Interconnected riverine–lacustrine systems as sedimentary repositories: Case study in southeast Michigan using Pb and Cs-based sediment accumulation and mixing models

Particle-reactive nuclides, such as ²¹⁰Pb and ¹³⁷Cs, serve as powerful chronometric tools in the investigations and reconstruction of historical contamination in coastal marine and lacustrine systems. Towards the first systematic establishment of sediment chronologies of river channel sediments, a set of seven sediment cores from Clinton River and Lake St. Clair riverine–lacustrine system were collected and analyzed for ²¹⁰Pb, ²²⁶Ra, and ¹³⁷Cs activities. Measured inventories of ²¹⁰Pb_xs and ¹³⁷Cs were ~ 2 and ~ 9 times higher than that expected from atmospheric fallout. From the measured ²¹⁰Pb_xs/¹³⁷Cs inventory ratios, erosional input of ¹³⁷Cs was found to be significantly higher than that of ²¹⁰Pb indicating that anthropogenic watershed disturbances have resulted in accelerated sediment erosion. Good agreement between accumulation rates using ²¹⁰Pb_xs and ¹³⁷Cs using four different age models were obtained for four of the seven cores in the riverine–lacustrine environment. Average sediment mass accumulation rates, based on the ²¹⁰Pb_xs CFCS model, in the lower Clinton River (mean: 0.91 g cm⁻² yr⁻¹) were generally higher than those in Lake St. Clair (mean: 0.55 g cm⁻² yr⁻¹) due to a higher sediment flux and the unique riverine system characteristics. Sediment mixing coefficients, based on a ²¹⁰Pb_xs mixing model, were much higher in the river (mean: 64.9 cm² yr⁻¹) compared to the lake (mean: 4.7 cm² yr⁻¹), as was expected due to the frequency of perturbation and resuspension. Net accumulation of datable sediments in the Clinton River indicates that similar river channel deposits may act as repositories for the reconstruction of historical contamination and environmental changes.     

Anthropogenic Copper Inventories and Mercury Profiles from Lake Superior: Evidence for Mining Impacts

During the past 150 years, the mining industry discharged more than a billion tons of tailings along Lake Superior shorelines and constructed numerous smelters in the watershed. Given the vast size of Lake Superior, were sediment profiles at locations far offshore impacted by nearshore activities? Did copper and associated precious metal mining modify regional fluxes for copper and mercury? Samples from thirty sediment cores document that background concentrations of copper are high (mean 60.9 ± 7.0 μg/g), due to the proximity of natural ore sources. Anthropogenic inventories uncorrected for focusing also are high, ranging from 20 to 780 μg/cm² (mean 187 ± 54 μg/cm²). Focusing factor corrections decrease the mean estimate and reduce variance (144 ± 24 μg/cm²). Several approaches to estimating inputs suggest that only 6 to 10% of historic copper deposition originated directly from atmospheric sources, emphasizing terrestrial sources. Moreover, coastal sediment cores often show synchronous early increases in copper and mercury with buried maxima. Around the Keweenaw Peninsula, twenty-two cores trace high copper and mercury inventories back to mill and smelting sources. Direct assays of ores from thirteen mine sites confirm a natural amalgam source of mercury in the stamp mill discharges. Core records from inland lakes (Michigamme Project) also reveal patterns of copper and mercury inputs from a variety of mining sources: historic tailing inputs, amalgam assay releases, and atmospheric smelter plumes.     

Silica Depletion in Lake Victoria: Sedimentary Signals at Offshore Stations

Six short sediment cores from offshore stations in Lake Victoria (East Africa) were analyzed for evidence of recent change in the lake's pelagic ecosystem. Three stations were located on a NW-SE transect between 48 m water depth, near the present upper limit of seasonal hypolimnetic oxygen depletion, and the deepest point of Lake Victoria at 68 m. Four stations formed a NE-SW transect across the east-central zone of maximum Holocene sediment accumulation below 64 m water depth. ^2I0Pb dating of two cores from deepwater stations established average recent sediment-accumulation rates of 0.032 ± 0.001 g/cm²/yr and 0.028 ± 0.001 g/cm²/yr. Although the deepest part of the basin has been subject to an event of possibly widespread sediment erosion dated to the mid-1920s, core correlation based on the stratigraphy of biogenic Si above this unconformity indicates that deepwater stations have accumulated representative high-resolution archives of lake history over the past 70 years. The sedimentary record of biogenic-Si accumulation in deepwater cores reflects a sequence of events in which progressive enrichment of Lake Victoria with essential nutrients other than Si first led to increased diatom production, until the combination of excess Si demand and greater burial losses of diatom-Si resulted in depletion of the dissolved-Si reservoir and a transition to Si-limited diatom growth. Available sediment chronologies infer that increased diatom production in offshore areas started between the 1930s and early 1950s, and that the recently documented phytoplankton transition to year-round dominance by cyanobacteria started in the late 1980s. Excess diatom production over the past half century has led to significantly higher burial losses of biogenic Si only in the depositional center of the basin at water depths below 60 m.     

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

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

Annual and Offshore Changes in Bacterioplankton Communities in the Western Arm of Lake Superior during 1989 and 1990

A shallow site in the western arm of Lake Superior near Duluth, Minnesota was sampled bimonthly from May to October during 1989 and 1990 to identify seasonal and annual changes in bacterioplankton communities. The greatest change in bacterioplankton abundance was between 1989 (1.48 × 10⁹/L ± 0.06 SE) and 1990 (1.14 × 10⁹/L ± 0.06 SE). The majority of bacterial cells (65%) were cocci. Individual cells were larger during 1989 (0.067 μm³ ± 0.007 SE) than 1990 (0.025 μm³ ± 0.002 SE). Although the rate of thymidine incorporation varied from 0.2 to 47.0 pmol/L/h over both years (mean = 12.1 pmol/L/h ± 1.3 SE), no consistent temporal or spatial changes were detected. Bacteria were more abundant (∼2×) and productive (∼10×) at the mouth of the Lester River than offshore of this site. During July and August, a benthic nepheloid layer (BNL) formed at shallow offshore sites but bacterioplankton abundance and production in this BNL were usually similar to values measured in the hypolimnion. Three additional sites from the Duluth basin northeast to the Chefswet basin were sampled during late summer (Aug-Sept) 1990 to identify spatial differences in bacterioplankton communities. Although the number of bacteria was often greater at shallower sites compared to deeper sites further offshore, a strong gradient was not found and bacterial production was similar at all sites. These results may be due in part to the lake basin morphology in this region of Lake Superior, as well as the time when these additional offshore sites were investigated.     

Nitrogen Dynamics in Sandy Freshwater Sediments (Saginaw Bay,Lake Huron)

Sediment-water nitrogen fluxes and transformations were examined at two sites in Saginaw Bay, Lake Huron, as a model for sandy freshwater sediments. Substantial ammonium release rates (74 to 350 μmole NH₄⁺/m²/h¹) were observed in flow-through cores and in situ benthic chamber experiments. Sediment-water ammonium fluxes were similar at the inner and outer bay stations even though inner bay waters are enriched with nutrients from the Saginaw River. The high net flux of remineralized ammonium into the overlying water from these sandy sediments resembles typical data for marine systems (11 to 470 μmole NH₄⁺/m²/h¹) but were higher than those reported for depositional freshwater sediments (0 to 15 μmole NH₄⁺/m²/h¹; Seitzinger 1988). Addition of montmorillonite clay (ca. 1 kg dry weight/m²) to the top of the sandy cores reduced ammonium flux. Mean “steady-state” ammonium flux following clay addition was 46 ± 2 (SE) % of the initial rates as compared to 81 ± 8% of the initial rates without clay addition. Zebra mussel excretion dominanted ammonium regeneration in the inner bay where the bivalve was abundant, but addition of zebra mussel feces/psuedofeces (3.0 g dw/m²) to sediments did not increase ammonium or nitrate flux. Partial nitrification of ammonium at the sediment-water interface was suggested by removal of added ¹⁵NH₄⁺ from lake water passing over dark sediment cores. Sediment-water fluxes of nitrogen obtained from flow-through sediment cores resembled those from in situ benthic chambers. However, extended static incubations in gas-tight denitrification chambers caused more of the regenerated nitrogen to be nitrified and denitrified than occurred with the other two measurement systems.     

Estimation of Zooplankton Mean Length for Use in an Index of Fish Community Structure and its Application to Lake Erie

Mills et al. (1987) developed an index of zooplankton mean size to assess the state of fish communities. The use of this index was evaluated in an assessment of the fish community structure in 1993 at nearshore and offshore sites in the three Lake Erie basins. Mills et al.’s index was developed using a 153-μm mesh net, while the samples in this study have been collected with 64-μm and 110-μm mesh size nets. Two methods were used to convert the data to 153-μm equivalent collections: (a) regression relationships based on simultaneous collections with three mesh sizes, and (b) elimination of smaller organisms that would have passed through the 153-μm mesh by determining the minimum length of inclusion (MLI). The regressions employed for the conversion of zooplankton mean length (ZML) between the nets were: ZML₁₅₃ = 0.137 + 0.988 ZML₁₁₀ (mm) (r²= 0.804) (n = 10) and ZML₁₅₃ = 0.042 + 1.330 ZML₆₄ (mm) (r² = 0.931) (n = 9). The MLI that resulted in the same mean length as the 153-μm sample averaged (± 1 SE) 0.267 ± 0.016 mm (n =19).The comparison between zooplankton mean length and fish community structure in the western basin of Lake Erie in 1993 showed good agreement with Mills et al.’s index. However, the same was not true for the 1988 to 1990 data. Reasons for this discrepancy are discussed.     

Metallothionein-like Protein and Tissue Metal Concentrations in Invertebrates (Oligochaetes and Chironomids) Collected from Reference and Metal Contaminated Field Sediments

The relationships between metallothionein-like protein (MTLP) and tissue metal (Ni, Cd, Cu, Zn, Pb) concentrations were examined in Oligochaetes and Chironomid larvae collected from reference and metal contaminated sediments. Tissue metal concentrations, for example, Cd reaching 0.3 μmol/g in oligochaetes and Cu, reaching 0.8 μmol/g in chironomids collected from Great Lakes reference sites (Environment Canada) indicated the presence of bioavailable metal at some sites. The concentration of MTLP ranged from 3.3 (± 1.0) to 11.5 (± 4.5) nmol MTLP/g in oligochaetes, and 2.7 (± 1.0) to 8.0 (± 2.7) nmol MTLP/g in chironomids. Individually, tissue metal concentrations were relatively poor (r² = 0.002–0.52) predictors of MTLP, but when multiple metals were used the relationship became stronger (oligochaetes r² = 0.84, p < 0.001; chironomids r² = 0.61, p < 0.001), suggesting that MTLP was induced by multiple metals. Tissue metal and MTLP concentrations in invertebrates from the metal contaminated slips of Collingwood Harbour were not elevated above reference levels, indicating that only a fraction of the metal in the sediment is bioavailable. Chironomids collected from lakes contaminated by atmospheric fallout of smelting emissions (Rouyn-Noranda, QC) had elevated tissue metal concentrations but MTLP concentration did not consistently reflect elevated tissue metal. Overall, MTLP concentrations in invertebrates collected from metal-contaminated sites were not significantly higher than concentrations in reference site-collected invertebrates. The data presented here suggest that, at some sites, MTLP concentration in field-collected organisms would be a useful surrogate for tissue metal but, because of a lack of detailed taxonomy (organisms were designated as chironomids or oligochaetes) and the relatively small sample sizes of this study, no definitive conclusion regarding the suitability of MTLP concentration as a field-based biomarker of metal exposure can be made without further study     

Silicon Isotopic Fractionation in Lake Tanganyika and Its Main Tributaries

Silicon isotopic measurements in Lake Tanganyika were performed using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in dry plasma condition. Isotopic signatures are reported for dissolved ortho-silicic acid [Si(OH)₄] collected during a 1-year-long surface waters survey in the southern basin along with several of the major tributaries. Deep-water Si isotopic profiles from a north-south transect cruise conducted in July 2002 are also described. The nutrient-like shape dissolved Si profiles and the isotopic disequilibrium between surface (δ²⁹Si = 0.87±0.08 ‰) and deep waters (0.61 ± 0.05 ‰) suggest the occurrence of biological isotopic discrimination induced by diatoms biomineralisation in a fresh water system. Short-term surface water Si isotopic and diatom biomass variations obtained during the 1-year bi-weekly monitoring (2002–2003) in the south confirms this biological effect. Five epilimnion biogenic opal samples also were analyzed. Their signature (δ²⁹Si of 0.28 ± 0.12‰) compared to those of surrounding waters are consistent with the diatom isotopic fractionation effect measured on marine tropical diatoms. This demonstrates the species and temperature independent character of the silicon isotope fractionations by diatoms. River signatures present variable dissolved Si concentrations which were positively correlated to δ²⁹Si values in the range of previously published world river data. Because of its fast response to climate variability, nutrient dynamics, and limnological changes, δ²⁹Si in siliceous organisms should be very useful in studying environmental changes and particularly the recent decline of diatom Si utilization in Lake Tanganyika.     

Sedimentation Rates and Depositional Processes in Lake Superior from 210Pb Geochronology

Sedimentation rates range from 0.01 to 0.32 cm/yr in 17 sediment box cores from Lake Superior, as determined by ²¹⁰Pb geochronology. Shoreline erosion and resuspension of nearshore sediments causes moderate to high (0.05–0.11 cm/yr) sedimentation rates in the western arm of Lake Superior. Sedimentation rates are very high (> 0.15 cm/yr) in marginal bays adjoining Lake Superior; and moderate to very high (0.07–0.19 cm/yr) in open lake regions adjacent to marginal bays. Resuspension of nearshore and shoal top sediments in southern and southeastern Lake Superior by storms is responsible for depositional anomalies in ²¹⁰Pb profiles corresponding to 1905, 1916–1918, and 1940 storms. Sedimentation rates are very low (0.01–0.03 cm/yr) in the central basins due to isolation from sediment sources. These data indicate that sedimentation rates and processes vary significantly in different regions of Lake Superior. The sedimentation rates provided by this study, in conjunction with previously-reported sedimentation rates, yield a better understanding of the Lake Superior depositional environment.     

Measures of larval lake whitefish length and abundance as early predictors of year-class strength in Lake Michigan

Many lake whitefish stocks in Lake Michigan have experienced substantial declines in growth and condition since the 1990s. Reduced growth and condition could result in reduced quality or quantity of eggs produced by spawning females, which in turn could negatively impact recruitment. We evaluated the potential for reduced recruitment by measuring early life stage density and length, and we discuss the utility of these measures as early indicators of lake whitefish year-class strength. Overall, mean larval density (number per 1000 m³ ± SE) in Lake Michigan was greater in 2006 (373.7 ± 28.3) than in 2005 (16.6 ± 24.8); whereas, mean length (mm ± SE) of larval lake whitefish was smaller in 2006 (12.87 ± 0.07) than in 2005 (14.38 ± 0.13). The ratio of zooplankton to fish density did not show an expected relationship with larval fish density or length. Rather, variation in larval density was best explained by a multiple-regression model that included larval length, spring wind intensity, and adult stock density as predictor variables. Our results suggest that the density of larval lake whitefish is not directly regulated by temperature or zooplankton density at the time of emergence, but that a potential for density-dependent regulation exists when larval emergence rates are high. We conclude that the observed declines in growth and condition of adult lake whitefish are not resulting in substantial reductions in recruitment.     

Occurrence of Zebra Mussels in Near-shore Areas of Western Lake Erie

We measured biomass, percent coverage, and length-frequency of zebra mussels in near-shore areas of western Lake Erie between 16 September and 10 November 1993 as part of a larger study on the ecological relationship between diving ducks and zebra mussels. Wet weight biomass of zebra mussels, determined by SCUBA diving, ranged from 0 to 3,611 g/m² and averaged ( ± 1 SE) 1,270 ± 380 g/m² (n = 11). Percent coverage of lake bottom by zebra mussels ranged from 0 to 70% and averaged 17 ± 4.0% (n = 27). Percent coverage of zebra mussels was relatively high in the northern portion (28–70% coverage) and in the southwestern portion (18–40%), but relatively low ( < 5%) in the southeastern portion of the study area. Percent coverage by zebra mussels, determined from underwater videography, was highly correlated (r² = 0.96) with zebra mussel biomass. Analysis of length-frequency data indicated that there was prominent recruitment of juvenile zebra mussels at only three of eight sites. Average shell length ranged from 11 mm to 15 mm at the other five sites. The non-uniform distribution of zebra mussels, as determined from biomass and videography, may have important ramifications when assessing zebra mussel impacts on waterfowl. These data may also be used when assessing impact of zebra mussels on other aquatic organisms in the near-shore areas of western Lake Erie.     

Temporal trends of phytoplankton and zooplankton stable isotope composition in tropical Lake Malawi

Stable isotope ratios of three seston size classes (20–100 μm, 2–20 μm, and 0.2–2 μm) and zooplankton species were analyzed to determine the plankton food web structure of Lake Malawi. Over an annual cycle, seston δ¹³C varied between ?20.41‰ and ?27.43‰ with a mean value of ?24.27‰ ± 1.2 while δ¹³C values for zooplankton fluctuated between ?22‰ and ?25‰ with a mean of ?23.84‰ ± 0.77. Seston δ¹³C fluctuations appeared to be related to changes in physical and meteorological conditions in the lake that ultimately control nutrient availability. The highest seston δ¹³C values observed during the rainy and mixed seasons likely result from high phytolankton growth rates. δ¹⁵N of plankton was temporally variable, suggesting short term changes in N cycling dynamics that control the supply of N to phytoplankton. Very low seston δ¹⁵N values recorded during the mixing season suggest excess NO₃^? availability resulting from upwelling and vertical mixing. In contrast to expectations the calanoid Tropodiaptomus cunningtoni appeared to feed at a trophic level higher than that of all other zooplankton species, including the cyclopoid, Mesocyclops aequatorialis aequatorialis. δ¹⁵N values indicate that zooplankton were nearly 2 trophic levels above seston in the early stratified season. This implies that adult zooplankton could be utilizing forms of food other than phytoplankton during this period, such as nauplii or protozoans. This extra step in the food web, and the trophic positions of large zooplankton species, may alter estimates of food web efficiency and potential fish production for Lake Malawi.     

Measurement of Diporeia respiration rate for Lake Superior

The freshwater amphipod Diporeia is a dominant macroinvertebrate species in Lake Superior’s benthic community and an important prey item for many fish. A capacity to predict growth and production rates of Diporeia using a bioenergetics model requires information on physiological processes of the species. The objective of this study is to quantify oxygen consumption of Lake Superior Diporeia and to determine if respiration rate changes with body length. Diporeia were collected from Lake Superior and kept over natural sediment maintained at 4 °C. Dissolved oxygen levels for groups of immature (2 mm), juvenile (4 mm), and adult (6 mm) Diporeia in 20 ml microcosms were measured using a polarographic microelectrode. Mass-specific respiration rates for Lake Superior Diporeia ranged from 32.0 to 44.7 mg O₂ g DW^− 1 day ⁻¹. A significant relationship between body length and mass-specific respiration rate (p > 0.1) was not found. The estimate of Diporeia respiration presented here is significantly higher (p < 0.05) than previous findings from populations in Lakes Michigan and Ontario. This study provides new data on respiration rates of Lake Superior Diporeia and compares findings to studies for other connecting Great Lakes.     

Heterotrophic Bacterioplankton Dynamics at a Site off the Southern Shore of Lake Superior

Evolution of the paradigm regarding the relative importance of allochthonous and autochthonous sources of organic matter in aquatic systems has rekindled interest in the role of bacteria in energy transfer. The development of material budget calculations characterizing conditions of net heterotrophy requires knowledge of spatiotemporal dynamics in the bacterial community. Here we present results from three years of measurement of bacterioplankton abundance and production at three locations on the south shore of Lake Superior. In general, bacterial numbers (0.63 × 10⁶ ± 0.29 × 10⁶ cells·mL⁻¹) and production (0.037 ± 0.055 mgC·m⁻³·hr⁻¹) were comparable to those reported previously for Lake Superior and were consistent with the system's place along the trophic gradient. Interannual differences in abundance and production were apparently related to the timing and magnitude of seasonal phytoplankton dynamics. There was no inter-transect variation or systematic nearshore-offshore gradients in bacterial activity despite substantial differences in proximity to sources of terrigenous materials (between transects) and in phytoplankton biomass (with distance offshore). The most striking signals in bacterial activity were those evidenced by peaks in bacterial production associated with the deep chlorophyll maximum and with the decline in the phytoplankton community with the approach to turnover. It is hypothesized that bacterioplankton activity in Lake Superior exists in a near steady state, fueled by labile organic matter produced through phytoplankton excretion and the photolytic processing of refractory terrigenous materials. Superimposed on this rather stable signal are peaks in bacterial production apparently related to senescence of the phytoplankton community and, perhaps, the generation of nutrients by the grazing community.     

Accumulation of Contaminant Metals in the Amphipod Diporeia spp. in Western Lake Ontario

Sediment and amphipods (Diporeia spp.) were collected in areas in western Lake Ontario suspected of containing a range of sediment metal contents. The total metal contents (Al, As, Cd, Cr, Cu, Ni, Fe, V, and Zn) of depurated Diporeia tissues were measured and compared to the total metal contents of the surrounding sediment. The fractional bioavailability of As, Cu, Fe, and Zn was determined in those same sediments using a sequential chemical extraction (SCE) procedure and correlated with depurated Diporeia tissue metal contents. Results of these analyses were then used to assess the ability of Diporeia to accumulate sediment metal contamination. A comparison of metal biota-sediment accumulation factors (BSAFs) and sediment metal enrichment factors (EFs) for each metal yielded separate metal groups showing different behavior for tissue accumulation. Aluminum, Cr, Fe, Ni, and V were not enriched in the sediments and were not accumulated in Diporeia tissues, while As, Cd, Cu, and Zn were enriched in the sediments and accumulated in Diporeia tissues. SCE results showed that Cu (3.2 to 8.4 %) and Zn (13 to 19 %) contents in the easily extractable sediment fractions (MgCl₂ and Na-acetate) were significantly correlated with Cu (r = 0.86; P<0.01) and Zn (r = 0.69; P<0.05) contents in depurated Diporeia tissue. In addition, Cu and Zn in Diporeia tissues are also correlated to the total sediment metal content for each respective metal (Cu: r = 0.67, P < 0.05; Zn: r = 0.66, P < 0.05). The high BSAFs for Cu and Zn, in combination with the association of tissue metal contents with easily extracted sediment metal fractions, suggest that Diporeia may be a potential bioindicator for Cu and Zn contamination in sediments.     

Effect of the urbanized embayment Toronto Harbour on the composition and production of zooplankton

To better understand zooplankton dynamics in Lake Ontario’s Toronto Harbour and adjacent coastal area (CA), we sampled zooplankton, phytoplankton, nutrients and physical parameters on six dates in 2016. Despite higher levels of nutrients, chlorophyll and primary production in the inner harbor (IH), the areas supported similar May to November zooplankton biomass (IH = 32 ± 7 and CA = 42 ± 10 mg/m³). IH values were much lower than other nutrient-enriched embayments in Lake Ontario, yet CA biomass was twice that of nearshore sites away from Toronto. Small zooplankton such as rotifers and Bosmina dominated IH; and large taxa (Daphnia, calanoids and predatory cladocerans) were more important in the CA. Daphnia, Bosmina, cyclopoids and calanoids were larger in the CA, and adult cyclopoids had higher egg ratios. This led to low annual IH production estimates for both cyclopoid and calanoid copepods. Total phosphorus and chlorophyll did not appear to regulate zooplankton biomass, but positive relationships were found with bacterial biomass in the IH and with temperature in the cool season. Atypically high fish planktivory rates likely suppressed larger IH zooplankton in 2016, allowing small, resilient Bosmina to flourish and contribute 84% of total production in the IH. Comparing 2016 data to previous zooplankton surveys revealed considerable inter-annual variation in proportions of Daphnia, Bosmina and predatory cladocerans over the 1994 to 2016 period, and the strong top-down controls observed in 2016 were not typical. Elevated microbial production may serve as an important alternate trophic pathway supporting cladoceran populations in Toronto Harbour.     

Horizontal and Vertical Distributions of Mercury in 1983 Lake Superior Sediments with Estimates of Storage and Mass Flux

Persistent elevated mercury concentrations in some species of Lake Superior fish and the general lack of information on mercury storage in and fluxes to the lake's sediments prompted the analysis of samples that were collected in 1983. Results of the analyses support the conclusion that Lake Superior sediments have mercury concentrations above background levels at all sites sampled. For those cores which penetrated the sediment deeply enough (the majority of the cores), background mercury concentrations ranged between 0.016 and 0.048 mg/kg. Mercury concentrations in surficial sediments ranged between 0.027 and 0.96 mg/kg. The maximum mercury concentration found in sub-surface sediments (2 to 20 cm deep) was 6.5 mg/kg. The surficial 20 cm of sediment contained 342 metric tons of mercury of which 51% or 174 metric tons was anthropogenic. The surface 2 cm of sediment contained 29 metric tons of mercury of which 76% or 22 metric tons was anthropogenic. Estimated total mercury fluxes to surficial sediments ranged between 0.1 and 10 ng/cm²/y with a mean of 3.2 ng/cm²/y. Background total mercury fluxes to the lake ranged between 0.20 and 0.72 ng/cm²/y with a mean of 0.48 ng/cm²/y. Estimated fluxes of anthropogenic mercury to surficial sediments ranged between –0.42 and 10 ng/cm²/y with a mean of 2.7 ng/cm²/y. The fluxes reported here are only the second known reporting of mercury fluxes to Lake Superior sediments. The inventory of mercury in the sediments is the first reported.     

The Effect of a Large Resuspension Event in Southern Lake Michigan on the Short-term Cycling of Organic Contaminants

In January and March, 1998, a series of intense, northerly wind-driven storms suspended sediment over the entire coastline of the southern basin of Lake Michigan. The effect of large scale resuspension on organic contaminant cycling was investigated using a two-pronged sample collection strategy that included analysis of settling sediment trap material and discrete air and water samples collected before and after a major resuspension event. It was found that major resuspension events result in a large flux of contaminants. For example, 6.2 ng/cm² ΣPCB (sum of 89 congener peaks) and 175 ng/cm² ΣPAHs (sum of 31 compounds) fell through the water column in the southern basin between November and May but almost half of that occurred in the month of March after a series of intense storms induced a largescale resuspension event in that month. Assuming the concentration of contaminants in settling sediments is similar throughout the basin, the March event brought ∼400 kg of ΣPCBs and ∼13,000 kg ΣPAHs into the water column. Furthermore, the data indicate that concentrations of dissolved phase ΣPCB and ΣPAHs declined significantly (α = 0.05) after the event and after resuspended sediment had settled from the surface waters. As a result of the depressed dissolved concentrations at the surface, the potential for gas-phase input to the lake increases on the southwestern coastal region near Chicago, IL and Gary, IN. The potential input of gas-phase contaminants was 8 kg for ΣPCBs and 2,200 kg for ΣPAHs over the 40-day lifetime of the near-shore event.     

Effects of lake-basin morphological and hydrological characteristics on the eutrophication of shallow lakes in eastern China

The Yangtze River floodplain contains numerous oxbow or riverine lakes, all of which were openly connected with the Yangtze River or its major tributaries prior to 1950s. However, stresses resulting from human settlement and utilization of catchment resources have exerted great pressures on these lake ecosystems changing their morphology, connectivity and trophic state lakes. This study examined the interaction of these changes and their impact on eutrophication for 90 shallow lakes in eastern China in 2008 to 2011. TN and TP in the study lakes had negative relationships with mean water depth (Z_mean), but no single lake-basin characteristic was found to dominate chlorophyll-a (Chl-a) growth. Instead, water depth and surface area were found to interactively affect Chl-a concentrations in smaller lakes. That is, Chl-a concentration in the lakes with Z_mean > 2 m and surface area (SA) ≤ 25 km² was significantly higher than that in relatively larger lakes with Z_mean > 2 m and SA > 25 km² (p-value ≤ 0.038). Chl-a concentration was higher in the lakes located within the lower Yangtze River basin which had longer retention times, than in the lakes located within the middle Yangtze River basin, where flow velocity is relatively larger. As expected, the water quality was found to be better in the lakes hydraulically connected with rivers than in those isolated from the river. This study revealed that lake-basin morphology and hydrology dominated algal blooms in the highly eutrophic shallow lakes in eastern China.