Detrended Correspondence Analysis of Phytoplankton Abundance and Distribution in Sandusky Bay and Lake Erie

The purpose of this study was to determine if phytoplankton communities in Sandusky Bay were distinct from those of Lake Erie. Samples were taken from 11 sites along a 50 km transect extending from the lower reaches of the Sandusky River, through Sandusky Bay and into the western basin of Lake Erie to identify factors correlated with identifiable patterns in distribution and abundance of summer phytoplankton. Detrended correspondence analysis (DCA), an ordination technique used to describe patterns in complex data sets, arranged the sample sites along an ordination axis that explained 76% of the variation in the phytoplankton abundance data matrix, and produced the following sequence of ordination axis scores: Sandusky Bay→ Lake Erie→ Sandusky River. DCA axis I scores strongly correlated with total phosphorus, soluble reactive phosphorus, algal phosphatase activity, dissolved oxygen, conductivity, turbidity, and alkalinity, but not chloride concentration, suggesting that phytoplankton abundance and distribution were related to phosphorus availability and not simply due to the passive movement of water along the transect. Bacterial abundance correlated with DCA axis I, suggesting that phytoplankton-bacterial interactions may be important in understanding distributional patterns of Sandusky Bay phytoplankton.     

Phytoplankton Productivity in Saginaw Bay,Lake Huron: Effects of Zebra Mussel (Dreissena polymorpha) Colonization

Phytoplankton photosynthesis-irradiance parameters, chlorophyll concentrations, underwater extinction coefficients (kPAR), and surface irradiance were determined at 8–10 sites on 27 occasions in Saginaw Bay from spring 1990 through fall 1993 corresponding to a period before and after the establishment of large zebra mussel populations (began in summer 1991). Similar measurements, with the exception of the photosynthetic parameter, α, had also been made in 1974/75 at eight sites on nine occasions. In inner Saginaw Bay where zebra mussels were primarily found, chlorophyll and kPAR values decreased, while the photosynthetic parameters, P_max and α, increased after zebra mussel colonization. At sites in the outer bay where no zebra mussels were found, chlorophyll and kPAR values did not change after zebra mussel colonization, whereas photosynthetic parameters increased. Decreases in chlorophyll and kPAR in the inner bay were related to the zebra mussel, but increases in photosynthetic parameters in both the inner and outer bay were not. Areal-integrated and volumetric phytoplankton productivity decreased by 38% and 37%, respectively, in inner Saginaw Bay after the establishment of zebra mussels; phytoplankton productivity at outer bay control sites was similar during the same period. Decreased phytoplankton productivity in the inner bay was attributable to the large decrease in chlorophyll as increases in underwater irradiance (increased kPAR) and photo synthetic parameters could not compensate for the chlorophyll effect. Increase in underwater irradiance produced a significant increase in light to the benthic region and contributed to increased benthic primary productivity; ratio of photic zone to station depth increased in inner Saginaw Bay, from 0.6–0.8 before the zebra mussel colonization (1974–1990) to 1.1–1.3 after colonization (1992–1993). Overall, primary productivity in the inner bay did not exhibit a notable change after zebra mussel colonization as decreases in phytoplankton productivity were accompanied by increases in benthic primary productivity. Thus, zebra mussels altered inner Saginaw Bay from a pelagic-dominated system to a benthic/pelagic system which will have long-term effects on food web structure and productivity at higher trophic levels.     

Analysis of the Impacts of the Zebra Mussel, Dreissena polymorpha, on Nutrients,Water Clarity,and the Chlorophyll-Phosphorus Relationship in Lower Green Bay

Zebra mussels, Dreissena polymorpha, invaded Green Bay, Lake Michigan in the early 1990s. In 1986, prior to zebra mussel invasion, the Green Bay Metropolitan Sewerage District initiated a long-term water quality monitoring program involving 12 stations in three distinct zones along a trophic gradient in lower Green Bay. We analyzed this data set pre and post invasion using various regression models to determine the impacts of the zebra mussel on water clarity, nutrient concentrations, and the relationship between chlorophyll and phosphorus in this system. Following zebra mussel invasion, Secchi depths did not change in all three zones. Chlorophyll a concentrations decreased post zebra mussels in all zones. These differences were attributed to the filter feeding abilities of zebra mussels. Lower Green Bay exhibits a strong trophic gradient and zebra mussel impacts on the chlorophyll-phosphorus relationship differed between the three zones. We saw no changes in the chlorophyll-phosphorus relationship in zone 1, zone 2 appeared to be a transition zone with slight changes in the chlorophyll-phosphorus relationship, and in zone 3 there was evidence of an altered chlorophyll-phosphorus relationship post zebra mussels. These results indicate that the impact of zebra mussels on water quality parameters and on chlorophyll-phosphorus dynamics may differ depending on initial trophic status and on zebra mussel densities.     

Benthos of Green Bay,Lake Michigan revisited after 40 years: A temporal update and assessment of environmental associations

Benthos of southern Green Bay, Lake Michigan have not been comprehensively examined since 1978. Since then, invasive species appeared, urbanization intensified, and restoration efforts were implemented, which likely altered the benthic macroinvertebrate community. Further, current benthos are subjected to dynamic factors including eutrophication, sedimentation, and periodic hypoxia. Understanding community responses to these anthropogenic stressors and natural habitat gradients is imperative to preserving biological integrity within Green Bay. Therefore, the objectives of this project were to describe the current macroinvertebrate community, examine changes since 1978, and assess the roles of productivity, substrate type, water depth, and hypoxia in structuring macroinvertebrate communities. Benthos were sampled at 197 stations, including 97 also sampled in 1978 by Markert (1982) and 100 that were added to increase spatial resolution. We collected 93 macroinvertebrate taxa in southern Green Bay with the community dominated by Chironomus and immature tubificid worms. Nonmetric multidimensional scaling (NMDS) ordination distinguished present and historical communities. Although oligochaete worms and chironomids remained dominant over time, Chironomus abundance increased and characterized the present community, whereas benthos were historically more diverse. The magnitude of temporal change varied spatially among zones of Green Bay, with larger differences concentrated in the Middle Bay and the Inner Bay remaining comparable to 1978. Present-day assemblages were most associated with the trophic gradient driven by Green Bay’s southernmost tributary, the Fox River, but also differed with substrate type and had similar structures in areas subjected to frequent hypoxia. Routine monitoring should continue to track changes while accounting for spatial effects.     

Food web structure and mercury trophodynamics in two contrasting embayments in northern Lake Victoria

Nearshore regions of lakes are important sources of fish, and can be strongly influenced by anthropogenic inputs of nutrients as well as contaminants. This study characterizes food web structure, mercury concentrations, and biomagnification of mercury in two embayments in northern Lake Victoria that differ in their connectivity to the open lake, trophic status, and the influence of local anthropogenic pollution. Murchison Bay is a semi-confined hypereutrophic bay in a densely populated region, while Napoleon Gulf is mesotrophic and is well flushed with water from the open lake. Based on stable carbon and nitrogen isotope analysis, food web structure was similar at both sites, with short food chains and conspecific fish occupying similar trophic positions. However, there were strong differences in net phytoplankton δ¹⁵N and δ¹³C between sites; net phytoplankton δ¹³C was largely related to trophic status, while δ¹⁵N values appeared to be influenced by inputs of human waste and the prevalence of biological nitrogen fixation. Total mercury (THg) concentrations in fish were consistently below 200 ng/g wet weight, and despite elevated THg concentrations in water in Murchison Bay, THg concentrations in net phytoplankton and fish from both embayments did not differ, highlighting that THg in water is not always a good predictor of concentrations in fish. We also observed that biomagnification of mercury was occurring at a lower rate in Murchison Bay than in Napoleon Gulf, and we propose that the hypereutrophic state of Murchison Bay may be acting to reduce potential Hg exposure for higher trophic level fish.     

Spatial analysis of toxic or otherwise bioactive cyanobacterial peptides in Green Bay,Lake Michigan

Cyanobacterial harmful algal blooms (cyanoHABs) are a growing problem in freshwater systems worldwide. CyanoHABs are well documented in Green Bay, Lake Michigan but little is known about cyanoHAB toxicity. This study characterized the diversity and spatial distribution of toxic or otherwise bioactive cyanobacterial peptides (TBPs) in Green Bay. Samples were collected in 2014 and 2015 during three cruises at sites spanning the mouth of the Fox River north to Chambers Island. Nineteen TBPs were analyzed including 11?microcystin (MC) variants, nodularin, three anabaenopeptins, three cyanopeptolins and microginin-690. Of the 19 TBPs, 12 were detected in at least one sample, and 94% of samples had detectable TBPs. The most prevalent TBPs were MCRR and MCLR, present in 94% and 65% of samples. The mean concentration of all TBPs was highest in the Fox River and lower bay, however, the maximum concentration of all TBPs occurred in the same sample north of the lower bay. MCs were positively correlated with chlorophyll and negatively correlated with distance to the Fox River in all cruises along a well-established south-to-north trophic gradient in Green Bay. The mean concentration of MC in the lower bay across all cruises was 3.0?±?2.3?μg/L. Cyanopeptolins and anabaenopeptins did not trend with the south-north trophic gradient or varied by cruise suggesting their occurrence is driven by different environmental factors. Results from this study provide evidence that trends in TBP concentration differ by congener type over a trophic gradient.     

Proposals for inhibiting abundant phytoplankton growth at the head of a river reservoir

In a river reservoir, the Ishitegawa Dam Reservoir in Japan, sites of abundant phytoplankton growth were investigated in connection with the water movement in the reservoir from January 1982 to January 1984 by comparing the distributions of chlorophyll a, water temperature, and coliform bacteria (Project A). The results distinguished three types of phytoplankton growth, of which one type, the abundant surface growth at the head of the reservoir, was most frequently observed. This case was considered to be caused by mixing of the surface lake water with the inflowing nutrient-rich river water at the shallow lake-head. Subsequently the effects of dissolved calcium and magnesium on the chlorophyll a and phytoplanktonic particulate phosphorus concentrations at the head of the reservoir were investigated until December 1987 (Project B). A trophic index, named the Ca-Mg index, was found which correlated positively with the chlorophyll a and phytoplanktonic particulate phosphorus concentrations in a logarithmic scale and varied in relation to the changes in the same index of the inflowing river water. Based on the ecological results of these two projects, two proposals are presented for inhibiting abundant phytoplankton growth at the head of a river reservoir. Proposal A: make the head deep and, in addition, create a strong near-bottom underflow of the inflowing river water. Proposal B: make a bypass in order to cut off the inflow of river water in the case of a high Ca-Mg index.     

Feeding ecology of Limnocalanus macrurus in the Laurentian Great Lakes

The zooplankton communities of several Laurentian Great Lakes have shifted toward greater biomass of calanoid copepods, particularly Limnocalanus macrurus, since the 1990s. Limnocalanus is an omnivore that feeds on large phytoplankton cells, ciliates, rotifers, and small crustacean zooplankton, especially copepod nauplii, and it may be an increasingly important zooplanktivore in these systems. Although there is previous research examining Limnocalanus predation rates on nauplii, we do not know if the presence of phytoplankton affects predation rates. Our initial experiments confirmed Limnocalanus preference for nauplii over small copepodites. Additional experiments showed that Limnocalanus feeding rates on nauplii decreased by 50% at the highest phytoplankton concentrations tested. Limnocalanus fed more on the larger algae tested (Cryptomonas, Cryptophyta, 40 µm) than on the smaller taxa (Scenedesmus, Chlorophyta, 10 µm). We used stable isotope analysis to infer Limnocalanus trophic position in the five Laurentian Great Lakes by comparing Limnocalanus with simultaneously captured Leptodiaptomus sicilis, another calanoid copepod known to feed on phytoplankton and microzooplankton. This analysis showed Limnocalanus at higher trophic positions in the more oligotrophic lakes Huron, Michigan, and Superior than in lakes Ontario and Erie. Summer Limnocalanus trophic position was inversely related to both the site-specific concentration of algae in the deep chlorophyll layer and a trophic state index based on spring chlorophyll and total phosphorus. Our results indicate that predation by Limnocalanus on zooplankton depends on lake algal abundance, and that feeding rates on nauplii by an individual Limnocalanus adult are likely higher in the more oligotrophic lakes.     

Wave exposure and hydrologic connectivity create diversity in habitat and zooplankton assemblages at nearshore Long Point Bay,Lake Erie

During an 11-day period in August 2008, we visited 102 sites along the nearshore (~ 60 km) of Long Point Bay. The purpose of our study was to evaluate the effects of wave exposure and hydrologic connectivity on zooplankton distributions. Long Point is located within the UNESCO Long Point Biosphere Reserve (26,250 ha) and encompasses the largest wetland complex in the Great Lakes system. We sampled for zooplankton, aquatic vegetation, temperature, specific conductance, pH, dissolved oxygen, dissolved organic carbon, water clarity, total nitrogen and depth. We evaluated the impacts of exposure using wind and fetch data to calculate a Relative Exposure Index (REI). Ordination techniques revealed a large variation in physical disturbance, water clarity, nutrient concentrations, water chemistry and aquatic vegetation that explained the distribution pattern of zooplankton at the 102 sites. Gradients of REI are strongly positively correlated with environmental variables, such as pH, dissolved oxygen and temperature and highly negatively correlated with conductivity and dissolved organic carbon. Visual inspection of the ordination site scores revealed the 102 sites clustering into six main groups based on spatial location and degree of surface-water connectivity to Long Point Bay. Sheltered sites (low REI) have much higher abundance of zooplankton whereas sites that have high REI scores are characterized by relatively low zooplankton abundance with a high prevalence of Polyarthra sp. This is the largest study on the distribution pattern of zooplankton in Long Point Bay, and it highlights the importance of wave exposure and hydrologic connectivity in structuring the zooplankton community.     

Zooplankton-phytoplankton interactions in Green Bay,Lake Michigan: Lower food web responses to biological invasions

As the largest freshwater estuary in the Laurentian Great Lakes, Green Bay, Lake Michigan (USA) is an important ecosystem presenting both challenges and opportunities for investigating changes in the face of multiple anthropogenic stressors. We collected new data from 2000 to 2007 to assess changes in lower food web interactions after establishment of invasive species (Bythotrephes longimanus and Morone americana in 1988 and Dreissena polymorpha in 1993) and nutrient reductions (1990s). Phytoplankton and zooplankton biomass and composition, as well as primary productivity and zooplankton community grazing rates, were determined along the previously well-studied trophic gradient from the shallow Lower bay to the stratified, open-water Middle bay. A clear trophic gradient still occurred during 2000–2007, with higher nutrients, phytoplankton and zooplankton in Lower bay compared to Middle bay. Phytoplankton abundance and cyanobacteria dominance increased significantly compared to earlier studies. However, integrated primary productivity did not change significantly at either Lower or Middle bay. Zooplankton standing stock decreased in Lower bay, driven primarily by reductions of bosminids, chydorids, and cyclopoid copepods, but did not change in Middle bay. Zooplankton community grazing rates did not change significantly, but shifts in magnitude and seasonality of net phytoplankton growth rates are consistent with increased phytoplankton standing stocks. Changes in zooplankton composition indicate increased predation by invertebrates and decreased fish predation. Shifts in both bottom-up and top-down factors have occurred, with Lower and Middle bay regions more eutrophic and similar to each other as a result of changes in this highly productive Great Lakes embayment.     

Diversity and ecology of phytoplankton in Lake Edward (East Africa): Present status and long-term changes

Lake Edward is one of the African Rift Valley lakes draining into the Nile River basin. We conducted three sampling series in Lake Edward in October-November 2016, March-April 2017 and January 2018, in distinct seasonal conditions and in several sites varying by depth and proximity to river outlets, including the Kazinga Channel, which connects the hypertrophic Lake George to Lake Edward. The phytoplankton was examined using microscopy and marker pigment analysis by high performance liquid chromatography (HPLC) and subsequent CHEMTAX processing for estimating abundance of phytoplankton groups. Chlorophyll a concentration in the pelagic and littoral open lake sites barely exceeded 10 µg L⁻¹ whereas, in contrast, in the semi-enclosed Bay of Katwe influenced by the Kazinga Channel chlorophyll a was up to 100 µg L⁻¹. Despite substantial seasonal variations of limnological conditions such as photic and mixed layer depths, cyanoprokaryotes/cyanobacteria represented on average 60% of the phytoplankton biomass, followed by diatoms, which contributed ~25% of chlorophyll a, and by green algae, chrysophytes and cryptophytes. 248 taxa were identified with clear prevalence of cyanobacteria (104 taxa), from the morphological groups of coccal and filamentous species (non-heterocytous and heterocytous). The high proportion of heterocytous cyanobacteria, along with a relatively high particulate organic carbon to nitrogen (C:N) ratio, suggest N limitation as well as light limitation, most pronounced in the pelagic sites. During the rainy season, the most abundant diatoms in the plankton were needle-like Nitzschia. Comparison with previous studies found differences in water transparency, total phosphorus, and phytoplankton composition.     

Phytoplankton Composition and Biomass in the Offshore Waters of Lake Erie: Pre- and Post-Dreissena Introduction (1983–1993)

Phytoplankton was collected in all basins of Lake Erie during 42 cruises during the spring and summer from 1983 to 1993—a period that spans the Dreissena mussel invasion. Two potential impacts of Dreissena on the phytoplankton community of the western, central, and eastern basins of Lake Erie were evaluated: Was selective feeding occurring as observed in Saginaw Bay and were reductions in biomass evident in the offshore regions of the three basins of Lake Erie? In the western basin, significant summer decreases in Chlorophyta, Bacillariophyta, Cyanobacteria, and total phytoplankton biomass were observed after Dreissena introduction. Similarly in the spring, Bacillariophyta and total phytoplankton biomass and chlorophyll a concentrations decreased significantly. Since several divisions of phytoplankton did not decrease in phytoplankton biomass in the western basin, and spring Cyanobacteria biomass increased significantly while other divisions decreased in biomass, selective feeding on the phytoplankton community was suggested. Where significant reductions in biomass were observed in the offshore waters of the western basin, they were approximately 50% of the reduction observed at the nearshore sites in Lake Erie by other workers.Dreissena impact on the phytoplankton community of the pelagic waters of the central and eastern basin appeared to be minimal. Pre- and post-Dreissena total phytoplankton biomass and chlorophyll a concentrations were not significantly different or increased significantly after the Dreissena invasion. Biomass of several divisions of phytoplankton significantly increased after Dreissena introduction in the central and eastern basins. These included Bacillariophyta (central basin), Cyanobacteria (central and eastern basin), Chrysophyta (eastern basin), Chlorophyta biomass (eastern basin) and phytoplankton biomass (central basin) and chlorophyll a (central basin) in the spring, and Chrysophyta (eastern basin) and Cryptophyta biomass (central basin) in the summer. Generally, a reduction in phytoplankton biomass would be expected as a result of Dreissena grazing, not an increase in biomass. Dreissena-mediated changes in phytoplankton have generally occurred in shallow, well-mixed lakes, ponds, and embayments, not in deeper waters such as the central and eastern basins of Lake Erie.     

Phytoplankton Distribution in Saginaw Bay

Phytoplankton cell counts were made on samples taken in Saginaw Bay during each of 11 cruises from April 1980 through November 1980. A spring assemblage numerically dominated by diatoms was followed by a brief period of dominance by green algae. Blue-green algae were numerically dominant from July through October when diatoms again became the dominant taxonomic group. Two regions of the bay were recognized based on the results of cluster analysis and principal component analysis of the average phytoplankton assemblage. The inner bay was typified by algal species with a Saginaw River origin and/or species typical of eutrophic waters in the Great Lakes. The outer region was typified in part by species of open Lake Huron. Wild Fowl Bay and the area to the southwest of Oak Point each have unique phytoplankton assemblages and are apparently not well integrated with the rest of the bay. On occasion, stations at or near the eastern side of the Saginaw Bay-Lake Huron interface have phytoplankton assemblages similar to inner bay and/or Wild Fowl Bay or Oak Point waters. Taken together, these findings indicate a possible strong nearshore effect on transport of Saginaw Bay populations to Lake Huron. The nearshore effect modifies the effect of Saginaw River on the phytoplankton of the Saginaw Bay-Lake Huron system.     

Benefits to 0+ fishes of connecting man‐made waterbodies to the lower River Trent,England

Floodplain waterbodies are reputed to enhance recruitment of riverine fish populations via provision of spawning and nursery habitat, refuge from floods, and increased availability of planktonic food resources compared with main river channels. Notwithstanding, there have been few parallel studies of fishes and their food resources at both main river and floodplain sites. Thus, this study investigated the 0+ fishes, zooplankton and phytoplankton (chlorophyll a) at four main river and four (man‐made) floodplain sites on the lower River Trent, England, between May 1999 and October 2004 inclusive. All sites shared the same key fish species, and there were no consistent differences in the densities, growth or condition of 0+ fishes from main river and floodplain sites. Similarly, all sites shared the same key zooplankton taxa. However, zooplankton densities, notably of large‐bodied cladocerans, and chlorophyll a concentrations, were significantly higher at floodplain sites than at main river sites. Thus, connection of man‐made waterbodies has the potential to enhance recruitment of riverine fish populations via provision of important spawning and nursery habitat, and superior feeding opportunities for 0+ fishes compared with main river channels. Copyright © 2007 John Wiley & Sons, Ltd.     

Phytoplankton community composition of Saginaw Bay,Lake Huron,during the zebra mussel (Dreissena polymorpha) invasion: A multivariate analysis

The colonization of the zebra mussel (Dreissena polymorpha) in Saginaw Bay dramatically altered the phytoplankton community composition resulting in exclusion of light sensitive species and dominance of species with oligotrophic preferences and light resistance. In 1990, the NOAA Great Lakes Environmental Research Laboratory initiated a 7-year survey program to monitor changes in the lower food web of Saginaw Bay, where zebra mussels became established in the fall of 1991. To investigate shifts in the phytoplankton community composition over the 7-year period from 1990 to 1996 we searched for clusters of similar composition using multivariate principal component analysis (PCA) on proportions of 22 taxonomic groupings of the total phytoplankton density (cells per milliliter). We then used an agglomerative hierarchical clustering analysis of the PCA scores. We identified five characteristic phytoplankton communities in configurations that allowed recognizing four distinct periods in Saginaw Bay linked to the zebra mussel invasion. Significant changes were indicative of increased water clarity and eutrophic conditions being replaced by more oligotrophic conditions as clusters dominated by light sensitive species, such as the cyanobacteria Oscillatoria redekii, became immediately rare and clusters dominated by diatoms such as Cyclotella spp. became common. Microcystis spp., a light tolerant cyanobacteria not grazed by zebra mussel, dominated assemblages after 1994. The shifts in phytoplankton composition confirm that zebra mussels effects on phytoplankton communities are mediated by both direct (filtration) and indirect (nutrient cycling) mechanisms and also suggests that increased light penetration is an important mechanism behind some changes.     

Results from the U.S. Great Lakes Fish Monitoring Program and Effects of Lake Processes on Bioaccumulative Contaminant Concentrations

An analysis of composite samples of 820 lake trout, walleye, steelhead, Chinook, and coho from the Laurentian Great Lakes reveals differences in contaminant processing among and between lakes which results in differing concentrations of bioaccumulative contaminants. Generally, contaminants are most concentrated in fish from Lake Michigan and least concentrated in fish from Lake Superior, with the notable exceptions of toxaphene and alpha-HCH. Differences in contamination patterns, however, are apparent not only among the lakes but between sites within a lake or even fish within a site. Lake trout composites from Lake Superior show an increase in the degree of chlorination of PCBs with increasing total PCBs. The PCB congener profile of lake trout from the Sturgeon Bay site of Lake Michigan is substantially different from that of the Saugatuck site of Lake Michigan, possibly due to the influence of contamination from nearby Green Bay. Finally, the ratios of selected PBDE and PCB congeners are much different in Lake Superior fish compared to fishes from all the other lakes. We hypothesize that this is a result of the colder temperatures and associated lower plankton growth rates in Lake Superior allowing PCB and PBDE uptake by phytoplankton to reach near equilibrium, thus enhancing the relative concentrations, in phytoplankton and the food web in general, of congeners that may be kinetically limited in other lakes.     

High nutrient loading and climatic parameters influence the dominance and dissimilarity of toxigenic cyanobacteria in northern bays of Lake Victoria

Eutrophication of Lake Victoria led to changes in its phytoplankton communities. However, different levels of eutrophication exist in the open lake and the bays, and between embayments. This study utilized spatial and temporal sampling of Napoleon Gulf and Murchison Bay, exhibiting different trophic conditions. Over one year, we investigated phytoplankton biomass, richness, diversity and dissimilarity, and related the dynamics of the dominant species to the limnological and climatic conditions. The results confirmed that Napoleon Gulf and Murchison Bay showed large differences in eutrophication status, with lower nutrient concentrations in Napoleon Gulf than in Murchison Bay, where a strong gradient was observed from inshore to offshore areas. These nutrient dynamics resulted in a 4 to 10 fold higher chlorophyll-a in Murchison Bay than in Napoleon Gulf. From the embayments, 135 phytoplankton taxa were recorded with no significant differences in alpha diversity. However, high dissimilarity in community structure was observed in beta diversity, mostly due to a turnover among the dominant toxigenic species. Thus, from a similar species pool, there was a shift in the dominant toxigenic cyanobacteria from Microcystis flos-aquae and M. aeruginosa in Murchison Bay, Dolichospermum circinale and Planktolyngbya circumcreta in Napoleon Gulf to D. circinale in the offshore stations. These cyanobacteria are toxigenic taxa with known health hazards. Using partial least square models, we showed that both climatic variables (e.g. wind, solar radiation) and levels of inorganic dissolved nutrients (e.g. SRP, NO₃^–, and NH₄⁺) are the main drivers of differences and dominance in cyanobacteria communities in northern Lake Victoria.     

Diet composition and overlap for adult walleye,lake whitefish,and yellow perch in Green Bay,Lake Michigan

Interspecific interactions among walleye Sander vitreus, lake whitefish Coregonus clupeaformis, and yellow perch Perca flavescens in Green Bay could influence the population status of each species, but potential trophic interactions are poorly understood. Our objectives were to determine if diet assemblages for each species and diet overlap among species varied spatially and temporally within Green Bay. Adult walleye (≥381 mm total length (TL); N = 981), lake whitefish (≥432 mm TL; N = 1507), and yellow perch (≥150 mm TL; N = 1174) were collected during May-October of 2018 and 2019 from multiple locations in southern and northern Green Bay. Diet assemblages of each species varied between northern and southern Green Bay, but walleye diets were more temporally variable (among months within zones and between years) than diets of lake whitefish or yellow perch. Lake whitefish represented a seasonally important prey item for walleye in southern Green Bay, composing 10 % and 41 % of walleye diets by weight in May and June, respectively. Yellow perch generally composed <15 % of walleye diets by weight but were consumed at a broader spatiotemporal scale than lake whitefish. Diet overlap between walleye and both lake whitefish and yellow perch was generally weak or moderate, whereas diet overlap between whitefish and perch was generally strong. Our assessment of adult trophic interactions suggests that changes in the population status of one species could influence fisheries for all three, and we identify additional research questions to address potential population-level effects of these trophic interactions.     

Statistical Analysis of the Spatial and Temporal Variability of the Ratio Chlorophyll A to Phytoplankton Cell Volume in Saginaw Bay,Lake Huron

Chlorophyll a has traditionally been used as an indicator of phytoplankton concentration in natural waters. This parameter has also been widely used by phytoplankton modelers for the purpose of comparing model output with field data. Examination of 1974 field data from Saginaw Bay indicated that chlorophyll a concentrations were inconsistent with phytoplankton cell volume concentrations. Statistical procedures were employed to determine the significance and source of the observed variability in the ratio of chlorophyll a to cell volume concentrations.The procedures employed included t-tests, analysis of variance and polynomial regression. The hypothesis that the natural log of the ratio of chlorophyll a to cell volume was constant in time was tested on six different sampling station groupings representing various spatial divisions of Saginaw Bay. In all cases, this ratio was found to vary significantly at the 95% confidence level. The variability of the ratio decreased when only data from the outer portion of Saginaw Bay were used. Greater than 60% of the variability of the ratio can be removed by using a linear correlation of the natural log chlorophyll a to cell volume ratio with the fraction of diatom cell volume.     

Copper-rich “Halo” off Lake Superior’s Keweenaw Peninsula and how Mass Mill tailings dispersed onto tribal lands

Over a century ago, shoreline copper mills sluiced more than 64 million metric tonnes of tailings into Lake Superior, creating a “halo” around the Keweenaw Peninsula with a buried copper peak. Here we examine how tailings from one of the smaller mills (Mass Mill, 1902–1919) spread as a dual pulse across southern Keweenaw Bay and onto tribal L’Anse Indian Reservation lands. The fine (“slime clay”) fraction dispersed early and widely, whereas the coarse fraction (stamp sands) moved more slowly southward as a black sand beach deposit, leaving scattered residual patches. Beach stamp sands followed the path of sand eroding from Jacobsville Sandstone bluffs, mixing with natural sands and eventually adding onto Sand Point, at the mouth of L’Anse Bay. Dated sediment cores and a multi-elemental analysis of the buried Cu-rich peak in L’Anse Bay confirm a tailings origin. Copper concentrations are declining in the bay, yet copper fluxes remain elevated. The spatial and temporal studies underscore that enhanced sediment and copper fluxes around the Keweenaw Peninsula largely reflect historic mining releases. Mercury is correlated with copper, yet mercury concentrations and fluxes remain relatively low in Keweenaw Bay compared to nearby Superfund sites (Torch and Portage Lakes), perhaps reflecting the absence of smelters on Keweenaw Bay. Tribal efforts to remediate contamination are progressing, but are hindered by recent high water levels plus severe storms. The long-term repercussions of Mass Mill discharges caution against mine companies discharging even small amounts of tailings into coastal environments.