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Stable isotope analyses offer a useful means for quantifying ecological niche dimensions, though few studies have examined isotopic response of an ecological community with respect to resource gradients such as fluctuations in prey availability. Stable carbon and nitrogen isotopes were measured for Lake Michigan salmonines and their prey collected from 2014 to 2016. Bayesian ellipse and mixing model analyses were used to quantify isotopic niche characteristics and diets, respectively, among species and years. During the 3-year study period, abundance and size structure of preferred alewife (Alosa pseudoharengus) prey changed substantially and offered an opportunity to explore predator isotopic niche response and diet shifts along a prey resource gradient. Results suggested increased reliance on alewives, especially small alewives, over the study period and were consistent with greater availability of this prey. However, differential use of alewife size classes and alternative prey sources by salmonine predators was apparent, which suggested possible resource partitioning. Characterization of ecological niche overlap using stable isotopes likely requires consideration of shared resource availability as well as specific prey and habitat preferences.

Atmospheric deposition of nutrients, particularly nitrogen and phosphorus has the capacity to significantly affect the productivity and biogeochemistry of aquatic ecosystems. The objective of this study was to assess the impact of land use/cover types on the atmospheric deposition of nutrients around Lake Kivu. Dry and wet atmospheric deposition samples were collected from four different land use/cover types (forest, wetland, agricultural and urbanized area) at four stations (Goma, Lwiro, Bukavu and Iko) around the Lake Kivu basin. The highest annual loading of dry total phosphorus (TP) was recorded at a station located in an urban area at Goma (4.4 ± 3.9 μmol/m2/yr) and the highest dry deposition of total nitrogen (TN) was recorded at Iko (84.5 ± 41.2 μmol/m2/yr). High wet TP and TN were at Bukavu (0.7 ± 1.1 μmol/m2/yr) and Iko (21.7 ± 34.7 μmol/m2/yr) respectively. High dry TP loads were recorded in the forest area of Goma and the highest dry TN at Lwiro. High wet TP loads were record in agriculture at Goma and high values of wet TN in agriculture at Iko. Phosphorus and nitrogen deposition rates around Lake Kivu were similar to those reported for other African lakes. The highest rate of TP was recorded mainly in the forest area for dry deposition while high TN was recorded in all types of land use/cover in the basin. This study found out that forest traps high concentration of nutrient than other land uses/covers.

Lake Michigan salmon and trout populations are important species for recreational fisheries and food web management, and are largely supported through stocking efforts, with varying degrees of natural recruitment. Ongoing fisheries management of these salmonine populations is dictated by relationships between predator and prey abundance as well as community structure within the lake. However, while prey fish biomass has declined, and species composition has changed in recent decades, knowledge of prey consumption by the salmonine community has lagged. Herein, we explore trophic relationships using fatty acids profiles, which offer insights into the foraging habits and energy pathways relied on over weeks to months prior to collection. Fatty acids of the prey base for salmonines in Lake Michigan indicate a gradient of foraging habits that range from pelagic (typified by alewife and rainbow smelt) versus benthic (i.e., slimy sculpin and round goby) resource use. Fatty acids implied that there was more variation in foraging habits among individual lake trout and brown trout compared to Chinook salmon, coho salmon and rainbow trout, which appeared to all rely almost exclusively on pelagic prey. Fatty acid profiles also indicated size-based shifts in foraging habits; for example, larger lake trout consuming a greater proportion of benthic prey than smaller individuals. Data herein suggest that Chinook and coho salmon, as well as rainbow trout, are more likely to experience competitive interactions during times of low pelagic prey-fish abundance in Lake Michigan, whereas brown and lake trout are able to utilize benthic resources to a greater degree.

In Lake Michigan, the unintended introduction of invasive species (e.g., zebra mussel, Dreissena polymorpha; quagga mussel, D. rostriformis bugensis; round goby, Neogobius melanostomus) and reduced nutrient loading has altered nutrient dynamics, system productivity, and community composition over the past two decades. These factors, together with sustained predation pressure, have contributed to declines of several forage fish species, including alewife (Alosa pseudoharengus), which has dominated diets of the five primary salmonine species of Lake Michigan for the last 50 years. Salmonines that have inflexible, less complex diets may struggle if alewife declines continue. We analyzed stomach contents of salmonines collected throughout the main basin of Lake Michigan in 2015 and 2016 to investigate diet composition, diet diversity, and individual variation of alewife lengths consumed. Chinook salmon (Oncorhynchus tshawytscha) almost exclusively consumed alewife and had lower diet diversities compared to the other four species, which consumed relatively high frequencies of round goby (brown trout, Salmo trutta; lake trout, Salvelinus namaycush), aquatic invertebrates (coho salmon, Oncorhynchus kisutch) and terrestrial invertebrates (rainbow trout, Oncorhynchus mykiss) along with alewife. Although clear spatio-temporal feeding patterns existed, much of the variation in diet composition and diet diversity was expressed at the individual level. Salmonine populations consumed the entire size range of alewife that were available, whereas individual stomachs tended to contain a narrow range of alewife sizes. Due to their reliance on alewife, it is likely that Chinook salmon will be more negatively impacted than other salmonine species if alewife abundance continues to decline in Lake Michigan.

The invasion of dreissenid mussels has profoundly altered benthic physical environments and whole-lake nutrient cycling in the Great Lakes over the past several decades. The resurgence of the filamentous green alga Cladophora appears to be one of the consequences of this invasion. Sloughed Cladophora deteriorates water quality, fouls recreational beaches, and may contribute to outbreaks of avian botulism, which have been especially severe in the Sleeping Bear Dunes National Lakeshore (SLBE) region of Lake Michigan. To help determine the fate of sloughed Cladophora, a Lagrangian particle trajectory model was developed to track the transport of Cladophora fragments in the nearshore zone based upon a physical transport-mixing model. The model results demonstrate that the primary deposition sites of sloughed Cladophora within the SLBE region are mid-depth sites not far away from their initial growth area. Because of high algae production in the nearshore waters and limited exchange between the inner and outer bay, the shoreline beach of Platte Bay appears to be particularly vulnerable to fouling, with overall three times as many accumulated particles as those along the Sleeping Bear Bay and Good Harbor Bay. The results of this model may be used to guide regional environmental management initiatives and provide insights into the mechanisms responsible for avian botulism outbreaks. This model may also inform the development of whole-lake ecosystem models that account for nearshore-offshore interactions.

Nutrient loading into Lake Michigan can produce algal blooms which in turn can lead to hypoxia, beach closures, clogging of water intakes, and reduced water quality. The Great Lakes Water Quality Agreement targets for Lake Michigan are 5600 MT annually for total phosphorus (TP) loading, 7 μg L−1 lake-wide mean TP concentration, and a chlorophyll-a concentration of 1.8 μg L−1. However, in light of the recent resurgence of nuisance algal (Cladophora sp.) growth in the nearshore zone, the validity of these targets is now uncertain. The occurrence and abundance of Cladophora in the nearshore area depends primarily on the availability of dissolved phosphorus, light, and temperature. The availability of dissolved phosphorus is a potentially useful indicator of nearshore areas susceptible to excessive Cladophora growth and impaired water quality. Regulating agencies are looking for guidance in determining phosphorus loading rates that minimize local exceedance of the lake target concentration. In this study, the lake assimilative capacity was quantified by applying a biophysical model to estimate the area required for mixing and diluting wastewater treatment plant outfall TP loadings to the level of the lake target concentration during the Cladophora growing season. Model results compared well with empirical measurements of particulate and dissolved phosphorus as well as Cladophora biomass and phosphorus content. The model was applied to test scenarios of wastewater treatment plant phosphorus loading in two different years, in order to help establish phosphorus discharge limits for the plant.

Nearshore water clarity, as measured by remotely sensed Kd(490), and stable C and N isotopes of several nearshore fishes differed across the Lake Michigan basin. Values of δ13C of round goby (Neogobius melanstomus), yellow perch (Perca flavescens), and spottail shiner (Notropis hudsonis) were depleted in the southeast where water clarity was low relative to the southwest where water clarity was greater. Bayesian analyses were used to evaluate spatial variation in diet composition and quantify the relationship between water clarity and the proportional importance of pelagic energy in fish diets. Water clarity in nearshore areas is likely related to variable riverine inputs, resuspension, and upwelling processes. While these processes may not directly impact δ13C or δ15N of nearshore fishes, we hypothesize that water clarity differentially affects benthic and pelagic algal production. Lower water clarity in the benthos and subsequently lower benthic productivity may be related to regional diet differences and increased reliance on pelagic energy sources. Mobile fishes such as alewife (Alosa pseudoharengus) may not be in isotopic equilibrium with regional prey sources and depart from spatial patterns observed in other nearshore fishes.

Ecological complexity may improve ecosystem function, stability and adaptability to natural and anthropogenic disturbances. Intraspecific trophic variation can represent a significant component of total community variation and can influence food web structure and function. Thus, understanding how trophic niches are partitioned between intraspecific and interspecific processes could improve our understanding of food web dynamics. We examined gut contents, fatty acids and stable isotope ratios in round goby (Neogobius melanostomus) and yellow perch (Perca flavescens) across six sites in Lake Michigan, USA, to determine patterns in intra‐ and interspecific trophic composition (i.e., mean gut or fatty acid composition) and diversity (i.e., the diversity of gut items or fatty acids). We also examined relationships between fatty acid diversity and gut content characteristics to understand potential mechanisms shaping individual trophic phenotypes. There was significant variation in both trophic composition and diversity among sites, and individual and spatial variation was as important to total trophic variation as species identity. Round goby that consumed dreissenid mussels had more diverse fatty acid profiles than those that consumed other benthic invertebrates, whereas yellow perch fatty acid diversity was not related to gut content composition. Our results confirm that intraspecific variation in resource use can be as important to trophic dynamics as interspecific variation, and that spatial variation in lower level food web processes or habitat may strongly structure local food web dynamics. Individual‐level examination of trophic diversity, in concert with trophic composition, could provide additional information about the resilience, function and adaptability of local food webs.

Residents along the shoreline of Lake Malawi depend on nearshore water for drinking, cooking, and bathing. Despite the importance of clean nearshore waters to public health, we are aware of no published studies of shoreline water quality in the lake. To address this gap, we explore seasonal and temporal trends of the fecal indicator bacteria Escherichia coli (E. coli) in nearshore water and sand. E. coli concentrations in sand ranged from 0 to 17,600 colony forming units (CFU)/100 ml, and in water concentrations ranged from 0 to 21,200 CFU/100 ml. Fifty-three percent of water samples exceeded the U.S. Environmental Protection Agency Recreational Water Quality Criteria of 126 CFU/100 ml, and 90% exceeded the World Health Organization drinking water standard of 0 CFU/100 ml. Distance from shore was the variable most predictive of E. coli concentration, with the level of beach use also playing a significant role. At 15 m from the shore, E. coli concentrations dropped to between 0.3% and 17% of shoreline values. Results suggest that the collection of water at distances >15 m from the beach could substantially decrease exposure to fecal bacteria. Further studies are needed to identify sources of fecal pollution and to determine the utility of E. coli as a predictor of the potential for waterborne disease.

Differences in chemical composition among zooplankton and their food sources can have important consequences for nutrient cycling in lakes. To date, studies of the interaction between plankton stoichiometry and nutrient cycling have focused primarily on temperate lakes. Here we explore the implications of plankton stoichiometry for nutrient cycling in tropical Lake Malawi. Three seston size fractions (picoplankton, nanoplankton and net plankton) displayed seasonal variations in C:P, C:N and N:P ratios. On average, picoplankton displayed lower C:P, C:N, and N:P ratios than nanoplankton and net plankton. Average C:N and C:P ratios for all seston size classes were much higher than Redfield ratios (C:N:P = 106:16:1). The elemental composition of individual species of crustacean zooplankton showed little temporal variation. However, there were large inter-species differences in P and N content, which can be expected to result in differences in N:P recycling ratios. The zooplankton community of Lake Malawi is dominated by copepods (>80% of total zooplankton biomass), which have a relatively low P content and high N:P ratio, resulting in a relatively low N:P recycling ratio. This might be expected to promote N limitation of phytoplankton and dominance of N-fixing cyanobacteria, but in some seasons the effects of zooplankton nutrient cycling appears to be masked by nutrient inputs from rivers, the atmosphere and vertical mixing.

Invasive dreissenid mussels have altered plankton abundance and nutrient cycling in the Great Lakes. In this study, a 1-D hydrodynamic-biogeochemical coupled model is developed to investigate their effects at a mid-depth offshore site in Lake Michigan. Model simulation shows that water surface temperature and vertical thermal structure can be well reproduced. Driven by the simulated vertical mixing, the biological model solves the transport and transformation of nutrients, plankton and detritus in the water column. Mussel grazing and excretion are added at the bottom boundary. The biological model predicts a notable decline of phytoplankton biomass and considerable increase of dissolved phosphorus (DP) in the entire water column at the end of spring. However, the reduction of phytoplankton and the increase of DP are limited to the bottom 20 m in summer as a result of the strong stratification. Model results also show that mussels can maximize particle delivery to the benthos, as the modeled benthic diffusive flux of particulate phosphorus exceeds the passive settling rate by 4.2× on average. Model simulation over a 10-month period indicates that profundal mussels have the potential to significantly change the distribution of energy and nutrients in the water column, even in a deep and stratified environment.

The recent decline in Lake Michigan productivity is often attributed to filter feeding by invasive quagga mussels, but some studies also implicate reductions in lake-wide nutrient concentrations. We use a 3D coupled hydrodynamic - biogeochemical model to evaluate the effect of changing nutrient concentrations and quagga mussel filtering on phytoplankton production and phytoplankton and zooplankton biomass. Sensitivity experiments are used to assess the net effect of each change separately and in unison. Quagga mussels are found to have the greatest impact during periods of isothermal mixing, while nutrients have the greatest impact during thermal stratification. Quagga mussels also act to enhance spatial heterogeneity, particularly between nearshore-offshore regions. This effect produces a reversal in the gradient of nearshore-offshore productivity: from relatively greater nearshore productivity in the pre-quagga lake to relatively lesser nearshore productivity after quaggas. The combined impact of both processes drives substantial reductions in phytoplankton and zooplankton biomass, as well as significant modifications to the seasonality of surface water pCO2, particularly in nearshore regions where mussel grazing continues year-round. These results support growing concern that considerable losses of phytoplankton and zooplankton will yield concurrent losses at higher trophic levels. Comparisons to observed productivity suggest that both quagga mussel filtration and lower lakewide total phosphorus are necessary to accurately simulate recent changes in primary productivity in Lake Michigan.

Understanding trophic interactions of non-native species is a key step in elucidating their ecological role in recently invaded systems. The benthic fish species round goby ( Neogobius melanostomus ) has successfully established in aquatic systems across the world, with abundances increasing dramatically over relatively short time periods. Though this (at times) voracious benthivore can become an increasingly important forage fish for piscivores, relatively little is known about how prey and production pathways that support round gobies vary in space and time. In 2010, we collected round gobies from ten nearshore sites, over three seasons, in Lake Michigan, U.S.A. Due to recent changes in Lake Michigan, the dynamic nearshore region may be crucial for stability of the whole-lake food web. We assessed the role of round gobies in the nearshore Lake Michigan food web using stomach contents, fatty acid profiles, and δ 13 C and δ 15 N stable isotopes. Patterns in all of these measures were highly influenced by site, suggesting that local conditions, such as substrate composition or proximity to riverine inputs, were important in structuring round goby trophic interactions. By contrast, season of sampling and depth of collection had relatively weak associations with observed patterns. Few broad, regional patterns were evident, including a relatively high reliance on benthic production pathways on the western side of Lake Michigan. The observed variety in feeding patterns of round gobies, including potential for exploitation of different production pathways, may contribute to long-term persistence of this aquatic invader in new habitats.

Water movement can influence the distribution of benthos, in part, by increasing food delivery; however, the impact of advective transport and turbulent diffusion on organic matter flux to nearshore benthic communities is not well quantified. In this study, we measured the vertical particulate organic carbon (POC) and particulate phosphorus (PP) flux in nearshore Lake Michigan using two naturally occurring daughter/parent radionuclide pairs (234Th/238U and 90Y/90Sr) and compared these fluxes to coincident benthic chamber estimates of respiration and total phosphorus efflux by quagga mussels on the lakebed. We found that advective onshore transport and vertical convective mixing increased POC and PP flux to the nearshore benthos by a factor of ∼ 15 and ∼ 30 over offshore trap‐derived estimates of flux. From these results, we hypothesize that high benthos population densities are related to an edge effect created when the dominant mechanism of particle delivery transitions from gravitational settling to convection.

Relative to other sites studied in Africa, precipitation on the southwestern shore of Lake Malawi is dilute with regard to most constituents, except Ca2+ and HCO 3 − https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203748978/6f625601-b027-4df5-afef-f7e4441303c5/content/eq131.tif"/> , and has a chemical composition similar to that observed for remote, non-marine locations. Relatively high NH 4 + https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203748978/6f625601-b027-4df5-afef-f7e4441303c5/content/eq132.tif"/> /cation and NO 3 − https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203748978/6f625601-b027-4df5-afef-f7e4441303c5/content/eq133.tif"/> /anion ratios indicate a moderate influence of biomass burning on rain chemistry, while a high mean Ca2+ concentration suggests extensive soil deflation. Rain pH averaged ~6, and was controlled almost entirely by Ca(HCO3)2 concentration. Phosphorus in rain appears to originate as aerosols, but the ultimate source is uncertain. It is suggested that soluble nitrogen and sulfate, which were correlated, are produced by biomass burning. Atmospheric deposition is estimated to account for 33% of new P and 72% of new N input into Lake Malawi (excluding N fixation). If the upward flux of nutrients from the monimolimnion is accounted for, atmospheric deposition accounts for 6% of P input to the surface mixed layer, while the atmospheric N contribution remains 72%. A comparison of nutrient input ratios with nutrient concentration ratios in the surface mixed layer suggests that nitrogen is recycled more efficiently than phosphorus within the surface waters of Lake Malawi.

Invasive dreissenid mussels have successfully established dense population at nearshore and mid-depth regions, and significantly altered the ecosystem and food web dynamics in Lake Michigan. In this work, we used a high-resolution hydrodynamic-biogeochemical model to evaluate how these extensively colonized benthic mussels will affect the dynamics of phosphorus cycling in Lake Michigan. Model results demonstrated the important role of mussels in re-engineering of phosphorus cycling, with their filtering and excretion activities causing increasing lake-wide ratio of dissolved (DP) and particulate (PP) phosphorus. The diffusive flux at the bottom of mid-depth site was significantly enhanced compared to the sedimentation due to the benthic mussels, with an average effective flux 12.1 times greater than the passive setting, which was consistent with the field observations. Nearshore-offshore phosphorus transport displayed substantial spatial and temporal variabilities, and the nearshore was identified as an overall source/sink of PP/DP to the offshore region. The simulation results with mussels revealed that the mussel could effectively reduce the PP transport to the offshore region, potentially risking the pelagic fish abundance. This study provides key quantitative estimates for the impact of benthic mussels on vertical PP delivery and lake-wide nearshore offshore phosphorus transport, and could inform potential ecosystem management strategies of Lake Michigan and other dreissenid mussel invaded lakes.

In aquatic systems, food web linkages are often assessed using diet contents, stable isotope ratios, and, increasingly, fatty acid composition of organisms. Some correlations between different trophic metrics are assumed to be well-supported; for example, particular stable isotope ratios and fatty acids seem to reflect reliance on benthic or pelagic energy pathways. However, understanding whether the assumed correlations between different trophic metrics are coherent and consistent across species represents a key step toward their effective use in food web studies. To assess links among trophic markers, we compared relationships between major diet components, fatty acids, and stable isotope ratios in three fishes: yellow perch (Perca flavescens), round goby (Neogobius melanostomus), and spottail shiner (Notropis hudsonius) collected from nearshore Lake Michigan. Yellow perch and spottail shiner are native in this system, while round goby are a relatively recent invader. We found some evidence for agreement between different trophic metrics, especially between diet components, n-3:n-6 fatty acid ratios, and stable isotope ratios (δ13C and δ15N). However, we also observed significant variation in observed relationships among markers and species, potentially due to taxonomic variation in the specific diet items consumed (e.g., chydorid microcrustaceans and Dreissena mussels) and species-specific biochemical processes. In many of these latter cases, the invasive species differed from the native species. Understanding the effects of taxonomic variation on prey and predator signatures could significantly improve the usefulness of fatty acids in food web studies, whereas diet contents and stable isotopes appear to be reliable indicators of trophic niche in aquatic food webs.

Recent spread of invasive mussels in Lake Michigan has altered primary productivity in the nearshore zone, resulting in proliferation of filamentous benthic green algae (Cladophora glomerata). In areas of dense Cladophora and quagga mussel (Dreissena bugensis) assemblages, as well as in regions where sloughed Cladophora accumulates, methylmercury (MeHg) production is enhanced. A shoreline transect from a river mouth through waters overlying Cladophora/quagga-rich zones showed that aqueous MeHg concentrations increased, despite river dilution. Cladophora, as primary producers, ranged from 0.6 to 7.5 ng g(-1) MeHg [4-47% of total mercury (Hg) as MeHg], and were higher than MeHg concentrations in offshore-collected seston. Concentrations of MeHg in decaying Cladophora accumulated onshore ranged from 2.6 to 18.0 ng g(-1) MeHg (18-41% as MeHg) and from 0.1 to 3.0 ng g(-1) MeHg (2-21% as MeHg) in deposits of recently sloughed and accumulated Cladophora in a nearshore topographical depression. Relative to offshore open waters, interstitial waters within decaying Cladophora from onshore and nearshore deposits were elevated in MeHg concentration, 1000- and 10-fold, respectively. Percent Hg as MeHg was also elevated (65-75% and 9-19%, respectively for onshore interstitial water and nearshore interstitial water, compared to 0.2-3.3% as MeHg for open water). Quagga mussels collected within growing Cladophora beds in the nearshore zone were significantly higher in MeHg than offshore counterparts. Our combined results suggest that recent changes in nearshore primary production contributes to MeHg production and bioaccumulation in Lake Michigan.

Ocean acidification will likely result in a drop of 0.3–0.4 pH units in the surface ocean by 2100, assuming anthropogenic CO2 emissions continue at the current rate. Impacts of increasing atmospheric pCO2 on pH in freshwater systems have scarcely been addressed. In this study, the Laurentian Great Lakes are used as a case study for the potential for CO2-induced acidification in freshwater systems as well as for assessment of the ability of current water quality monitoring to detect pH trends. If increasing atmospheric pCO2 is the only forcing, pH will decline in the Laurentian Great Lakes at the same rate and magnitude as the surface ocean through 2100. High-resolution numerical models and one high-resolution time series of data illustrate that the pH of the Great Lakes has significant spatio-temporal variability. Because of this variability, data from existing monitoring systems are insufficient to accurately resolve annual mean trends. Significant measurement uncertainty also impedes the ability to assess trends. To elucidate the effects of increasing atmospheric CO2 in the Great Lakes requires pH monitoring by collecting more accurate measurements with greater spatial and temporal coverage.

Air quality forecast models typically predict large summertime ozone abundances over water relative to land in the Great Lakes region. While each state bordering Lake Michigan has dedicated monitoring systems, offshore measurements have been sparse, mainly executed through specific short-term campaigns. This study examines ozone abundances over Lake Michigan as measured on the Lake Express ferry, by shoreline differential optical absorption spectroscopy (DOAS) observations in southeastern Wisconsin and as predicted by the Community Multiscale Air Quality (CMAQ) model. From 2008 to 2009 measurements of O3, SO2, NO2 and formaldehyde were made in the summertime by DOAS at a shoreline site in Kenosha, WI. From 2008 to 2010 measurements of ambient ozone were conducted on the Lake Express, a high-speed ferry that travels between Milwaukee, WI, and Muskegon, MI, up to six times daily from spring to fall. Ferry ozone observations over Lake Michigan were an average of 3.8 ppb higher than those measured at shoreline in Kenosha, with little dependence on position of the ferry or temperature and with greatest differences during evening and night. Concurrent 1–48 h forecasts from the CMAQ model in the upper Midwestern region surrounding Lake Michigan were compared to ferry ozone measurements, shoreline DOAS measurements and Environmental Protection Agency (EPA) station measurements. The bias of the model O3 forecast was computed and evaluated with respect to ferry-based measurements. Trends in the bias with respect to location and time of day were explored showing non-uniformity in model bias over the lake. Model ozone bias was consistently high over the lake in comparison to land-based measurements, with highest biases for 25–48 h after initialization.

The lakewide seasonal carbon cycle of Lake Michigan is poorly quantified and lacks the mechanistic links necessary to determine impacts upon it from eutrophication, invasive species, and climate change. A first step toward a full appreciation of Lake Michigan's carbon cycle is to quantify the dominant mechanisms of its internal carbon cycle. To achieve this, we use the MIT general circulation model configured to the bathymetry of Lake Michigan and coupled to an ecosystem model to simulate the seasonal cycle of productivity, temperature, circulation, and the partial pressure of CO2 in water (pCO2). This biogeochemistry is designed to be appropriate for the prequagga mussel state of the lake. The primary mechanism behind the seasonal cycle of primary productivity is lake physics. The offshore spring phytoplankton bloom begins following a reduction in deep vertical mixing and ends with the depletion of nutrients via thermal stratification. The exception is the western shoreline, where summer winds drive coastal upwelling, providing hypolimnetic nutrients and generating significant productivity. Surface pCO2 is controlled by the net effect from temperature on solubility, and is modulated by biological uptake of dissolved inorganic carbon (DIC) and isothermal mixing of DIC-rich water in winter. Temperature tends to have the greatest seasonal impact in nearshore regions, while local DIC has the greatest impact in offshore regions. Lakewide, the model suggests that carbon is absorbed from the atmosphere during the spring bloom and released to the atmosphere during winter mixing and when summer surface temperatures are at their maximum.

Recent studies suggested that under low to moderate wind conditions without bubble entraining wave breaking, the air-water gas transfer velocity k+ can be mechanistically parameterized by the near-surface turbulence, following the small eddy model (SEM). Field measurements have supported this model in a variety of environmental forcing systems. Alternatively, surface divergence model (SDM) has also been shown to predict the gas transfer velocity across the air-water interface in laboratory settings. However, the empirically determined model coefficients (α in SEM and c1 in SDM) scattered over a wide range. Here we present the first field measurement of the near-surface turbulence with a novel floating PIV system on Lake Michigan, which allows us to evaluate the SEM and SDM in situ in the natural environment. k+ was derived from the CO2 flux that was measured simultaneously with a floating gas chamber. Measured results indicate that α and c1 are not universal constants. Regression analysis showed that α∼log(e) while the near-surface turbulence dissipation rate e is approximately greater than 10−6 m2 s−3 according to data measured for this study as well as from other published results measured in similar environments or in laboratory settings. It also showed that α scales linearly with the turbulent Reynolds number. Similarly, coefficient c1 in the SDM was found to linearly scale with the Reynolds number. These findings suggest that larger eddies are also important parameters, and the dissipation rate in the SEM or the surface divergence β′ in the SDM alone may not be adequate to determine k+ completely.

The objectives of this study were to describe the current trophic structure of Lake Michigan's upper food web using stable C and N isotopes. We describe ontogenetic diet patterns for common Lake Michigan species as revealed by stable isotope analysis and compare these patterns during a major transitional period (2002–2003 to 2010–2012) for most trophic levels. Proportional prey assimilation for various fishes was described using a Bayesian mixing model approach, and ontogenetic patterns were described using an analysis of covariance. δ 13 C results reveal distinct separation between the offshore and nearshore food webs. Within the offshore food web, invertebrates and prey fishes exhibited a large range in mean δ 15 N values which resulted in an apparent overlap between these trophic levels. This overlap may be due to spatiotemporal variability in δ 15 N at the base of the food web. Within the nearshore food web, more distinct delineation of trophic levels was apparent in δ 15 N values. Non-dreissenid nearshore benthic invertebrates were very isotopically similar and were the primary prey for non-piscivorous nearshore fishes. Ontogenetic shifts in diet were observed for alewife, bloater, deepwater sculpin, lake trout, round goby and yellow perch. Furthermore ontogenetic diet patterns changed significantly during the study period for alewife and bloater. Our findings provide a trophic structure baseline that can inform management action and be used to evaluate further ecological change as community structure continues to evolve.

Within the last few decades Lake Michigan's plankton community has undergone substantial changes. Oligotrophication of the pelagic zone has led to a decrease in zooplankton biomass, while species composition has shifted, with copepod species becoming more dominant. Although these observations have been relatively well documented, their implications for the food web have not been well addressed. To define feeding relationships within the zooplankton community, we measured the stable carbon and nitrogen isotope composition and biomass of major zooplankton species, three seston size classes, and seston collected from three depths in southeastern Lake Michigan. Nitrogen stable isotope ratios suggest a complex plankton food web with zooplankton appearing to occupy at least three distinct trophic positions. The large calanoid copepod Limnocalanus macrurus occupied a tertiary consumer trophic position, more than one trophic position above all other zooplankton species sampled. Carbon stable isotopes revealed significant differences among zooplankton species. Though the mechanisms responsible for these differences are not clear, these results suggest that there is significant partitioning of resources at the base of the food web.

In the past 10 to 15 years, excessive growth of Cladophora and other attached algae in the nearshore regions of Lake Michigan has re-emerged as an important resource management issue. This paper considers the question, “What information is needed to predict the response of Cladophora production in Lake Michigan to management variables, such as nutrient loading, and to additional environmental variables that are outside of management control?” Focusing on Lake Michigan, while drawing on the broader literature, we review the current state of information regarding 1) models of Cladophora growth, 2) models that simulate the physical environment, 3) models that simulate nearshore and whole-lake nutrient dynamics, with a specific focus on the role of dreissenid mussels, and 4) monitoring of Cladophora abundance. We conclude that while substantial progress has been made, considerable additional research is required before reliable forecasts of Cladophora response to nutrient loads and other environmental variables are possible. By providing a detailed outline of this complex, multidisciplinary problem, we hope that this paper will aid in coordinating collaborative research efforts toward the development of useful predictive models.

Respiration rates of the quagga mussel (Dreissena rostriformis bugensis) were determined for the shallow and profunda morph phenotypes from in situ and laboratory experiments under a range of temperature (4–20°C), shell size, and food and oxygen availability conditions. Temperature-normalized oxygen consumption was significantly lower for the hypolimnetic profunda phenotype than for the shallow type. Mass-normalized respiration rates were inversely related to mussel size. Mussels adjusted their oxygen consumption in response to food enhancement and deprivation, lowering their respiration to a basal metabolic rate 18 hours after food deprivation. In response to decreasing ambient oxygen, quagga mussels exhibited first-order reaction kinetics, with mass-normalized respiration rate at a dissolved oxygen concentration of 0.002 mol L− 1 being 1% of that at saturation. Using published data on quagga mussel energy budgets and respiratory quotients, oxygen consumption rates were converted to organic carbon consumption rates. Using these values, along with data on mussel density and size frequency distribution, it is estimated that quagga mussels consume 54% of annual phytoplankton production, from 1.4–4.1 times the offshore annual settled organic carbon in the southern basin of Lake Michigan, and from 2.4–5.5% of offshore areal particulate carbon in the water column. Dreissenids appear to exert significant direct influences on benthic oxygen dynamics, and consume enough organic carbon to have a significant effect on energy flow in the Lake Michigan ecosystem.

On-going ecosystem alterations within Lake Michigan have drastically transformed species interactions and food-web assembly. Description of trophic interactions across broad spatial regions is likely necessary to fully appreciate the structure of this emerging food web. Spottail shiners, Notropis hudsonius, are numerically abundant in the nearshore zone of Lake Michigan, but their trophic interactions are under-described. To that end, we described fatty acid profiles of spottail shiner through spring, summer, and fall across western and eastern shorelines of Lake Michigan's southern basin. Fatty acids, used as dietary tracers, suggested a shift from benthic-based diets in spring to more pelagic-associated diets in summer and a reversal in fall. When time lag of fatty acid accumulation is accounted for in interpretations, diets likely follow spring plankton and summer/fall benthic invertebrate maxima. Fatty acid profiles also indicated differences in diet composition based on geographic location, with benthic tracers more prevalent among spottail shiner inhabiting the western shoreline. These interpretations were generally supported by stomach content data, with high Chironomidae consumption in spottail shiners from western waters. The presence of Coleoptera, Hymenoptera, and Odonata in spottail shiner stomach contents throughout the lake highlights its reliance on nearshore and potentially inshore areas. This study offers one of the most spatially broad depiction of spottail shiner foraging habits in Lake Michigan.

The effects of dreissenid mussels on plankton abundance and nutrient cycling in shallow, productive waters of the Great Lakes have been well-documented, but the effects of their more recent expansion into offshore regions have received much less attention. Understanding quagga mussel impact on Lake Michigan's phosphorus (P) fluxes is critical in assessing long-term implications for nutrient cycling and energy flow. In this study, P excretion and egestion rates were determined for mussels in the hypolimnion of Lake Michigan. Constant low temperatures and limited food supply contributed to a lower basal P excretion rate in profunda quagga mussels compared to the shallow phenotype. The P excretion:egestion ratio was approximately 3:2, highlighting the need to consider both of these pathways when assessing the effect of these filter feeders on nutrient dynamics. Total dissolved P (TDP) excretion rates ranged from 0.0002 to 0.0124 μmol mgDW − 1 d − 1 , soluble reactive P (SRP) excretion rates ranged from 0.0002 to 0.0061 μmol mgDW − 1 d − 1 , and particulate P (PP) egestion rates (feces + pseudofeces) ranged from 0.0007 to 0.0269 μmol mgDW − 1 d − 1 . The ability of profunda mussels to alter P cycling dynamics is reflected in an increased hypolimnetic dissolved:particulate P ratio and the disappearance of the benthic nepheloid layer. On an areal basis, mussel P recycling rates are up to 11 times greater than P settling rates as determined by sediment traps, suggesting that mussel grazing has resulted in an increased delivery rate of P to the deep benthos and a shorter P residence time in the water column.

Describing food web structure through either direct or indirect diet analysis is often a fundamental step in elucidating ecosystem dynamics and developing resource management goals. The present study examines spatial trophic connections in an opportunistic forager, juvenile yellow perch ( Perca flavescens ), through the concomitant use of stomach content, fatty acid profiles, and stable isotope ratio methods. During September 2010, yellow perch were collected at nine coastal locations representative of Lake Michigan's habitat heterogeneity. The three diet assessment methods revealed differential levels of spatial diet heterogeneity. In general, yellow perch relied on pelagic prey more along the eastern shoreline, and over rocky substrates within each shoreline grouping. Conversely, high benthivory was noted in yellow perch from sandy substrates and western locations. Intra-population spatial diet dissimilarity may be common yet over looked among other species within large systems. We further advocate the concurrent examination of chemical ecological tracers (e.g., stable isotopes and fatty acid profiles) and stomach contents to investigate diet patterns of predators.

An in situ free-floating underwater miniature particle image velocimetry (UWMPIV) system is developed and applied to measure the structure of turbulence in the aqueous side of the wind wave surface boundary layer. The UWMPIV system provides a direct way to measure the aqueous side turbulence dissipation rate and vortex structures immediately below the air–water interface, which are important parameters that determine the gas exchange rate across the air–water interface subjected to a low-to-moderate wind shear. The impact of platform motion on the measurement of small-scale turbulence is discussed and found to be insignificant. A series of field experiments under a near “zero-fetch” wind wave condition and one open water experiment under a low wind condition were conducted on Lake Michigan to demonstrate the capabilities of the free-floating particle image velocimetry (PIV) system. The dissipation rate estimated with a “direct method” and with a “spectra fitting” method are compared. Vertical profiles of the turbulence dissipation rate suggest a power-law dependency with depth below the water surface. Surface shear velocities estimated through the aqueous side Reynolds stress distribution agreed well with wind stresses estimated by the classic drag law for zero-fetch wind wave conditions, where the primary source of turbulence was wind shear. For the open water experiment under a very low wind condition, a high dissipation rate was observed near the water surface, suggesting a high turbulence production rate by surface waves, and the profile of dissipation rate showed a slower decay rate with depth in the presence of waves.

A battery-powered in situ Underwater Miniature PIV (UWMPIV) has been developed and deployed for field studies. Instead of generating high-energy laser pulses as in a conventional PIV system, the UWMPIV employs a low-power Continuous Wave (CW) laser (class IIIb) and an oscillating mirror (galvanometer) to generate laser sheets. In a previous version of the UWMPIV, the time between exposures of a pair of particle images, \(\Updelta t\), could not be reduced without loss of illumination strength. This limitation makes it unsuitable for high-speed flows. In this paper, we present a technique to solve this problem by adopting two CW lasers with different wavelength and two CCD cameras in a second-generation UWMPIV system. Several issues including optical alignment, non-uniform distribution of \(\Updelta t\) due to the varying speed of the scanning beam and local flow velocities are discussed. The timing issue is solved through a simple calibration procedure that involves the reconstruction of maps of laser beam arrival time. Comparison of the performance between the new method and a conventional PIV system is presented. Measurements were performed in a laboratory open-channel flume. Excellent agreement was found between the new method and the standard PIV measurement in terms of the extracted vertical profiles of mean velocity, RMS fluctuation, Reynolds stress and dissipation rate of turbulent kinetic energy.

Nuisance growth of the attached, green alga Cladophora was considered to have been abated by phosphorus management programs mandated under the Great Lakes Water Quality Agreement. The apparent resurgence of nuisance growth in Lakes Erie, Michigan and Ontario has been linked conceptually to ecosystem alterations engineered by invasive dreissenid mussels (Dreissena polymorpha and Dreissena bugensis). Here, we apply contemporary modeling tools and historical water quality data sets in quantifying the impact of long-term changes in phosphorus loading and dreissenid-mediated changes in water clarity on the distribution and production of Cladophora. It is concluded that reductions in phosphorus loading in the predreissenid period achieved the desired effect, as model simulations were consistent with the biomass declines reported from the early 1970s to the early 1980s. These declines were, however, largely offset by dreissenid-driven changes in water clarity that extended the depth of colonization by C...

A recent review of the Great Lakes Water Quality Agreement has concluded that while controls on phosphorus inputs to Lake Michigan achieved the desired effect in offshore waters, the nearshore region continues to suffer from elevated phosphorus levels. Failure to achieve trophic state goals in the nearshore is manifested in nuisance growth of Cladophora and attendant impacts on property owners, utilities, and the public health and welfare. This study focuses on a site in Lake Michigan near Milwaukee, Wisconsin, where nuisance growth of Cladophora and associated beach fouling occur regularly. A mechanistic model simulating Cladophora growth, suitable for guiding nutrient management in the Great Lakes nearshore, is presented. The model represents an update of the Canale and Auer framework, reflecting current understandings of Cladophora ecology and offering a user-friendly interface making the software more widely available to decision makers. This Great Lakes Cladophora Model (GLCM) is first validated for the Auer/Canale data set collected in 1979 at a site on Lake Huron and then for a data set developed in 2006 for a site on Lake Michigan. Model performance under the strikingly different forcing conditions (depth, light, phosphorus levels) characteristic of these two sites affirms the widespread applicability of the tool. The GLCM is then extended to examine the impacts of ecosystem perturbation (dreissenid colonization) on Cladophora growth and to future approaches to monitoring and management.

A low-cost self-contained underwater particle image velocimetry (UWPIV) system has been developed to measure small-scale turbulent flow structures in situ. The UWPIV employs a compact continuous-wave laser and an optical scanner to deliver a light sheet that illuminates naturally occurring particles. Particle images are taken by a CCD camera along with an ultra-compact PC. The nontethered and compact design can be fit in two small underwater housings with all components powered by batteries: an ideal design for a variety of in situ deployments. The system has been field-tested in the coastal zone of Lake Michigan. Turbulent flow structures of the wave-current bottom boundary layer are measured right above the nearshore lakebed, which is densely covered by quagga mussels (Dreissena bugensis). Vertical profiles of mean velocity, Reynolds stresses, dissipation rate of the turbulent kinetic energy, turbulent viscosity, plankton particle concentration, and the turbulent flux of particles are presented and discussed.

A method is described for achieving increased confidence in the selective determination of PCBs using capillary gas-liquid chromatography with tandem mass spectrometry (GC-MS/MS). It is well known that quantitation of PCBs by MS is susceptible to a false positive interference that arises from the co-elution of a higher PCB homolog with a lower PCB homolog (i.e., M-Cl+ from a higher homolog is M+ for a lower homolog). Because the elution order of the PCB congeners is not exactly proportional to increasing Cl content, frequent switching from MS/MS windows for specific homologs must take place. This approach can yield significant errors when matrix-induced retention time shifts occur. We therefore explored an alternative approach that involves repetitive analyses of a single extract. We developed and optimized a method that requires three injections, with homolog classes sequentially monitored as: 1-4-7-10, 2-5-8, and 3-6-9, respectively. The sequential design of the method entails the use of separate, broad...

In southern Lake Malawi, seasonal pelagic chlorophyll means were 1.0 ± 0.3 μg L−1 in the deep mixing season (DMS) (May–August), 0.8 ± 0.3 μg L−1 in the dry stratified season (DSS) (September to November) and 0.7 ± 0.3 μg L−1 in the wet stratified season (WSS) (December to April). Despite the low variability in chlorophyll, there was a wide range in chlorophyll specific photosynthetic activity. The photosynthetic parameters, Pbm (the light saturated rate) and αb (the light limited slope), varied significantly among seasons and were highly positively correlated, with lowest values in the DSS and highest values in WSS. During deep mixing, Pbm did not covary with αb; and the light saturation index, Ek (=Pbm/αb), varied in response to changes in αb rather than in Pbm. Phytoplankton appeared to be nutrient deficient at all times but less deficient during deep vertical mixing in the DMS. Average daily rates of integrated phytoplankton primary productivity were lowest in the DSS (337 mg C m−2 d−1) and hi...

All African lakes, large and small, have great significance to their riparian populations and the nations in which they lay for providing protein from fisheries, water for agricultural, domestic and drinking use, transportation for commerce, and recreational use. The three largest lakes in Africa, Victoria, Tanganyika, and Malawi/Nyasa (hereafter referred to as Malawi) offer similar benefits, and their very size certainly amplifies the numbers of people and the number of countries that depend upon them for their beneficial uses. The Victoria catchment is shared by approximately 30 million people living in five countries, and it exports critical quantities of water downstream to the Nile and fishes to a global market. The other two lakes are shared by three (Malawi) and five (Tanganyika) countries and are headwater lakes to the Zambesi and the Congo Rivers whose outflows sustain downstream benefits to many other users outside their catchments. These three great lakes in addition have evolved remarkable endemic biodiversity with nearly 10% of all the planet’s freshwater fish species occurring within them making them globally significant gene banks. The multi-national distribution of benefits, and the potential losses of such benefits if the lakes are degraded, places these lakes in a class of their own with regard to management challenges. Lakes everywhere come under stress as human populations in their catchments increase, demands for natural resource extraction grow and industrialization impacts spread through economic development, and this is equally true on these great lakes. In recognition of the international scope of the management challenge posed by these great lakes and the global benefits they provide, the Global Environmental Facility (GEF) and its national and international funding partners have funded extensive studies and analysis of these lake ecosystems through the mid to late 1990’s extending up to the present. The achievements and shortcomings of those GEF projects are reviewed in case studies presented at this workshop. In aggregate, those studies and other studies around the world have revealed that some of the challenges to successful management of these great lakes extend beyond their catchments and that regional or even global action will be necessary to maintain or restore many of the lakes’ beneficial uses. Smaller lakes are likely also being impacted by these regional and global scale threats although other more immediate impacts may be of greater concern at present. The thesis of this paper is that there is a global dimension to lake management in Africa, and elsewhere, that will require concerted action by, not only individual riparian states, but also regional, continental and global communities. Current threats arise from global climate change, land degradation and contaminants, and they share the common feature that the atmosphere is the vector that spreads their impact over large areas and to many lakes. 2. Hydrologic sensitivities

Haloacetic acids (HAAs) are a family of compounds whose environmental concentrations have been extensively studied, primarily in Europe. Depending on the compound, their sources are believed to be both natural and anthropogenic. To better understand possible sources and contribute to the knowledge of the global distribution of these compounds, especially between the Northern and Southern Hemispheres, samples of precipitation, soils, and conifer needles were collected from Canada, Malawi, Chile, and the U.K. Precipitation samples exhibited highest HAA concentrations in collections from Canada, and lowest in those from Malawi. Malawi samples contained measurable levels of monobromoacetic acid (MBA) (56 ng/ L) unlike those from most other locations (< 9 ng/L). Soil HAA concentration levels were highest in the U.K. (e.g., 7.3 ng/g average TCA) and lowest in Malawi (0.8 ng/g average TCA), with Chile having higher levels (4.8 ng/g average TCA) than Canada (3 ng/g average TCA). Malawi soils contained small amounts of MBA (2 ng/g), in common with the two most southern of the 11 Chilean sites. Analysis of soil cores (10-cm depth sliced at 1 cm) from sites in Malawi and Chile showed that trichloroacetic acid (TCA) generally declined with depth while mono- and dichloroacetic acid (MCA and DCA) showed no trend. MCA, DCA, and TCA concentrations in archived U.K. soil samples increased by factors of 2, 4, and 5-fold over 75 years while TFA showed no consistent trend. Monochloroacetic acid (MCA) was detected in pine needles collected from Malawi. U.K. needle samples had the highest concentrations of all chloroacetic acids (CAAs): MCA, 2-18 ng/g; dichloroacetic acid (DCA), 2-38 ng/g; and trichloroacetic acid (TCA), 28-190 ng/g. Conifer needles from Canada and Chile contained CAAs at levels ranging from < 2 to 16 ng/g wet wt. Trifluoroacetic acid concentrations generally declined with increasing elevation in the samples from the Rocky Mountains in western Canada. The results indicate that concentrations of HAAs are greatest in the industrialized Northern Hemisphere but there are significant amounts of these compounds in the less industrialized Southern Hemisphere.

Temporal diet patterns were studied for nine species of haplochromine cichlids in Lake Malawi, East Africa, using a combination of stable isotope and stomach content analyses. The results illustrate the complementarity of the two approaches in elucidating dietary differences. Evidence was found that benthic algal production contributed to the energy requirements of offshore fishes that occupy depths between 10 and 30 m. The importance of the lake fly Chaoborus edulis as a food source for demersal fishes was confirmed, supporting the recent hypothesis that deep demersal fish production is principally sustained through the pelagic food chain rather than from benthic detrital sources. Isotopic differences were observed among species with apparently similar diets, feeding behavior, and depth preferences, suggesting that important resource partitioning exists among Lake Malawi benthic haplochromine cichlids.