Environmental drivers of spatial and temporal water quality variability in four coastal wetlands of Lake Ontario

Great Lakes coastal wetlands serve as mediation zones between the land and the lake, regulating the fate of materials received from tributaries prior to discharge to the lake nearshore zone. To improve our understanding of water quality processing and nutrient fate in coastal wetlands, we evaluated within- and across-wetland water quality as a function of environmental drivers over a decade (2009–2018) in three drowned river mouth (Carruthers, Duffin’s, and Rouge) and one barrier lagoon (Frenchman’s Bay) wetlands on the north shore of Lake Ontario. Overall, land-use had a weak relative association with most water quality parameters, reflecting no appreciable changes in land-use across the study years. The barrier lagoon wetland Frenchman’s Bay had a distinctly different water quality pattern from the drowned river mouth wetlands, where water quality followed a high to low concentration gradient from near the tributary confluences (high) to the lake-wetland confluence (low) (permutational analysis of variance p-value < 0.001). Notably, we observed significant differences among celled (i.e., natural ponds in wetlands) and non-celled sites in Duffin’s and Rouge marshes, primarily attributed to strong covariation among phosphorus, phosphate, and organic nitrogen concentrations (permutational analysis of variance p-value < 0.001). This water-quality signature seemed to be driven by solar radiation and lake level variability (i.e., seiche inundation), inferring that wetlands may be important sites for the mobilization of legacy phosphorus in sediments under certain climatic conditions, and following seiche events.     

Temporal Dynamics of Nutrients (N and P) and Hydrology in a Lake Superior Coastal Wetland

Coastal wetlands on Lake Superior are hydrologically complex ecosystems situated at the interface of upland catchments and the oligotrophic lake. Little is known about nutrient dynamics within coastal wetlands or their role in modifying or contributing to nutrient fluxes from watersheds to Lake Superior. We conducted an intensive study of Lost Creek Wetland (LCW) near Cornucopia, WI, with the objective of determining influences of temporal variability in hydrology on dynamics and retention of N and P. We measured hydrologic inputs and distributions of inorganic and organic forms of nitrogen and phosphorus within LCW under hydrologic conditions ranging from summer base flow to spring snow melt. Our study confirms that the interrelationship between hydrologic connections to lake and tributary and seasonal variations in hydrology can regulate internal nutrient dynamics of coastal wetlands. The strength of hydrologic linkage of LCW to Lake Superior and tributary varied greatly among seasons, resulting in shifts in the relative importance of these nutrient sources and influencing spatial distribution of nutrients within the wetland. Ratios of inorganic nitrogen and phosphorus in the wetland were consistently low (< 16) indicating a potential for nitrogen limitation. Retention of inorganic nitrogen ranged from 11% to 94% and was positively related to hydraulic residence time which ranged from < 1 day during snow melt to 26 days in summer. Retention of total and soluble reactive phosphorus was generally lower than retention of inorganic nitrogen and was not related to hydraulic residence time.     

Hydrodynamic modeling and characterizing of Lagrangian flows in the West Scott Creek wetlands system,South Carolina

With increased recognition of the value of tidal wetlands and their defining hydrology, the need for better understanding of tidal flow and mixing characteristics is vital to any wetland restoration and enhancement projects and studies. A numerical modeling study was carried out to study the mixing and transport processes in the tide dominated West Scott Creek Estuary, South Carolina. The West Scott Creek estuary is a system of meandering tidal creeks and salt marsh between Edisto Island and the Edisto Beach barrier island. A coupled, depth-integrated hydrodynamic and particle transport model was developed. The model was calibrated and verified against a set of field-measured hydrodynamic data and the model-predicted water elevations, velocities and discharges were in good agreement with the field measurements. The hydrodynamically calibrated model was coupled with a particle tracking module to quantify the residence time distribution and associated transport mechanisms in a spatially varying situation. Computed residence time distribution, under tidal forcing, indicates a strong spatial variation and a non-monotonic distribution along the thalweg of the creek. The model computed a residence time of 4.92 days at the head of the creek. Numerical analysis of the Lagrangian flow field indicated varying mixing regions (strong and weak) within the wetland and the result is anticipated to provide useful information to assess the impact of coastal development in and around the West Scott Creek estuary including the restoration of tidal hydrology.     

Hydrogeomorphic Classification for Great Lakes Coastal Wetlands

A hydrogeomorphic classification scheme for Great Lakes coastal wetlands is presented. The classification is hierarchical and first divides the wetlands into three broad hydrogeomorphic systems, lacustrine, riverine, and barrier-protected, each with unique hydrologic flow characteristics and residence time. These systems are further subdivided into finer geomorphic types based on physical features and shoreline processes. Each hydrogeomorphic wetland type has associated plant and animal communities and specific physical attributes related to sediment type, wave energy, water quality, and hydrology.     

Modelling the influence of seiche-events on phosphorous-loading dynamics in three Lake Ontario coastal wetlands

Coastal Wetlands (CWs) provide critical ecosystem services that maintain biogeochemical processes and habitats in the coastal zone of the Great Lakes. When nutrient-laden surface waters flow into CWs from their watersheds, internal physical, chemical, and biological processes can alter the final nutrient loadings to the lake. However, CWs can periodically be inundated with lake water from seiche events, and little is known about the impacts of seiches on nutrient processing and loadings from CWs. To evaluate the influence of lake seiches on CW phosphorous-loading dynamics, we built a multi group structural equation model (SEM) using climatic and wave data, and interannual (2009–2018) estimated sediment and phosphorous loadings from three CWs on the north-shore of central Lake Ontario (Rouge Marsh, Duffin’s Marsh, and Carruthers Marsh). Wind speeds, lake levels, and an increased peak period of wave spectra were significant explanatory variables of seiche events (p-value < 0.001). We identified that seiche events caused significant sediment resuspension (p-value < 0.001) in CWs, which contributed to a significant increase of phosphorous loading to the coastal zone of Lake Ontario (p-value < 0.001). Our results indicate that lake-seiche events can influence CW phosphorous-loadings to Lake Ontario, and should be considered when modelling water quality in the nearshore zone.     

Wind-driven Summertime Upwelling in a Fjord-type Lake and its Impact on Downstream River Conditions: Quesnel Lake and River,British Columbia,Canada

Observations and modeling results are presented to explore the response of a multi-basin, fjord-type lake to episodic wind forcing. Field observations show that abrupt cooling and warming events (magnitude greater than 5°C d-1) lasting 3–6 days in a large, salmon-bearing river (Quesnel River) are due to upwelling in its upstream lake (Quesnel Lake) during the summer, stratified season. Within the lake, vertical displacement of isotherms in the vicinity of the river mouth associated with this upwelling is shown to be forced by wind events longer than one quarter of the fundamental seiche period and of sufficient magnitude that the Wedderburn number approaches one. Upwelling occurs nearly-simultaneously throughout a smaller basin adjacent to the outflow (West Basin) that is separated from the Main Basin of Quesnel Lake by a sill and contraction. Wind-driven water fluxes across the sill are estimated using a conceptual model based on volume and heat budgets. These estimates provide an upper bound for flow across the sill and suggest that exchange flow may at times be internally hydraulically controlled, with epilimnetic velocities of up to ∼25 cm/s. Computed fluxes suggest the West Basin hypolimnion has a residence time of 6-8 weeks during the summer stratified period with each upwelling episode irreversibly exchanging 25–30% of the hypolimnetic volume with the rest of the lake. Implications of such events are profound for salmon bearing rivers wherein the thermal habitat is critical to migration success.     

Effects of Hydrological Flow Regime on Sediment-water Interface and Water Column Nitrogen Dynamics in a Great Lakes Coastal Wetland (Old Woman Creek,Lake Erie)

Sediment-water interface nitrogen (N) transformations and water column ammonium cycling rates were measured along a stream to lake gradient at three sites within Old Woman Creek (OWC) and one near-shore Lake Erie site during two hydrological regimes: one with open flow to the lake after a rain event (July 2003), and another with a sand barrier blocking flow (July 2004). Net N2 effluxes in OWC at all times and at the near-shore Lake Erie site in July 2003 suggest that sediments are a N sink via denitrification. Observed dissimilatory nitrate reduction to ammonium (DNRA) may counteract some of this N removal, particularly when the creek mouth is closed. Upstream, a closed creek mouth led to higher sediment oxygen demand, net N₂ flux, potential DNRA, and potential denitrification rates. The lake site exhibited lower rates of these processes with the creek mouth closed except denitrification potential, which was unchanged. Denitrification in OWC appeared to drive N limitation in the lower wetland when the sand barrier was blocking flow to the lake. Higher potential versus in situ denitrification estimates imply that water column NO₃⁻ limits and drives denitrification in OWC. Water column to sediment regeneration ratios suggest that sediment recycling may drive primary production in the OWC interior when the creek mouth is closed and new N inputs from runoff are absent, but more data are needed to confirm these apparent trends. Overall, hydrological regime in OWC appeared to have a greater impact on sediment N processes than on water column cycling.     

Watershed influences on the transport, fate and bioavailability of mercury in Lake Superior: Field measurements and modelling approaches

The issuance of fish consumption advisories in US states bordering Lake Superior has heightened the need for understanding the biogeochemical cycling and transformations of mercury in this great lake. Major routes of mercury (Hg) transport to lakes include atmospheric deposition (wet and dry), direct discharges and riverine (watershed) inputs. The specific objectives of this ongoing study are to: (i) determine the speciation and bioavailability of Hg transported to Lake Superior (ii) determine the importance of watershed‐specific characteristics that control physical and chemical forms of Hg; (iii) identify key mechanisms controlling Hg bioavailability and speciation in near‐shore zones relative to open lake regions; and (iv) provide process‐level information to compliment concurrent development of Hg fate and transport models of the Lake Superior ecosystem. Three tributaries of Lake Superior were chosen for detailed field study based on previous results and particular watershed characteristics. Mixing zones represent a potentially important zone of Hg entry into the food chain, and were sampled for biota, water and sediment in transects between the tributary mouth and the lake end member. Sampling of open‐water sites was conducted during Environmental Protection Agency‐sponsored cruises on the research vessel Lake Guardian. Results from our work on Lake Superior tributaries demonstrate that dissolved organic carbon and methylmercury (MeHg) export was greatest from watersheds containing wetlands. In Lake Superior, concentrations of Hg species were small at pelagic stations (total mercury (HgT) averaged 0.49 ng/L, MeHg averaged 6.4 pg/L). In contrast, MeHg concentrations in tributaries ranged from 100 to 250 pg/L. Watershed sites dominated by wetlands exhibited the greatest MeHg concentrations, occasionally in excess of 12 ng/L. Samples of bulk zooplankton ranged from 20 to 130 ng/g dry weight HgT and from 15 to 45 ng/g dry weight MeHg, and demonstrated typical bioacculmulation of mercury as well as distinct seasonality in concentrations.     

Influence of lake levels on water extent,interspersion, and marsh birds in Great Lakes coastal wetlands

Coastal wetlands in the Laurentian Great Lakes undergo frequent, sometimes dramatic, physical changes at varying spatial and temporal scales. Changes in lake levels and the juxtaposition of vegetation and open water greatly influence biota that use coastal wetlands. Several regional studies have shown that changes in vegetation and lake levels lead to predictable changes in the composition of coastal wetland bird communities. We report new findings of wetland bird community changes at a broader scale, covering the entire Great Lakes basin. Our results indicate that water extent and interspersion increased in coastal wetlands across the Great Lakes between low (2013) and high (2018) lake-level years, although variation in the magnitude of change occurred within and among lakes. Increases in water extent and interspersion resulted in a general increase in marsh-obligate and marsh-facultative bird species richness. Species like American bittern (Botaurus lentiginosus), common gallinule (Gallinula galeata), American coot (Fulica americana), sora (Porzana carolina), Virginia rail (Rallus limicola), and pied-billed grebe (Podilymbus podiceps) were significantly more abundant during high water years. Lakes Huron and Michigan showed the greatest increase in water extent and interspersion among the five Great Lakes while Lake Michigan showed the greatest increase in marsh-obligate bird species richness. These results reinforce the idea that effective management, restoration, and assessment of wetlands must account for fluctuations in lake levels. Although high lake levels generally provide the most favorable conditions for wetland bird species, variation in lake levels and bird species assemblages create ecosystems that are both spatially and temporally dynamic.     

Breeding birds and anurans of dynamic coastal wetlands in Green Bay,Lake Michigan

Breeding birds and anurans (frogs and toads) in coastal wetlands of Green Bay, Lake Michigan vary dynamically with changing water levels, habitat type, and geography. We describe species assemblages over a seven-year period (2011–2017) beginning with historic low water levels followed by an increase in average lake level of 0.85?m. In general, species richness and abundance of marsh-obligate species responded positively to increasing water levels, although several species of shallow wetlands (sandhill crane, sedge wren, swamp sparrow, and American toad) showed the opposite trend. Anuran assemblages were more diverse in the middle and upper bay, coinciding with a well-established nutrient gradient from the hypereutrophic lower bay to more oligotrophic waters of the upper bay. Three marsh-obligate bird species (black tern, sandhill crane, and sedge wren) were significantly more abundant in the middle or upper bay while sora, American coot, and common gallinule were more abundant in the eutrophic lower bay. Our findings have several important implications for conservation. Inland wetlands near the coast (including diked wetlands) might play a key ecological role by providing refugia for some species during low water years. However, the importance of shallow coastal wetlands and nearshore gradients of wetland habitat might be overlooked during low water years; when high water returns, these areas can become extremely productive and species-rich.     

Fish Habitat Use Within and Across Wetland Classes in Coastal Wetlands of the Five Great Lakes: Development of a Fish-based Index of Biotic Integrity

The relative importance of Great Lake, ecoregion, wetland type, and plant zonation in structuring fish community composition was determined for 61 Great Lakes coastal wetlands sampled in 2002. These wetlands, from all five Great Lakes, spanned nine ecoregions and four wetland types (open lacustrine, protected lacustrine, barrier-beach, and drowned river mouth). Fish were sampled with fyke nets, and physical and chemical parameters were determined for inundated plant zones in each wetland. Land use/cover was calculated for 1- and 20-km buffers from digitized imagery. Fish community composition within and among wetlands was compared using correspondence analyses, detrended correspondence analyses, and non-metric multidimensional scaling. Within-site plant zonation was the single most important variable structuring fish communities regardless of lake, ecoregion, or wetland type. Fish community composition correlated with chemical/physical and land use/cover variables. Fish community composition shifted with nutrients and adjacent agriculture within vegetation zone. Fish community composition was ordinated from Scirpus, Eleocharis, and Zizania, to Nuphar/Nymphaea, and Pontederia/Sagittaria/Peltandra to Spargainium to Typha. Once the underlying driver in fish community composition was determined to be plant zonation, data were stratified by vegetation type and an IBI was developed for coastal wetlands of the entire Great Lakes basin.     

Numerical study of hydrodynamic process in Chaohu Lake

In this paper, the hydrodynamic characteristics of water flow in Chaohu Lake are studied by using the finite volume coastal ocean model(FVCOM), which is verified by the observed data. The typical flow field and the 3-D flow structure are obtained for the lake. The flow fields under extreme conditions are analyzed to provide a prospective knowledge of the water exchange and the transport process.The influence of the wind on the flow is determined by the cross spectrum method. The results show that the wind-driven flow dominates most area of the lake. Under prevailing winds in summer and winter, the water flows towards the downwind side at the upper layer while towards the upwind side at the lower layer in most area except that around the Chaohu Sluice. The extreme wind speed is not favorable for the water exchange while the sluice's releasing water accelerates the process. The water velocity in the lake is closely related with the wind speed.     

Gyres and Seiches in a Large and Shallow Lake

Gyres and seiches are two prominent features of lakes. Gyres largely transport sediments, nutrients, and algae in the horizontal direction. Seiches, on the other hand, can contribute to the vertical mixing in lakes. Theoretical analysis, statistical methods, and numerical models are used to investigate gyres and seiches in Lake Okeechobee, the largest subtropical/tropical lake in North America. The lake has a 1,730-km2 surface water area, a typical length of more than 50 km, and a mean depth of 3.2 m. Both the Empirical Orthogonal Function (EOF) method and the numerical model results indicated that lake circulation is typically dominated by a two-gyre pattern, especially in the winter. The northwest wind or southeast wind leads to a cyclone (a counterclockwise rotation gyre) in the southwest and an anticyclone (a clockwise rotation gyre) in the northeast. Because the mean velocity field in the lake is very weak, the first two EOF modes play an important role in lake transport. The mechanism of gyre formation in the lake is clearly explained in a theoretical analysis. Power spectra analysis on measured and modeled water elevations at four stations revealed that Lake Okeechobee has a seiche signal of 5 hours or so. The seiche range is typically around 10 cm. Results from the theoretical analysis, power spectral analysis, and numerical modeling all agree with each other very well. The findings in this study should be useful to understand the lake processes, to guide field data collection programs, and to assist decision making on lake management.     

我国湖泊湿地面临的问题及其对策研究

在综合分析国内外湖泊湿地保护、治理及管理等研究的基础上,比较系统地讨论了我国湖泊湿地面临的问题:泥沙淤积;湖泊面积萎缩;水位下降;湖泊水质恶化等。提出了一系列湖泊湿地管理对策,如建立湿地自然保护区、开展湿地生态系统健康评价、湿地生态功能区划及可持续管理对策研究等,为湖泊湿地的可持续发展和管理提供理论指导和科学依据。     

The effect of tectonism on deltaic wetland migration: A case study from Bendimahi River Delta System (Lake Van-Eastern Turkey)

Coastal wetlands are in decline globally because of climatic and/or hydrological changes, and anthropogenic activities. Deltaic wetlands are complex environments formed by different water sources, particularly when ionically rich soda water and freshwater and converge and influence the presence and diversity of biological activity. Lake Van is a terminal lake, 1650 meters above the sea level, surrounded by high mountains. Because of its alkaline water, vegetation and biological activity are generally rich in the deltaic areas. In this study, geological evolution of Bendimahi River Delta System, located on northeastern part of the Lake Van, is assessed as to constructive and destructive temporal factors, determined with satellite images and field studies. Bendimahi river channel is significantly deformed by regional tectonic activity and this deformation has caused migration of deltaic wetland. That migration process has resulted in abandoned deltaic area and a new delta formation. Hydrological and biological results of the migration of the Bendimahi River Delta System clearly indicate the importance of wetlands on coastal ecology, especially alkaline lakes.     

Seasonal patterns in hydrochemical mixing in three Great Lakes rivermouth ecosystems

Rivermouth ecosystems in the Laurentian Great Lakes represent complex hydrologic mixing zones where lake and river water combine to form biologically productive areas that are functionally similar to marine estuaries. As urban, industrial, shipping, and recreational centers, rivermouths are the focus of human interactions with the Great Lakes and, likewise, may represent critical habitat for larval fish and other biota. The hydrology and related geomorphology in these deltaic systems form the basis for ecosystem processes and wetland habitat structure but are poorly understood. To this end, we examined hydrogeomorphic structure and lake-tributary mixing in three rivermouths of intermediate size using water chemistry, stable isotopes, and current profiling over a five-month period. In rivermouths of this size, the maximum depth of the rivermouth ecosystem influenced water mixing, with temperature-related, density-dependent wedging and layering that isolated lake water below river water occurring in deeper systems. The inherent size of the rivermouth ecosystem, local geomorphology, and human modifications such as shoreline armoring and dredging influenced mixing by altering the propensity for density differences to occur. The improved scientific understanding and framework for characterizing hydrogeomorphic processes in Great Lakes rivermouths across a disturbance gradient is useful for conservation, management, restoration, and protection of critical habitats needed by native species.     

Fen development along the southern shore of Lake Ontario

Fen development along a drowned-river-mouth tributary to Braddock Bay, Lake Ontario was studied to address its formation. Nested piezometers were installed to assess groundwater contributions and obtain water chemistry samples. Soil and geology information came from existing sources. We converted paleo lake levels from published reports to IGLD1985 and calendar years BP for use in analyzing vegetation changes over time using a combination of peat-core plant macrofossils and modern surveys. Piezometer data showed upward discharge, water at 3-m depth had pH 6.9, specific conductivity of 508 µS/cm, and alkalinity 206 mg/L as CaCO₃. Hydraulic head and mineralized water chemistry decreased at shallower depths. Vegetative development began 1790 cal yr BP with sedges and brown moss when land surface was 0.135 m above lake level. Lake levels increased, and by 1590 cal yr BP, water was 0.17 m deep and sedges were joined by shoreline emergent species. Water depth then increased to 0.525 m but began decreasing as lake levels fell. Peatland species appeared around 810 cal yr BP when water depth was reduced to 0.225 m. About 585 cal yr BP, additional peatland species appeared when land surface was 0.075 m above lake level. Sphagnum became prominent 80 cal yr BP (0.81 m above lake level), representing 67 % mean cover in modern vegetation. Isolation of the surface from calcareous groundwater resulted in transition from rich fen to poor fen. These wetlands are rare in the lower Great Lakes and deserve protection of their characteristic hydrology, water chemistry, and vegetation structure.     

洞庭湖典型洲滩湿地水分时空变化特征及其相关性分析

为了分析变化条件下洲滩内部土壤水分时空变化特征及不同深度土壤水分含量和湖水位的相关性,以洞庭湖典型洲滩断面为研究对象,利用原位监测装置连续2个月监测了湖水位、大气温度及土壤水分含量的动态变化过程。结果表明:洞庭湖洲滩水分场的分布在垂向上呈现明显的分层现象,同一位置的土壤水分含量的波动幅度随着深度的增加而逐渐减小,至深层处(大于50 cm)趋于稳定,同一深度的土壤水分含量的波动幅度随着与岸边距离的增加而逐渐减小。土壤水分含量与湖水位之间的相关性随着深度的增加呈先增强后减弱的趋势,浅层及深层的土壤水分含量和湖水位之间均呈无显著相关性,地表以下50~70 cm深度处土壤水分含量与湖水位相关性较高。研究结果有助于了解土壤水分在洲滩生态系统地下含水层-土壤-大气界面的相互作用机制,为洲滩水文过程及生态环境保护研究提供重要方法和理论参考。     

A hydrodynamic approach to modeling phosphorus distribution in Lake Erie

The purpose of this paper is to show how a high-resolution numerical circulation model of Lake Erie can be used to gain insight into the spatial and temporal variability of phosphorus (and by inference, other components of the lower food web) in the lake. The computer model simulates the detailed spatial and temporal distribution of total phosphorus in Lake Erie during 1994 based on tributary and atmospheric loading, hydrodynamic transport, and basin-dependent net apparent settling. Phosphorus loads to the lake in 1994 were relatively low, about 30% lower than the average loads for the past 30 years. Results of the model simulations are presented in terms of maps of 1) annually averaged phosphorus concentration, 2) temporal variability of phosphorus concentration, and 3) relative contribution of annual phosphorus load from specific tributaries. Model results illustrate that significant nearshore to offshore gradients occur in the vicinity of tributary mouths and their along-shore plumes. For instance, the annually averaged phosphorus concentration can vary by a factor of 10 from one end of the lake to the other. Phosphorus levels at some points in the lake can change by a factor of 10 in a matter of hours. Variance in phosphorus levels is up to 100 times higher near major tributary mouths than it is in offshore waters. The model is also used to estimate the spatial distribution of phosphorus variability and to produce maps of the relative contribution of individual tributaries to the annual average concentration at each point in the lake.     

Offstream movements of fish during drought in a regulated lowland river

Access to offstream habitats is vital for many freshwater fish, but details of their lateral movements are scarce. We describe the movements of fish between the channel of the River Murray and six perennially inundated wetlands in South Australia from August to November 2006. At this time there were unprecedented low flows in the river owing to the combined effects of river regulation, drought and over‐allocation to upstream users. Some 210 000 fish from 18 species (14 native, 4 alien) were recorded, including two uncommon native species listed by conservation agencies. Movements of juveniles and adults varied among wetlands despite the shared river reach and the proximity of the wetlands to each other, but showed no consistent directionality. This may reflect the prevailing low‐flow conditions, the virtually permanent connections between the wetlands and channel maintained by weirs, levees and barrages, and the dominance of ‘generalist’ species. We speculate that movements facilitate efficient resource utilization and nutrient exchange between homogenized river and wetland habitats in the absence of the flood‐pulse. We anticipate directional movements will become apparent when flows are increased, so that our data could provide a comparative baseline for future studies. As modifications to natural flow paths may impede access to/from wetlands by fish and other aquatic fauna, provisions for access should be incorporated into flow‐control structures, used locally to manipulate wetland hydrology. Copyright © 2010 John Wiley & Sons, Ltd.