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.     

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.     

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.     

Silica Regeneration Processes in Nearshore Southern Lake Michigan

The seasonal depletion of dissolved silica to levels that limit diatom production is particularly critical in Lake Michigan's nearshore zone where diatom biomass is greatest, and where silica regeneration from sediments is not well-understood. In our study, intact, medium-fine sand cores, collected from an 11 m deep site in nearshore Lake Michigan during July-August 1980, released soluble reactive silica (SRS) at a mean rate of 2,707 ± 122 (SE) μg Si cm^?2 yr^?1 when incubated in darkness and at 12° C. This measured SRS release was greater than a diffusive flux (270 ± 49 (90% C.I.) μg Si cm^?2 yr^?1) estimated from SRS pore water profiles and physical sediment properties. SRS release from individual cores was not correlated with abundance of most macroinvertebrates (chironomids, pisidiid clams, or oligochaetes). However, a significant (P < 0.05) and inverse relationship between SRS release and Pontoporeia hoyi densities implied that amphipods suppressed SRS release through mixing and burial of a surficial floe layer, where most dissolution of biogenic silica occurs. Moreover, SRS release rates measured from our coarse-grained nearshore sediments were comparable to rates reported for fine-grained offshore material and further implicate dissolution of surficial biogenic silica as the source of remineralized SRS. Because nearshore areas of Lake Michigan undergo strong seasonal variations in temperature and diatom production, and because significant riverine silica inputs exist, we cannot extrapolate our results on a lakewide, or season-long basis. The data, however, strongly imply that nearshore sediments are an important participant in the Lake Michigan silica cycle.     

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.     

Transport of Fine Sediment Through a Wetland Using Radionuclide Tracers: Old Woman Creek,OH

The Old Woman Creek estuary (OWC), a coastal wetland in Ohio, traps 47% of incoming suspended sediment and has a sedimentation rate of ∼1 cm/yr. Persistence of the OWC wetland and other coastal wetlands with high sedimentation rates seems problematic unless some previously trapped sediment is exported from the wetland.Suspended sediment, ⁷Be, and ²¹⁰Pb_xs budgets for a single runoff event in the OWC wetland were developed to understand short-term sediment dynamics. The budgets were balanced by subtracting the sum of the imports from the sum of the exports and attributing the difference to either deposition on, or resuspension from, the wetland bed. The wetland exported 118 ± 2%, 93 ± 1%, 74 ± 2% of the delivered sediment, ²¹⁰Pb_xs, and ⁷Be, respectively, during the studied event. The ⁷Be/²¹⁰Pb_xs ratios of the total suspended solids and bed sediment were distinct from one anther and used to quantify the relative proportions of recently delivered and resuspended bed material in the sediment efflux from the wetland. The ⁷Be/²¹⁰Pb_xs ratios suggest that 26 ± 20% of the sediment efflux was resuspended from the bed. While the wetland trapped 13 ± 3% of the sediment it received during the runoff event, resuspension and removal of previously deposited sediment in the wetland was sufficiently large to result in a net loss of sediment from the wetland during the event. Thus, the Old Woman Creek wetland is a sediment sink over the long-term, but can be a net exporter of sediment during single events.     

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.     

Contaminant Concentrations and Reproductive Rate of Lake Superior Bald Eagles, 1989–2001

We investigated the trend in contaminant concentrations in Lake Superior bald eagles (Haliaeetus leucocephalus) from 1989–2001, and examined the relationship of contaminant concentrations to eagle reproductive rate during that time. Concentrations of dichloro-diphenyl-dichloroethylene (DDE) and total polychlorinated biphenyls (PCBs) in nestling blood plasma samples decreased significantly from 1989-2001 (p = 0.007 for DDE, p = 0.004 for total PCBs). Mean contaminant concentrations in eaglet plasma, 21.7 μg/kg DDE (n=51) and 86.7 μg/kg total PCBs (n = 54), were near or below the estimated threshold levels for impairment of reproduction as determined in other studies. A preliminary assessment of polybrominated diphenyl ether (PBDE) concentrations indicated a mean of 7.9 μg/kg total PBDEs in Lake Superior eaglet plasma (n = 5). The number of occupied bald eagle nests along the Wisconsin shore of Lake Superior increased from 15 to 24 per year, between 1989 and 2001 (p < 0.001, r² = 0.70, n = 13 years). Eagle reproductive rate did not increase or decrease significantly between 1989 and 2001 (p = 0.530, r² = 0.037, n = 13 years, mean productivity = 0.96 young per occupied nest). The lack of correlation between reproductive rate and contaminant concentrations, as well as the comparison of contaminant concentrations to the estimated thresholds for impairment of reproduction, suggest that DDE and PCBs no longer limit the reproductive rate of the Lake Superior eagle population in Wisconsin.     

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.     

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     

Diatom-based Weighted-averaging Transfer Functions for Great Lakes Coastal Water Quality: Relationships to Watershed Characteristics

In an effort to develop indicators for Great Lakes near-shore conditions, diatom-based transfer functions to infer water quality variables were developed from 155 samples collected from coastal Great Lakes wetlands, embayments and high-energy shoreline sites. Over 2,000 diatom taxa were identified, and 352 taxa were sufficiently abundant to include in transfer function development. Multivariate data exploration revealed strong responses of the diatom assemblages to stressor variables, including total phosphorus (TP). Spatial variables such as lake, latitude and longitude also had notable relationships with assemblage characteristics. A diatom inference transfer function for TP provided a robust reconstructive relationship (r² = 0.67; RMSE = 0.28 log(μg/L); r²jackknife = 0.55; RMSEP = 0.33 log (μg/L)) that improved following the removal of 13 samples that had poor observed-inferred TP relationships (r² = 0.75; RMSE = 0.22 log(μg/L); r²_jackknife = 0.65; RMSEP = 0.26 log (μg/L)). Diatom-based transfer functions for other water quality variables, such as total nitrogen, chloride, and chlorophyll a also performed well. Measured and diatom-inferred water quality data were regressed against watershed characteristics (including gradients of agriculture, atmospheric deposition, and industrial facilities) to determine the relative strength of measured and diatom-inferred data to identify watershed stressor influences. With the exception of pH, diatom-inferred water quality variables were better predicted by watershed characteristics than were measured water quality variables. Because diatom communities are subject to the prevailing water quality in the Great Lakes coastal environment, it appears they can better integrate water quality information than snapshot measurements. These results strongly support the use of diatoms in Great Lakes coastal monitoring programs.     

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.     

Organochlorines,Mercury, and Selenium in Great Blue Heron Eggs from Indiana Dunes National Lakeshore,Indiana

In 1993, 20 great blue heron (Ardea herodias; GBH) eggs (one per nest) were collected from a colony at the Indiana Dunes National Lakeshore, Indiana (INDU). The eggs were artificially incubated until pipping and were then analyzed for organochlorines, mercury, and selenium. Livers of embryos were analyzed for hepatic microsomal ethoxyresorufin-O-dealkylase (EROD) activity. Brains were measured for asymmetry. Egg-laying began in early April and the mean clutch size was 4.2 eggs per clutch. Organochlorine concentrations were generally low (geometric mean p,p’-DDE = 1.6 /μg/g wet weight; polychlorinated biphenyl [PCB] = 4.9 μg/g); however, one egg had elevated concentrations of p,p -DDE (13 /μg/g) and PCBs (56 /μg/g). EROD activity in the embryos analyzed from INDU was not elevated. The frequency (11%) of brain asymmetry was low. Eggshells averaged 3.4% thinner than eggshells collected prior to the use of DDT. Mercury (geometric mean = 0.9 μg/g dry weight) concentrations in GBH eggs were within background levels. Selenium (4.0 μg/g dry weight) concentrations in eggs were above background levels, but below a concentration threshold associated with reproductive impairment.     

Anomalous Concentrations and Chlorination of Polychlorinated Biphenyls in Sediment Downwind of Lake Ontario

Relatively high concentrations (up to 98.5 ng/g dry wt) of polychlorinated biphenyls (PCBs) have been found in two cores penetrating fluvial sands east of Lake Ontario. One core was taken from the upper Salmon River reservoir on the Tug Hill Plateau and the other from Rice Creek near the lake at Oswego. In both instances, portions of the cores containing PCBs and other organochlorines (OCs) lack excess radiogenic ²¹⁰Pb and ¹³⁷Cs, implying depositional ages predating widespread OC commercial production, use, and release. Within each core there is little vertical variation in PCB composition and highly chlorinated congeners dominate the pattern. In addition, the PCB signature is analytically indistinguishable between cores despite collection over 50 km apart. This argues against local sources, and implies deposition and accumulation by processes operating over a substantial area and time period. Transport and introduction of a residual fraction of colloidal-bound PCBs to the sands via tannin-rich riverine waters is proposed.The PCB congener-specific pattern of air samples collected during the spring and summer of 1999 downwind of the lake at two locations (Rice Creek and Sterling) are similar to the sediments and display anomalous concentrations and chlorination with respect to other air sampling localities in the Great Lakes. A link between the residual fraction of PCBs observed in the sediment and air patterns via volatilization from the terrestrial surface is proposed. Elevated PCB concentrations and chlorination in non radiogenic fluvial sands and air may reflect regional accumulation and weathering processes operating over many decades and/or enhancement of contaminant deposition and partitioning downwind of the lake related to lake-effect precipitation.     

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.     

Factors in the Determination of Selected Trace Elements in Near-Shore U.S. Waters of Lakes Superior and Huron

Determinations of total As, Cr, Cu, Fe, Mn, and Pb in acidified unfiltered water samples from 48 stations representing the near-shore U.S. waters of Lake Superior, Lake Huron, and selected incoming streams gave overall mean concentrations and standard deviations (μg/L) of 1.0 ± 0.7, 0.9 ± 1.8, 2.0 ± 1.8, 236 ± 819, 14 ± 34, and 0.7 ± 0.8, respectively, using flameless atomic absorption analytical methods. Total concentrations of Ni and Hg were usually less than the instrumental detection limits (μg/L) of 0.8 and 0.02, respectively. Small losses of Hg, As, and Cr from solutions are possible in addition to a molecular spectral interference in the determination of As using the graphite furnace method. River waters usually were only slightly higher in total element concentrations than lake waters, except for Fe andMn. Nearshore waters of both lakes were very similar in elemental composition.     

Occurrence and distribution of organochlorine pesticides and polycyclic aromatic hydrocarbons in surface sediments from Qinghai Lake,northeast Qinghai–Tibet plateau,China

Organochlorine pesticides (OCPs) and polycyclic aromiatic hydrocarbons (PAHs) were analyzed in surface sediment samples collected from Qinghai Lake, the largest lake in China, which is situated in the northeast Qinghai–Tibet plateau at an altitude of 3200 m. The concentrations of these pollutants ranged from 0.02 to 1.00 ng/g for hexachlorocyclohexanes (HCHs), from not detected to 0.86 ng/g for dichlorodiphenyltrichloroethanes (DDTs), from 0.26 to 1.73 ng/g for OCPs, and from 366 to 966 ng/g for PAHs. The predominance of α-HCH suggests that long-range atmospheric transport is an important source for HCHs. A low α- to γ-HCH ratio (3.87 on average) indicates the possible usage of lindane in the drainage basin. The high percentage of p,p′-DDE and p,p′-DDD and the low percentage of o,p′-DDT indicated significant degradation from previous inputs, and no recent inputs of dicofol derived DDT. Based on the analysis of the component ratios, PAHs were found to be primarily from the combustion of biomass and coal-based fossil fuels. Using the Canadian sediment guidelines, PAHs are of greater ecological concern than OCPs in Qinghai Lake.     

Organochlorine Insecticides and PCB in the Surficial Sediments of Lake Superior (1973)

Analysis of 405 Lake Superior surficial sediments (0-3 cm depth) collected in 1973 were found to contain low levels of DDE, HEOD, and PCB, but no detectable mirex, heptachlor expoxide, endrin, endosulfan, or chlordane. p,p’-DDE was detected in 50%, HEOD in 9%, and PCB in 44% of the sediment samples at trace or low measurable levels. Mean residues for the whole lake were p,p’-DDE 0.71 ± 1.65 ng/g, HEOD below the detection level of 0.25 ng/g, and PCB 3.3 ± 5.7 ng/g. p,p’-DDE was more concentrated in depositional basins than in non-depositional zones while PCB levels were similar for the two zones.     

Spatial and temporal distribution of the cyanotoxin microcystin-LR in the Lake Ontario ecosystem: Coastal embayments,rivers, nearshore and offshore,and upland lakes

Cyanotoxins, a group of hepatotoxins and neurotoxins produced by cyanobacteria, pose a health risk to those who use surface waters as sources for drinking water and for recreation. Little is known about the spatial and seasonal occurrence of cyanotoxins in Lake Ontario and other lakes and ponds within its watershed. Within the embayments, ponds, rivers, creeks, shoreside, and nearshore and offshore sites of Lake Ontario, microcystin-LR concentrations were low in May, increased through the summer, and reached a peak in September before decreasing in October. Considerable variability in microcystin-LR concentrations existed between and within habitat types within the Lake Ontario ecosystem. In general, the average microcystin-LR concentration was two orders of magnitude lower in embayment (mean = 0.084 μg/L), river (mean = 0.020 μg/L), and shoreside (mean = 0.052 μg/L) sites compared to upland lakes and ponds (mean = 1.136 μg/L). Concentrations in the nearshore sites (30-m depth) and offshore sites (100-m depth) were another order of magnitude lower (mean = 0.006 μg/L) than in the creek/river, bay/pond, and shoreside habitats. Only 0.3% (2 of 581) of the samples taken in Lake Ontario coastal waters exceeded the World Health Organization (WHO) Drinking Water Guideline of 1 μg microcystin/L for humans. In contrast, 20.4% (20 of 98) of the samples taken at upland lakes and ponds within the watershed of Lake Ontario exceeded WHO Guidelines. No significant relationship between nitrate and microcystin-LR concentrations was observed in Lake Ontario even though a significant positive relationship existed between phosphorus and phycocyanin and microcystin-LR concentrations. At an upland lake site (Conesus Lake) in the Ontario watershed, the development of a littoral Microcystis population was not observed despite high nutrient loading (P and N) into the nearshore zone, well-developed nearshore populations of filamentous Spirogyra and Zygnema, the occurrence of Dreissena spp., and the known occurrence of Microcystis and microcystin production in the pelagic waters of Conesus Lake.