periods of time by a barrier beach. Seicheson Lake Erie can reverse the normal lake inflow,causing lake water to spill into the wetland. Aquatichabitats within the wetland include open waterplanktonic systems and American lotus (Nelumbo

556 W. J. MITSCH

kllomoter

$802488

01 23" 5mile

LANDSCAPE SCALE

• Hydroperiod• Water Quality• Wetland Sediments• Wetland Vegetation• Remote Sensing• Spatial Model

ECOSYSTEM SCALE

• Hydrology Budget• Aquatic Metabolism• Paleoecology• Ecological Model• Chemical Budgets• Chemical Processes

FIG. 4. Two scales ofhierarchy for study ofLake Eriewetlands in Ohio, including types of studies that werefound to be most appropriate at each level (Mitsch1989). Abbreviations in upper map refer to selected wet­lands: WPW = Winous Point West (diked); WPN =Winous Point North (diked); OSCB = Ottawa Shoot­ing Club Big Pond (diked); OSCA = Ottawa ShootingClub Allen Pond (diked); PCK = Pickerel Creek (natu­ral); WLP = Willow Point (natural); BVC = Bay ViewCenter (diked); BVB = Bay View B (diked); PLMB =Plumb Brook (natural); SHM = Sheldon Marsh (natu­ral); OWC = Old Woman Creek (natural).

lutea) beds throughout a significant portion of themarsh.

Ecosystem Simulation Models

Figure 5 shows a model of Old Woman Creek wet­land (Mitsch and Reeder 1991) with details of someof the processes in a wetland which contribute toits nutrient retention capability. Plant uptake, bothby plankton and macrophytes, sedimentation, andresuspension are probably the most significantprocesses involved in the wetland retaining and re-

leasing phosphorus. This conceptual model wasdeveloped into a simulation model based onknowledge of the cycling of phosphorus and en­ergy in wetland ecosystems. Unique to this wetlandmodel is the interaction of Lake Erie with the wet­land. The model was divided into three submodelsand simulated by the higher level simulation lan­guage STELLA™ (Mitsch and Reeder 1991).

A hydrology submodel of the simulation model isdesigned to depict the hydrologic budget of OldWoman Creek wetland with the only state variablefor this submodel being the volume of water in thewetland. Factors affecting the volume of water inthe marsh include rainfall, watershed inflow,evapotranspiration, and exchange with Lake Erie.The availability of inflow data, fairly good data onevaporation, and knowledge of the hydrologic forc­ing functions in the wetland allowed the develop­ment of an accurate hydrologic budget and model.An important part of the hydrodynamics is the tim­ing of an ephemeral barrier beach on the wetlandoutflow.

A primary productivity submodel includes thestate variables macrophyte biomass and planktonbiomass. These are both a function of sunlight, withthe major losses being respiration and sedimenta­tion. The productivity submodel is linked to thehydrology submodel in two ways (Fig. 5). First,plankton are exported to Lake Erie when waterfrom the wetland flows into the lake and the beachis open. This assumption is consistent with the fielddata. Second, macrophytes are assumed to be moreabundant when water levels are lower, and lessabundant when water levels are higher. This is basedon observations of other Lake Erie coastal marshes(Herdendorf 1987). Field measurements of grossprimary productivity (see below) as measured at OldWoman Creek wetland were in general agreementwith model simulations. Using chlorophyll a as acalibration variable, the model and field data werefound to be in agreement (within one standard devi­ation) 78070 of the time. Given the differences in thetwo field measurements and the variability ofenergy/chlorophyll ratios in aquatic systems (Vol­lenweider 1974), the model accurately predicted sea­sonal patterns of productivity and biomass (Mitschand Reeder 1991).

A phosphorus submodel was coupled with boththe hydrology and primary productivity submodelsand is incapable of running simulations withoutinput from these submodels. This submodel uti­lizes one phosphorus storage in the waters of thewetland and another in the sediments, with linear

ECOSYSTEM AND LANDSCAPE APPROACHES TO WETLANDS 557

--Energy

-- Phosphorus

Permanent Burialor Phosphorusin Sediments

FIG. 5. Conceptual model of Old Woman Creek wetland along Lake Erie (fromMitsch and Reeder 1991a). This model describes the major pathways in a simulationmodel developed for that wetland.

pathways between the two. The phosphorus sub­model includes a sedimentation pathway as de­fined for shallow lakes by Kamp-Nielsen (1983)and Henderson-Sellers (1984) with an average set­tling velocity of 0.03 m d-1

• Calibration was doneby varying the resuspension coefficient until themodel predicted phosphorus concentration resultssimilar to field data.

Hydrology, productivity, and phosphorus fielddata enabled model calibration and preliminary es­timations to be made of the role of phosphorussedimentation and phosphorus resuspension in theshallow wetland. Simulations show high levels ofsedimentation in the early spring, with resus­pension exceeding sedimentation through the re­mainder of the year (Fig. 6a). This excess of phos­phorus resuspension over sedimentation in the

model simulations is surpnsmg at first, but theproductivity estimates throughout the year clearlyillustrate that there is insufficient phosphorus inthe inflow to support the high level of productivityand the generally high phosphorus concentrationsexperienced in the wetland from May throughNovember.

The model also predicts the total phosphorussedimentation rate, including contributions fromplankton and macrophytes (Fig. 6b). Sedimenta­tion rates as high as 40 mg P m-2d- 1 are simulatedfor the spring, while the rate for the remainder ofthe year, when very little allochthonous inflows areexperienced, is around 10 mg P m-2d-1

• The simu­lated rates of sedimentation and resuspensiontranslate to a total net sedimentation of 0.8 g P m-2

for the 9-month study period. This contrasts to an

558 W. J. MITSCH

40000

30000

g-P/day20000

I resuspension2 sedimentation

10000

0.061 130 198 267 335

Time, days

IMar Apr I May Jun Jul I I

Aug Sep Oct Nov

0.0400

0.0300

g-P/m2-day

0.0200

Total Sedimentation Rate

0.01000

61

I Mar

130

Apr I May Jun

198Time

Jul Aug

267

Sep Oct Nov

335

FIG. 6. Calibration simulations from Old Woman Creek wetland model that dem­onstrate approximate rates and seasonal patterns ofsedimentation and resuspensionfrom March through November (Mitsch and Reeder 1991). Total sedimentation rateincludes contributions from planktonic system.

estimated annual retention of 5-7 g P m-2 predictedby us a few years earlier using a simple empiricalmodel (Mitsch et af. 1989). Because the model isbased on the year 1988 which had a significantdrought, we can expect this net sedimentation rateto be well below average. It is not unreasonable tosuggest that less than 20 percent of the normalsedimentation occurred in the wetland during ourcalibration year of extreme drought and as a resultof having a simulation for only nine of the twelvemonths in the year. Subsequent simulations

(Mitsch and Reeder 1991) show that higher inflowsfor the same 9-month period lead to proportion­ately higher net retention of phosphorus, approxi­mately 1.3 to 3.3 g P m-2

, respectively, for normaland wet years.

Ecosystem Metabolism

Diurnal patterns of dissolved oxygen can give indi­cations of systems-level parameters of wetlandsthat are useful in assessing biological metabolism

ECOSYSTEM AND LANDSCAPE APPROACHES TO WETLANDS 559

9 12nidniglt

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Ststlon 8, Old WomsnCr..k WstlsndJuly 11-12, 1988

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30 kcal m-2 d-1 in July yields an efficiency of 0.4 %.The average gross primary productivity of 8 kcalm-2 d-1 in October results in an efficiency of 0.3 ltfo.These productivities are significant for planktonicsystems and are comparable to those of productivewetlands in flow-through conditions (Mitsch andGosselink 1986). P/R ratios calculated from Julydata suggest an excess of production over respira­tion at six of the eight sites although, interestingly,there was greater respiration than production atthe two sites in the upstream reaches of the wet­land. All sites except one in October have P IRratios less than 1.0 as solar energy and tempera-

FIG. 7. Typical summer diurnal patterns of dissolvedoxygen, temperature, and rate of change of dissolvedoxygen for Old Woman Creek wetland. Note that dis­solved oxygen ranges from 3 to 15 mg L-1

•

and in calibrating ecosystem models. The rise andfall of dissolved oxygen in an aquatic system re­flect that system's metabolism. Diel patterns of ox­ygen have been used to determine the overall pri­mary productivity and respiration of estuarine,lacustrine, and riverine systems (e.g., Odum andHoskin 1958, Mitsch and Kaltenborn 1980, Meyerand Edwards 1990). Shallow plankton-dominatedwetlands such as the Old Woman Creek wetlandnear Lake Erie are ideal systems in which to mea­sure the diurnal patterns of oxygen and to usethose patterns to estimate productivity. The shal­low nature of the wetland limits the euphotic zoneto a narrow depth of high chlorophyll and dra­matic oxygen swings. The warm water tempera­tures in the summer season further enhance bio­chemical activity. Furthermore, the calm waters ofthe shallow wetland usually do not have high ratesof oxygen diffusion. Measurement of the diurnalpatterns of water chemistry parameters such as dis­solved oxygen and temperature is also important topredict redox conditions and the habitat value ofsuch wetlands for aquatic organisms.

Dissolved oxygen patterns were measured onseveral occasions with Winkler titration and metersin Old Woman Creek wetland (Mitsch 1989,Reeder 1990). Gross primary productivity and res­piration are calculated from rates of change of thedissolved oxygen, with nighttime readings givingan estimate of the hourly rate of respiration anddaytime changes reflecting both gross primary pro­ductivity and daytime respiration. For example,July 1988 dissolved oxygen data display a dramaticchange in oxygen over a 24-hour period (Fig. 7).Readings ranged from 2 mg L-l at 6 AM to peaksof 12 to 15 mg L-l at dusk. Water temperatureswere quite warm at 24 to 29°C. In contrast, thedissolved oxygen range in Old Woman Creek inearly October was less dramatic, ranging from 6 to10 mg L-l with a temperature range of 6 to 14°C.Water during July was clearly supersaturated inlate afternoon with dissolved oxygen and undersat­urated at dawn; numbers remained much closer tosaturation during the October readings.

Old Woman Creek wetland, with the higher thannormal depths prevalent during this study year, isprimarily a plankton-dominated system. Gross pri­mary productivity calculations, based on the Julydata, result in measures of 15 to 57 kcal m-2 d-1 oran average of 30 kcal m-2 d-1 for all sites. The Octo­ber data result in productivity calculations of 2 to19 kcal m-2 d-1 with an average of 8 kcal m-2 d- l forall sites. An average gross primary productivity of

560 w. J. MITSCH

tures decrease rapidly in October and the systemshifts to catabolism and lower productivities.

Hydrology Budgets

Old Woman Creek wetland has complex hydro­logic conditions typical of natural Great Lakeswetlands still connected to their lake. For example,the water level of the wetland reached a high of4.56 m above datum (approximately 1 m deep) on8 May 1988 before a 1.4 cm storm led to a break­through of the barrier beach and a water level dropto 55 cm in 2 days. The barrier beach was open formost of March, half of April, and for a short pe­riod during May. After those openings, the beachclosed for the remainder of the measurement pe­riod. It is normal for the wetland to be opened toLake Erie during the spring and then closed duringthe summer. The wetland was open to Lake Eriefor a total of 50 days, or 23 percent of the studyperiod. An estimate of the daily water budget forthat study period, calculated from the water levelsand the water budget equation, demonstrated thatsurface inflow from the watershed averaged 15,200m3 d-1 while Lake Erie provided 3,500 m3 d- I (Fig.8). The flow from Lake Erie represents 18070 of thewater flow into the wetland. As expected, evapo-

transpiration was higher than precipitation (by80%) because of the summer drought. The watervolume in the wetland actually increased slightlyduring the study period (March-November 1988)by 700 m3 d-I

, even with the drought conditionsbecause of the closed barrier beach between it andLake Erie and because of possible groundwaterinputs.

Chemical Budgets

Table 1 contrasts rates of phosphorus retention inOld Woman Creek from empirical model calcula­tions, from field data collected in 1988 (Reeder1990, Mitsch and Reeder 1992), and from the simu­lation model (Mitsch and Reeder 1991). An empiri­cal loading retention model of Richardson andNichols (1985) predicted 8-19 mg P m-2d-1 beingpermanently buried in the wetland sediments understandard phosphorus loading rates from Lake Eriewatersheds estimated from Johnson et af. (1978),IJC (1980), and Novotny (1986). This method ofestimating phosphorus retention does not take intoaccount any of the interactions between Lake Erieand the wetland nor does it consider any intrasys­tern interactions. A mass balance predicted fromfield data (Reeder 1990, Mitsch and Reeder 1992)

Precipitation

Units are m3/day x 1,000

Evapotranspiration

1.0 1.8

Lake Erie~17.2

Outflow toLake Erie

Inflow fromLake Erie

3.5

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15.2

Surface Inflow

FIG. 8. Approximate water budget for Old Woman Creek for March through November, 1988 (from Mitsch andReeder 1992).

ECOSYSTEM AND LANDSCAPE APPROACHES TO WETLANDS 561

TABLE 1. Comparison of nutrient retention capabilities of Old Woman Creek wetland by different measurements.Numbers except % retention are in mg P1m2-day.

Method Inflow Outflow Retention 070 Retention Source

Empirical Model v. I 36-63 14-19 30-39 Mitsch et al. 1989aEmpirical Model v. II 17-33 8-13 39-47 Mitsch and Reeder 1991Field Data 2.2 1.4 0.8 36 Reeder 1990, Mitsch

and Reeder 1992Sediment Core 22 Reeder 1990Simulation Modela

Dry Year 32.3 20.6-26.8 5.5-11.7 17-36 Mitsch and Reeder 1991Normal Year 45.5 23.6-33.1 12.4-21.9 27-48Wet Year 83.7 39.9-63.8 19.9-43.8 24-52

afor 9-month period only; high % retention in range is with high lake level, low % retention is with normal lake level

has a much lower loading rate primarily because thedata reflect drought year conditions and do not in­clude an entire calendar year. Phosphorus retentionfrom the field data was estimated to be 36070 of theinflow, nearly the same retention percentage as pre­dicted by the empirical models. The field dataresults indicate that the contribution of Lake Erie tophosphorus loading to the wetland is minimal(0.4% of watershed inflow) as assumed in themodels. Actual loading rates and retention may behigher than either the empirical model or field datapredict, because data from one recent sediment coreshow that an average of 22 mg P m-2d-1 has beendeposited over the past 180 years (Reeder 1990).However, the sediment coring site is in the area ofthe marsh with the highest sedimentation rate (Bu­chanan 1982), so the sedimentation data probablyrepresents a region of maximum retention.

The simulation model described above takes intoaccount the effects of hydrology on loading rate,the amount of transformation occurring due to thebiota, and the sedimentation and resuspension esti­mated from model calibration (Table 1). The simu­lation model predicts that 5.5 mg P m-2d-1 or lessthan 20% of the phosphorus inflow is permanentlyretained in the wetland - more than the field esti­mates suggest, but less than the empirical modeland sediment core predict. As expected, the pri­mary producers transform some bioavailable phos­phorus into non-bioavailable forms, but this con­tribution to the sediments is minimal in the model.Other simulations predicted that as much as 44 mgP m-2d-1 could be retained in the marsh, especiallyunder high water level conditions (Reeder 1990,Mitsch and Reeder 1991). The model estimates arereasonable considering the variability of the dataused for comparison, and probably represent the

best available estimate on phosphorus retention inthis Great Lakes coastal wetland.

LANDSCAPE-SCALE WETLAND STUDIES

The landscape-scale area of Lake Erie shoreline istoo large to address with ecosystem-level measure­ments and ecosystem models, but it is closer to thescale that is required to determine the importanceof coastal wetlands on Lake Erie. We initiallychose 11 wetland sites including Old Woman Creekfor synoptic studies of hydroperiod, water quality,sediments, and vegetation and for the applicationof remote sensing, geographic information sys­tems, and spatial modeling (Fig. 4). Several ofthese wetlands are diked to maintain artificial wa­ter levels, primarily to attract waterfowl. These in­clude Winous Point Shooting Club and OttawaShooting Club, private hunt clubs found in thewestern extreme of Sandusky Bay. Other wetlands,such as Pickerel Creek wetland and Willow Pointwetland, are undiked marshes found on the southshore of Sandusky Bay, while Plum Brook wetlandand Sheldon Marsh are undiked wetlands found inthe embayment behind the Cedar Point Amuse­ment Park peninsula.

Hydroperiod

Robb (1989) and Robb and Mitsch (1989, 1992)compared the hydroperiods of a number of dikedand undiked wetlands in the study region duringthe summer of 1988. Relative water levels of thesewetlands (Fig. 9) illustrate a number of points.First, the wetlands that are open to Lake Erie gen­erally dropped in water level through the growingseason due to decreasing lake levels during this pe­riod. During this period, the precipitation was

562 W. J. MITSCH

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o

8

OctSept

@ 8

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Winous Point West

Wmous Point North

@ 80 e0 0 0

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May June

50

em 30

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60

40

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Water Quality

Robb (1989) and Robb and Mitsch (1989, 1992)discussed a synoptic sampling and analysis of wa­ter quality in a number of wetlands in our studyarea of Lake Erie. Results that compare selectedwater quality parameters in diked and undikedwetlands are shown in Table 2. While no signifi­cant differences were inferred for all wetlands inLake Erie, there were significant differences be­tween wetland types for some parameters on a lo­cal scale. Turbidity and total phosphorus werehigher in natural (undiked) marshes while conduc­tivity, alkalinity, and orthophosphate were higherin diked or managed marshes. Biologically-avail­able nitrogen (nitrate + nitrite + ammonia) wasabout the same in the two wetland types. The waterquality data suggest that open wetlands, because ofgreater influence by runoff and streamflow, have

OctSept

Sheldon Marsh

Willow Point

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May

140 +-__-'--_--'----"'-----"'-----"------+

about 750/0 of normal (about 20 cm below normal;Robb 1989). One exception to this trend for man­aged wetlands was Old Woman Creek wetland,which generally maintained its water level duringthis drought year (see above) due to the formationof a barrier beach at its outflow. Second, the wet­lands that are diked generally maintained or evenincreased in water levels due to pumping. Dikedwetlands at Bay View, Ottawa, and Winous Pointreceived pumped water at some times during thesummer. According to Robb (1989), 65 to 75 cmwere added to the Ottawa marshes while 45 to 50cm were added to the Winous Point marshes.

FIG. 9. Patterns of water levels in selected wetlands inlandscape study scale of Ohio's coastal wetlands for: a)natural wetlands (not impounded); and b) diked wet­lands. Depths are relative scales and do not representabsolute average depths; multiple data points per dataindicate measurements at several locations. Curves arebest-fit polynomials (from Robb and Mitsch 1989).

50

em 30

10

50

em 30

10

70 0@

em 50

e 030

50

em 30

10

em

ECOSYSTEM AND LANDSCAPE APPROACHES TO WETLANDS 563

TABLE 2. Comparison of least-square means ofselected water quality parameters in diked and undikedwetlands of Ohio's coastal Lake Erie (Robb 1989).

Diked UndikedParameter Marshes Marshes

temperature, °C 20 22pH 7.67 8.10alkalinity, mg CaCO/L 161* 131conductivity, /Lmhos/cm 1,053* 766turbidity, NTU 26 57*nitrate, /Lg-N/L 330 360nitrite, /Lg-N/L 3 6ammonia, /Lg-N/L 50 40ortho-phosphate, /Lg P IL 73* 55total phosphorus, /Lg P/L 149 237*

*indicates significantly higher number in comparisonbetween diked and undiked marshes, p < 0.01

more suspended sediments and hence more totalphosphorus in the water column than do dikedwetlands which are isolated from such inflows.Lower water levels as a result of the 1988 droughtmade water sampling in natural wetlands difficult,and resuspension of sediments is likely in somecases. Water was generally more plentiful in thediked wetlands because of continual pumping.Furthermore, the presence of organisms that causebioturbation, particularly carp, is more likely inthe open wetlands, as they are specifically con­trolled in many of the diked wetlands in our studygroup. The higher alkalinity, conductivity, and or­thophosphate in diked wetlands suggest that thediked wetlands are like large evaporation basins inthe summer, particularly during a drought, and sa-

linity, particularly as calcium bicarbonate, in­creases as water with dilute salts is pumped into thewetlands and pure water is lost through evapotran­spiration.

Wetland Sediments

Very little has been published on the chemical qual­ity of sediments and their rates of accrual or deple­tion in coastal wetlands of the Great Lakes. Thesediments of Lake Erie coastal wetlands may bedifferent in chemical and physical characteristics,depending on whether they are more influenced byparent soils, by geologic exchange with Lake Erie,by import from upstream watersheds, or by vege­tative productivity in the wetlands themselves. Along history of diking wetlands, thereby isolatingthem from exchange with flooding rivers and theGreat Lakes, may ultimately lead to depauperatesediments and changes in wetland productivity.Several physical and chemical characteristics ofsediments were examined in nine coastal wetlands,five of which are diked wetlands and four of whichare undiked and therefore open to Lake Erie andwatershed exchanges. Table 3 summarizes averageconcentrations of phosphorus, potassium, cal­cium, magnesium, iron, and percent organic mat­ter. Undiked wetlands appear to have higher con­centrations of phosphorus and potassium, whilediked wetlands are higher in calcium and magne­sium. With the exception of Sheldon Marsh, whichhad high organic content (to a depth of 33 cm) inits sediments, the undiked wetlands appeared tohave lower organic content (10-12 percent) thatdid the diked wetlands (14-21 percent). This prob­ably reflects the openness of the undiked wetlands

TABLE 3. Selected average sediment concentrations from diked and undiked wetlands of Ohio's coastal Lake Erie.Wetland abbreviations refer to locations on Figure 4. Complete data Mitsch et al. (l989b).

SoilParameter

phosphorus, ppmpotassium, ppmcalcium, ppmmagnesium, ppmiron, ppmorganic content, 010bulk density, glcm3

pHcation exch cap, meq

OSCB

386235,5533288217.50.646.632

Diked WetlandsOSCA WPN BVC BVB

9 43 1 1966 844 99 414,669 4,289 9,783 9,229196 716 137 72119 90 4719.0 14.5 16.5 21.60.91 0.62 0.66 0.557.8 6.1 10.6 10.328 30 50 47

Undiked WetlandsOWC SHM PCK WLP

53 39 28 22868 921 727 8712,480 3,908 6,212 3,633183 278 266 23680 13 109 8710.2 25.5 11.1 11.70.85 0.50 0.97 1.008.1 7.6 8.3 7.115 23 35 22

Average ± std errorDiked Undiked

18±9 35±7515±190 847±426,705± 1,165 4,058±782290± 115 241 ±2185± 13 88±818±1 15±40.67±0.06 0.83±0.118.3±0.9 7.8±0.337±5 24±4

564 W. J. MITSCH

to flooding by inorganic sediments from upstreamwatersheds and export or organic detritus that oth­erwise would be building organic soils. Concentra­tions of phosphorus were highest in Old WomanCreek wetland, a wetland that has a significantinput of high phosphorus sediments (see above).The diked Bay View Marshes showed extremelylow concentrations of phosphorus, but no signifi­cant differences are noted between the undikedand diked wetlands. These diked wetlands also hadthe lowest concentrations of potassium and mag­nesium but the highest concentrations of calciumand the highest cation exchange capacity. The pHof the ashed sample solutions was also highest inthe Bay View Marsh samples. All of this suggeststhat the Bay View Marshes are heavily influencedby the limestone geology prevalent in this area ofLake Erie and are dominated by calcium carbonateand bicarbonate.

Wetland Vegetation

Lowden (1969), Meeks (1969), Stuckey (1975,1989), Marshall and Stuckey (1974), Farney andBookhout (1982), Balogh and Bookhout (1989),and Robb (1989) have all documented the vegeta­tion of various coastal wetlands of Lake Erie. Onelandscape-scale question is whether the altered hy­drologic conditions and hydrologic isolation im­posed by diking wetlands causes an enhancementor loss of vegetative diversity and productivity. Forexample, Stuckey (1975, 1989) has argued that thediversity of native wetland plants along the coastof Lake Erie has been diminished by extensive wet­land management such as diking and water levelmanipulation. Results from the study by Robb(1989) appear to indicate that the opposite may bethe case. Although the data are based on relativelyfew sample quadrats in each of the study wetlands,there were 32 aquatic plant species identified in thediked wetland sample stations and only 17 speciesin the undiked wetlands (Robb 1989). But she re­ported more floating-leaved macrophytes in theopen wetlands because of their ability to adapt tofluctuating water levels. When other measures ofplant net productivity are compared, the dikedwetlands appear to be more productive (Table 4).Net biomass production, as measured by peak bio­mass in August, is almost twice as high in the dikedwetlands as compared to the undiked wetlands(897 vs. 473 g dry wt m-Z

) and stem counts are overtwice as high (597 vs. 241 stems m-Z

). Biomass isnot an accurate measure of productivity, however,

and the open wetlands may be exporting more oftheir productivity to the lake during the growingseason.

Wetland Remote Sensing and Mapping

Previous studies of remote sensing have evaluatedthe capability to measure wetland variables, andmany efforts have demonstrated remote sensordata as input to wetland studies and wetland inven­tories (e.g., Shima et at. 1976, Lyon 1981, Mitschet at. 1983, Lyon et at. 1986, Hardisky et at. 1986).Most researchers agree that remote sensing pro­vides a time and cost effective method of mappingwetlands and it can provide various informationsuch as 1) the presence of wetlands, 2) knowledgeof adjacent land use which affect wetland areas,and 3) the change of wetland vegetation over timeassociated with other ecosystem changes. Variousmappings of Great Lakes wetlands have been exe­cuted, some by the use of remote sensing and oth­ers before remote sensing was generally available.For example, Lyon and Drobney (1984) describedthe area of wetlands as a function of water levels ofLake Michigan from seven sets of black and whiteaerial photographs taken between 1938 and 1977.The data showed that an increase in water level of30 cm resulted in a decrease of 18070 of the 438 haof vegetated wetlands and beaches in the Straightsof Mackinac. These data were subsequently usedto develop a predictive model of long-term effectsof water levels on coastal wetlands (Lyon et at.1986). Balogh and Bookhout (1989) used true­color 35-mm photographs (slides) taken at 1,500 min 1984 to map the distribution of purple loose­strife (Lythrum salicaria) in Ohio's southwest LakeErie marshes. Visual ground truth by low-altitudeflights was done during the flowering season ofAugust 1985 to improve the accuracy of the maps.Seventy percent of the loosestrife strands werefound within areas designated as wetlands onUSGS topographic maps.

In our study, vertical aerial photographs (slides)were taken of Lake Erie Wetlands on 22 August1988 by the Division of Soil and Water Conserva­tion of the Ohio Department of Natural Resources(Roush et at. 1989). Both 35-mm color and colorinfrared films were used. Approximately 270 pho­tos were taken over several flight lines at an ap­proximate scale of 1:39,000. Interpretation ofthese photographs is not complete, but Figure 10illustrates a preliminary interpretation of a section

ECOSYSTEM AND LANDSCAPE APPROACHES TO WETLANDS 565

TABLE 4. Selected average macrophyte measurements at peak biomass from diked and undiked wetlands of Ohio'scoastal Lake Erie. Wetland abbreviations refer to locations on Figure 4. Data from Robb (1989).

Average±std error

Measure ofVegetation

Biomass, g dry wt/m2

# species/plot*# stems/m2

OSCB

1,986

1.0170

Diked Wetlands

OSCA WPN WPW

527 590 560

2.0 1.5 3.01,476 92 463

BVC BVB

1,479 239

1.0 1.7474 907

Undiked WetlandsOWC SHM PCK WLP

187 327 859 599

1.0 1.0 1.3 2.3114 177 300 373

Diked(n = 6)

897 ± 277

1.7±0.3597±211

Undiked(n = 4)

473± 149

1.4±0.3

241 ± 59

*only species> 10070 by weight per plot. Plots were 0.5 m2 randomly placed in each wetland (3 to 6 per wetland)

FIG. 10. Example ofa) aerial photograph of landscape-level study ofLake Erie coastal wetlandsand b) subsequent interpreted wetland map (Roush et at. 1989). Photo was taken 22 August 1988by Ohio Department of Natural Resources.

of wetlands in the study area. We chose to usebroad categories such as open water (W), emergentwetlands (E), and floating-leaved wetlands (F). Afew areas are easily recognizable, such as areas of

open water which appear blue or black on the in­frared slides. Wooded areas are also relatively easyto recognize by the shapes of the trees. Cattails(Typha spp.) appear dark green on the conven-

566 w. J. MITSCH

Lake Erie WetlandsVickery Quad - OhioFlight 7 - Slides 21-24from Color and IR Aerial Photography byOhio Department of Natural ResourcesAugust 22, 1988

meters

a 200 400 600 800 1000

FIG. 10. b

Idlorneter

@- 0 2 4 6 8

o 1 2 345mile

WETLANDSW open waterE emergent wetlandsE-ct cattailsE-br bulrushE-m millet

F floating-leaved wetlandF-n water lotus

OTHER LAND USES

T forests (trees)X exposed landA agricultural landA-bw buckwheat

D dikes

ECOSYSTEM AND LANDSCAPE APPROACHES TO WETLANDS 567

Wetland Map of SandU3k.y Ba,

N

Ci J)OJ 4XO Netres fr\1-1 _---'__~I__~_....JI U

T..,r'let Jne;:r.dO'l

ooo

Bare soil

­!III­c:J

FIG. 11. Proposed spatial simulation model ofLake Erie coastal wetlands showing incorporation ofecosystem-levelmodel into geographic mapping system. Map by Gi-Chul Yi.

tional color slides and bright red on the infraredslides, and water lotus (Nelumbo lutea) appearsblue-green on the color slides and light pink on theinfrared slides, but there may be other types ofvegetation which exhibit the same characteristics.Wherever the general category of emergent wet­lands (E) was used, the color infrared slides ap­peared white. R. Kroll, Winous Point ShootingClub, (pers. comm.) suggested that this may indi­cate the flowering of Sagittaria spp. These mapdata will provide the spatial information necessaryfor building spatial databases and can be used tocalibrate and verify spatial models.

Future Studies - Spatial Modeling

The obvious next step in completing a systems viewof the wetlands in the Sandusky Bay region is tocombine the benefits of dynamic modeling as dem­onstrated by the ecosystem simulation model ofMitsch and Reeder (1991) with the spatial mappingcapabilities made available from remotely senseddata and geographic information system data­bases. We hope to develop a dynamic spatial modelof large-scale processes of wetland development.This idea is illustrated conceptually in Figure 11.Such a modeling approach would combine the ben-

568 w. J. MITSCH

efits of recent computer advances in displaying andmanipulating data sets over space with the predic­tive capabilities of dynamic simulations. Spatialmodeling of coastal wetlands has previously beenapplied to the coastal region of Louisiana (Cos­tanza et af. 1988, 1990) and to the examination ofthe effects of sea level rise on oceanic coastal wet­lands (Park et af. 1989). No such approach has yetbeen applied to the Great Lakes, although theyoffer an excellent test case for such models for anumber of reasons: 1) there is a relative abundanceof data on important forcing functions such asstreamflow, water quality, and Lake Erie waterlevels; 2) there is a long history of observation ofthe coastal wetlands along Lake Erie's southernshoreline, with some maps going back to the firsthalf of the 19th century; 3) the fluctuating waterlevels of the Great Lakes, the relatively rapid de­velopment of the shoreline, and the current intensemanagement of the wetlands have caused the wet­lands of the region to be very dynamic in nature,changing on almost a yearly basis as a result ofmany factors.

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Submitted: 1 May 1991Accepted: 29 June 1992