Recovery of Burrowing Mayflies (Ephemeroptera: Ephemeridae: Hexagenia) in Western Lake Erie

Burrowing mayflies (Hexagenia spp.) are native to western Lake Erie and were abundant until the 1950s, when they disappeared due to degraded water and sediment quality. Nymphs were absent from the sediments of most of western Lake Erie after the 1950s, although small, widely disjunct populations apparently persisted near shore. Sediment samples collected in 1993 revealed several small populations near the western and southern shores and beyond the mouths of the Detroit and Maumee rivers. A larger population was found in the southern island area, but nymphs were absent in the middle of the basin. By 1995, nymphs had spread throughout the western half and eastern end of the basin but remained absent from the middle of the basin. These data indicate that Hexagenia began recolonizing nearshore areas before offshore areas. Increasingly large swarms of winged Hexagenia on shore and over the lake between 1992 and 1994 further indicate that mayflies are recolonizing the basin. Factors that have permitted Hexagenia recovery in western Lake Erie probably include improved sediment and water quality attributed to pollution abatement programs implemented after the early 1970s, and perhaps environmental changes in the early 1990s attributed to effects of the exotic zebra mussel (Dreissena polymorpha)     

The effect of mayfly (Hexagenia spp.) burrowing activity on sediment oxygen demand in western Lake Erie

Previous studies support the hypothesis that large numbers of infaunal burrow-irrigating organisms in the western basin of Lake Erie may increase significantly the sediment oxygen demand, thus enhancing the rate of hypolimnetic oxygen depletion. We conducted laboratory experiments to quantify burrow oxygen dynamics and increased oxygen demand resulting from burrow irrigation using two different year classes of Hexagenia spp. nymphs from western Lake Erie during summer, 2006. Using oxygen microelectrodes and hot film anemometry, we simultaneously determined oxygen concentrations and burrow water flow velocities. Burrow oxygen depletion rates ranged from 21.7 mg/nymph/mo for 15 mm nymphs at 23 °C to 240.7 mg/nymph/mo for 23 mm nymphs at 13 °C. Sealed microcosm experiments demonstrated that mayflies increase the rate of oxygen depletion by 2–5 times that of controls, depending on size of nymph and water temperature, with colder waters having greater impact. At natural population densities, nymph pumping activity increased total sediment oxygen demand 0.3–2.5 times compared to sediments with no mayflies and accounted for 22–71% of the total sediment oxygen demand. Extrapolating laboratory results to the natural system suggest that Hexagenia spp. populations may exert a significant control on oxygen depletion during intermittent stratification. This finding may help explain some of the fluctuations in Hexagenia spp. population densities in western Lake Erie and suggests that mayflies, by causing their own population collapse irrespective of other environmental conditions, may need longer term averages when used as a bio-indicator of the success of pollution-abatement programs in western Lake Erie and possibly throughout the Great Lakes.     

Burrowing mayfly (Ephemeroptera: Ephemeridae: Hexagenia spp.) bioturbation and bioirrigation: A source of internal phosphorus loading in Lake Erie

Traditional lake eutrophication models predict lower phosphorus concentrations with decreased external loads. However, in lakes where decreased external phosphorus loads are accompanied by increasing phosphorus concentrations, a seeming “trophic paradox” exists. Western Lake Erie is an example of such a paradox. Internal phosphorus loads may help explain this paradox. We examined bioturbation and bioirrigation created from burrowing mayfly, Hexagenia spp., as a possible source of internal phosphorus loading. Phosphorus concentrations of experimental microcosms containing lake sediments, filtered lake water, and nymphs (417/m²) collected from western Lake Erie were compared to control microcosms containing sediments and lake water over a 7-day period. Phosphorus concentrations in microcosms containing Hexagenia were significantly greater than microcosms without nymphs. Further, we estimate the soluble reactive phosphorus flux from the sediments due to Hexagenia is 1.03 mg/m²/day. Thus, Hexagenia are a source of internal phosphorus loading. High densities of Hexagenia nymphs in western Lake Erie may help explain the “trophic paradox.” Furthermore, Hexagenia may be a neglected source of internal phosphorus loading in any lake in which they are abundant. Future studies of phosphorus dynamics in lakes with Hexagenia must account for the ability of these organisms to increase lake internal phosphorus loading.     

Spatial and seasonal variations and ecotoxicological significance of sediment trace metal concentrations in Kebir-Rhumel basin (Northeast of Algeria)

This study sought to assess sediment contamination by trace metals (cadmium, chromium, cobalt, copper, manganese, nickel, lead and zinc), to localize contaminated sites and to identify environmental risk for aquatic organisms in Wadis of Kebir Rhumel basin in the Northeast of Algeria. Water and surficial sediments (0-5 cm) were sampled in winter, spring, summer and autumn from 37 sites along permanent Wadis of the Kebir Rhumel basin. Sediment trace metal contents were measured by Flame Atomic Absorption Spectroscopy. Trace metals median concentrations in sediments followed a decreasing order: Mn > Zn > Pb > Cr > Cu > Ni > Co > Cd. Extreme values (dry weights) of the trace metals are as follows: 0.6-3.4 microg/g for Cd, 10-216 microg/g for Cr, 9-446 microg/g for Cu, 3-20 microg/g for Co, 105-576 microg/g for Mn, 10-46 microg/g for Ni, 11-167 microg/g for Pb, and 38-641 microg/g for Zn. According to world natural concentrations, all sediments collected were considered as contaminated by one or more elements. Comparing measured concentrations with American guidelines (Threshold Effect Level: TEL and Probable Effect Level: PEL) showed that biological effects could be occasionally observed for cadmium, chromium, lead and nickel levels but frequently observed for copper and zinc levels. Sediment quality was shown to be excellent for cobalt and manganese but medium to bad for cadmium, chromium, copper, lead, nickel and zinc regardless of sites.     

Distribution of Heavy Metals in Sediments of the Detroit River

The spatial distribution of 17 metals in the sediments of the Detroit River was established using metal concentrations from a river-wide survey. The survey (1999) was based on a stratified random sampling design that divided the river into upper, middle, and lower reaches and subsequently into U.S. and Canadian sides of the river. Results based on strong extraction, using concentrated acids, revealed that the Lowest Effect Level (LEL) for As, Cd, Cu, and Hg was exceeded at more than 75% of sampling sites and the Severe Effect Level (SEL) for As at 16.2% of sites. Most of the metals were homogenously distributed throughout reaches of the river, although sites with elevated concentrations were localized mainly along the middle and lower reaches as a result of a pattern of contamination sources and geographic complexity of the river, especially a spatial/temporal variability in water flow. A comparison of the results of a strong to a weak extraction (cold 5% acetic acid, to assess metal bioavailability) revealed two groups of sediment type. The first group with a “high” weak/strong ratio (bioavailable metals; about 1 for Ca, Mg, Na and from 0.6 to 0.4 for the rest of metals) was observed at sites with low flow velocities below 0.4 m s⁻¹. The “low” ratio (non-bioavailable metals; 0.25 for Ca, Mg, Na and from 0.15 to 0.05 for other metals) was observed at sites with flow velocities greater than 0.6 m s⁻¹. The data indicate that the sediment conditions, dependent on flow distribution, regulate not only the distribution of heavy metals but also can regulate metal bioavailability.     

Distribution and Abundance of Burrowing Mayflies (Hexagenia spp.) in Lake Erie, 1997–2005

Burrowing mayflies (Hexagenia limbata and H. rigida) recolonized sediments of the western basin of Lake Erie in the 1990s following decades of pollution abatement. We predicted that Hexagenia would also disperse eastward or expand from existing localized populations and colonize large regions of the other basins. We sampled zoobenthos in parts of the western and central basins yearly from 1997–2005, along the north shore of the eastern basin in 2001–2002, and throughout the lake in 2004. In the island area of the western basin, Hexagenia was present at densities ≤ 1,278 nymphs/m² and exhibited higher densities in odd years than even years. By contrast, Hexagenia became more widespread in the central basin from 1997-2000 at densities ≤ 48 nymphs/m² but was mostly absent from 2001-2005. Nymphs were found along an eastern basin transect at densities≤ 382/m² in 2001 and 2002. During the 2004 lake-wide survey, Hexagenia was found at 63 of 89 stations situated throughout the western basin (≤ 1,636 nymphs/m², mean = 195 nymphs/m², SE = 32, N = 89) but at only 7 of 112 central basin stations, all near the western edge of the basin (≤ 708 nymphs/m²), and was not found in the eastern basin. Hexagenia was found at 2 of 62 stations (≤ 91 nymphs/m²) in harbors, marinas, and tributaries along the south shore of the central basin in 2005. Oxygen depletion at the sediment-water interface and cool temperatures in the hypolimnion are probably the primary factors preventing successful establishment throughout much of the central basin. Hexagenia can be a useful indicator of lake quality where its distribution and abundance are limited by anthropogenic causes.     

Spatial-temporal patterns of recolonizing adult mayflies in Lake Erie after a major disturbance

This 12-year study of Hexagenia male imagos documents the recovery of two species of burrowing mayflies, Hexagenia limbata and Hexagenia rigida in western Lake Erie after a 30-year absence due to hypoxia, resulting from cultural eutrophication. Annual adult mayfly collections were made at night during the peak emergence period at four sites along the north shore of the western basin of Lake Erie, 1997 to 2008. H. rigida, the dominant species in upstream riverine waterbodies, was the early colonizer, representing about 90% of all male imagos sampled in 1997. In 2000, when the two species were co-dominant, both inland aerial dispersal (5.5 km) and lakeward (0.25 to 4 km) oviposition patterns confirmed species co-existence. Twice weekly collections throughout the extended emergence period at one site confirmed that H. rigida was the dominant species in 1997, H. limbata and H. rigida were co-dominant in 2000, and H. limbata was dominant in 2002. Once H. limbata became the dominant species (> 90%) in 2000 to 2002 (depending on the site), it remained so. Both species followed a similar inland dispersal pattern, decreasing in density with increasing distance from shore; most mayflies were present within 1 km from shore. There was no significant difference in mean egg density of the two species among the sites extending lakeward in 2000 when the two species were equally abundant. The transition from the dominance of H. rigida to H. limbata may have resulted from several factors, including differential competition and growth between species or predation effects.     

Spatial and temporal variation in the distribution of burrowing mayfly nymphs (Ephemeroptera: Hexagenia limbata and H. rigida) in western Lake Erie

In the early 1990s, burrowing mayfly species reappeared in sediments of the western basin of Lake Erie after an absence of over 30 years due to episodic hypoxia at the sediment–water interface. Long-term monitoring of adult mayflies at shoreline areas had revealed that Hexagenia rigida was more abundant than Hexagenia limbata during the initial recolonization period, but was gradually replaced by H. limbata. We hypothesized that this shift in dominance would be confirmed by the distribution and abundance of nymphs. We identified nymphs collected each spring throughout western Lake Erie from 1997 to 2004. The relative abundances of H. rigida and H. limbata nymphs exhibited the same temporal sequence as adults. Furthermore, the number of sites in the western basin in which H. rigida occurred decreased as the occurrence frequency of H. limbata increased. H. limbata were dominant in the basin by 2004. Hexagenia limbata nymphs persisted in the center-most part of the basin, whereas H. rigida did not, possibly due to differences in tolerance to hypoxia. There were no significant differences in body size between the two populations. Differences in dispersal distance from source populations and the timing and success of egg hatching likely accounted for the initial colonizing success of H. rigida, but the differential ability of H. limbata eggs to overwinter in sediments and possible tolerance of nymphs to hypoxia has possibly led to its current dominance in the western basin.     

The effects of dreissenid mussels on the survival and condition of burrowing mayflies (Hexagenia spp.) in western Lake Erie

Burrowing mayflies (Hexagenia limbata and H. rigida) are once again prominent members of the benthic community in western Lake Erie. However, this community is now dominated by dreissenid mussels. We conducted a laboratory experiment and field sampling to investigate whether survival and condition of Hexagenia were affected by the presence, density, and quality of dreissenid mussels. In a laboratory experiment, Hexagenia survival was higher in microcosms without dreissenid mussels. We also found Hexagenia density to be higher at field sites with low dreissenid density, suggesting that Hexagenia survival is higher in these areas as well. In microcosm treatments with low dreissenid density, Hexagenia survival was higher in treatments with live dreissenids than in treatments containing only dreissenid shells. These findings suggest that while dreissenid shells degrade the quality of soft sediments for Hexagenia, some of the negative effect is offset by the presence of live dreissenids. The positive effect of live dreissenids is likely due to additional food resources made available to Hexagenia by the filtering activity of dreissenids. Neither dreissenid density nor shell “type” (shells alone or live dreissenids in shells) had an effect on Hexagenia condition. Thus, the interactions between these dominant benthic invertebrates are complex. Recovery of Hexagenia populations in western Lake Erie is likely affected by both changing environmental conditions due to anthropogenic activities and the introduction of exotic species into the benthic community. The results are likely to be continued instability of the benthic food web and unpredictable consequences for human utilization of this ecosystem.     

A simple molecular assay differentiates two burrowing mayfly species, Hexagenia limbata and H. rigida (Ephemeroptera: Ephemeridae), in western Lake Erie

Burrowing mayflies (Hexagenia spp.) are important indicators of mesotrophic water conditions, and have been studied for several decades in conjunction with the changing trophic status of western Lake Erie. However, most studies on the biology of burrowing mayflies have failed to differentiate between immature H. limbata and H. rigida, the life stages that are most commonly collected. We developed a simple molecular assay to facilitate species-level identification of the two mayflies as an easier alternative to sequencing the COI gene. We analyzed a total of 152 cytochrome c oxidase, subunit I barcoding sequences from the Barcode of Life Data System (BOLD) for these mayfly species and developed a diagnostic primer set. We tested our primers on 46 random burrowing Lake Erie mayfly nymphs using both conventional PCR and qPCR. Results of the two methods were consistent and unambiguous. Sequencing of the COI barcoding region in the mayflies confirmed that the PCR assays correctly identified mayflies in all cases. Our assay is simple and effective, and will facilitate species-level identification of burrowing mayflies in future studies on their biology where the two species co-occur.     

Surficial Sediment Contamination in Lakes Erie and Ontario: A Comparative Analysis

Sediment surveys were conducted in Lakes Erie and Ontario to characterize spatial trends in contamination, to assist in elucidation of possible sources of contamination, and for identification of areas where contamination exceeded Canadian sediment quality guidelines for protection of aquatic biota. Sediment levels of metals including nickel, lead, zinc, chromium, and copper were compared to pre-colonial concentrations, and sediment enrichment factors, defined as the ratio of surficial concentrations to background concentrations determined from benthos cores, were calculated. Sediments in Lake Ontario exhibited elevated contamination compared to Lake Erie. The average enrichment factor for Lake Ontario (2.6) was comparable to the western basin in Lake Erie but greater than those for the central (1.3) and eastern (1.0) basins. There was a gradient toward decreasing sediment contamination from the western basin to the eastern basin of Lake Erie, and from the southern to the northern area of the central basin. Sediment contamination in Lake Ontario was similarly distributed across the three major depositional basins. The spatial distribution of metals was similar to those of other contaminants including mercury, polychlorinated biphenyls (PCBs), and polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/PCDFs). Lake-wide averages of sediment mercury, PCBs and PCDDs/PCDFs in Lake Erie were 0.185 μg/g, 96.5 ng/g, and 18.8 pg/g TEQs, respectively. Lake-wide averages of sediment mercury, PCBs and PCDDs/PCDFs in Lake Ontario were 0.586 μg/g, 100 ng/g, and 101 pg/g TEQs, respectively.     

A 200 year chronology of burrowing mayflies (Hexagenia spp.) in Saginaw Bay

After an absence of 50 years, burrowing mayflies (Hexagenia spp.) colonized western Lake Erie which led to interest in whether this fauna can be used to measure recovery in nearshore waters throughout the Great Lakes. However, in many areas we do not know if mayflies were native/endemic and thus, whether recovery is a logical measure to assess progress of recovery. In the present study, we construct a chronologic record of relative abundance of burrowing mayflies in Saginaw Bay by the use of mayfly tusks and radionuclides in sediments (i.e., a paleoecologic record) and historic records of mayfly nymphs in the bay. These records reveal that mayflies: (1) were few before 1799, which indicates that nymphs were probably native/endemic in the bay, (2) increased between 1799 and 1807 and remained at relatively high levels between 1807 and 1965, probably in response to increased nutrient run-off from the watershed, (3) declined dramatically between 1965 and 1973, probably as a result of excessive eutrophication in the mid-1950s; and, (4) were few and highly variable between 1973 and 2001, probably as a result of low and unstable abundances of mayfly nymphs. Historic records verify that nymphs disappeared in the bay in the late-1950s to early-1960s which is in agreement with the paleoecologic record. Reoccurrence of low abundances of nymphs in the bay between 1991 and 2008 and comparison of chronologic records of nymphs in Saginaw Bay and western Lake Erie suggest that mayflies may return to Saginaw Bay in the early-21st century. Undoubtedly, watershed conservation and three decades of pollution abatement have set the stage for a recovery of burrowing mayflies in Saginaw Bay, and possibly in other areas of the Great Lakes.     

Spatial Distributions and Temporal Trends in Sediment Contamination in Lake St. Clair

The sediments of Lake St. Clair were surveyed in 2001 for a range of compound classes including metals (such as total mercury and lead), polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins and dibenzofurans, organochlorine pesticides, polycyclic aromatic hydrocarbons, and short- and medium- chain chlorinated paraffins, in order to evaluate the spatial distribution and temporal trends of contamination. Concentrations of contaminants were generally low compared to the lower Great Lakes (Erie and Ontario), and were typically below the Canadian Sediment Quality Probable Effect Level (PEL) guidelines. The only exceptions were for mercury and DDE, where concentrations exceeded their respective PEL at one of the thirty-four sites sampled. With the exception of mercury, it was difficult to interpret spatial trends in contaminant concentrations due to these low levels, although relatively elevated concentrations of several contaminants were found in L’Anse Creuse Bay and at the outflow of the Thames River. In the case of mercury, historically-contaminated sediments in the St. Clair River associated with chlor-alkali production appeared to contribute to loadings to Lake St. Clair. There have been substantial reductions in sediment contamination in Lake St. Clair over the past three decades, as determined through sediment core profiles as well as through comparison of current data to those from historical surveys conducted in the early 1970s. These results indicate that management actions to reduce contaminant loadings to Lake St. Clair have been generally successful.     

Sediment Contamination in Lake Erie: A 25-Year Retrospective Analysis

Lake Erie sediment surveys were conducted in 1997 and 1998 to characterize spatial and temporal trends in contamination and for comparison with historical levels to assess the degree of improvement in environmental quality since the advent of measures to reduce impacts from sources. These surveys were also designed to assist in identification of possible sources of contamination and areas where contamination exceeded Canadian sediment quality guidelines for protection of aquatic biota. Encouragingly, lakewide contaminant concentrations were found to have significantly decreased from levels observed in samples collected in 1971 in previous Environment Canada surveys. The lakewide average polychlorinated biphenyl (PCB) sediment concentrations decreased from 136 ng/g in 1971 to 43 ng/g in 1997. This decreasing temporal trend was also evidenced by contaminant profiles of core samples from all three major basins. There was a lakewide spatial trend in increasing sediment contamination from the eastern basin to the western basin, and from the north-central basin to the south-central basin. Sediments in many areas of Lake Erie still exceeded Canadian Federal and Provincial sediment quality guidelines. However, exceedences of sediment guidelines describing contaminated environments in 1997/98 were largely restricted to the western basin and the southern portion of the central basin. Exceedences of Canadian Sediment Quality probable effects guidelines were most numerous for dioxins and furans (40%) followed by mercury (6%). The Canadian threshold effects guideline for PCBs (34.1 ng/g) and the Provincial lowest effect guideline (70 ng/g) were exceeded at 52% and 22% of the sites, respectively. Mercury, PCBs and polychlorinated dibenzo-p-dioxins and dibenzofurans are responsible for fish consumption advisories in Lake Erie.     

Spatial Distributions of Legacy Contaminants in Sediments of Lakes Huron and Superior

The sediments of Lake Superior, Lake Huron, and Georgian Bay were sampled in 2001 and 2002 in order to evaluate the extent of surficial sediment contamination of polychlorinated biphenyls (PCBs), organochlorine pesticides (OCs), polycyclic aromatic hydrocarbons (PAHs), and metals. Sediment concentrations of PCBs, OCs, PAHs, and mercury were generally low and up to 2 orders of magnitude less than in Lakes Erie and Ontario. In contrast, concentrations of metals such as arsenic, copper, and nickel were comparable to those in Lakes Erie and Ontario. These elevated Lakes Superior and Huron metal concentrations were attributed to naturally occurring metals within the bedrock, soil, and sediment of the study region. Concentrations of all contaminants were typically below the Canadian Sediment Quality Probable Effect Level (PEL) guidelines. With regard to spatial patterns, most contaminants were focused primarily in the depositional basins and atmospheric deposition was likely the major source of these chemicals to the lakes. The major exception was for metals (not including mercury) whose patterns were also influenced by natural sources as well as extensive mining activity. A comparison between surficial sediment contamination of samples collected as part of this survey and those collected in the late 1960s/early 1970s using similar methods showed that concentrations of DDT, PCB, lead, and mercury were generally similar between these two time periods. These results are not consistent with production and usage patterns that have declined substantially in the past 3 decades. We hypothesize that the lack of temporal trends is an artifact due to slow sediment accumulation rates as well as differences in analytical protocols between the two time periods.     

Toxicity of Detroit River Sediment-Bound Contaminants to Ultraplankton

Four sites in the Detroit River/Lake Erie western basin were evaluated for their toxicity. The evaluation was based on 1) bulk chemical characterization of the sediments, 2) chemical composition of the sediment elutriates, and 3) toxicity of the elutriates to ultraplankton and microplankton/net plankton. A sequence of decreasing contamination was determined from the chemical composition of the elutriates based on the elutriation release of metals such as Zn, Mn, Cd, Ni, and Co. Bioassessment of elutriate toxicity was determined by carbon-14 Algal Fractionation Bioassays (AFB's) which were conducted with various dilutions of standard and Chelex-100 treated elutriates. Site A (near Windsor, Ontario) and Site D (western Lake Erie) were found to be toxic to ultraplankton. The observed toxicity was attributed to the bioavailability and synergistic impact of elutriated metals on ultraplankton production. A direct relationship between the water soluble metal fraction and toxicity was observed. These results confirmed that sediment toxicity should not be evaluated solely on bulk chemical composition of the sediments. The AFB's have been proven useful in the bioassessment of sediments due to their rapidity/sensitivity and hence could be routinely used for the screening and early detection of contaminants affecting fast growing organisms which form the basis of the aquatic food chain.     

Biogeochemical Investigation of Big Creek Marsh,Lake Erie,Ontario

Geochemical composition of sediment, uptake of nutrients and metals by macrophytes, and nutrient and metal concentration in the water was investigated at six selected stations in Big Creek Marsh, Ontario. The maximum concentrations of Pb, Ni, Cu, Cr, and Zn in marsh sediment (86, 48, 38,119, and 383 μg/g dry weight, respectively), were lower than the concentration of these metals found in fine-grained surficial sediments of Lake Erie, and maximum concentration of As and Hg in marsh sediment (112 and 0.930 μg/g dry weight, respectively) were higher than As and Hg concentration in most of Lake Erie surficial sediments. Occurrence of DDT metabolites in marsh sediment reflects extensive use of DDT in the Big Creek drainage basin prior to 1970. Increase of total Kjeldahl Nitrogen and NH₄-N in marshwater was observed at a few sampling stations in November. Submerged macrophytes Myriophyllum, Chara, and Elodea sps. accumulated larger quantities of Pb, Cu, Ni, Cd, and Cr than emergent macrophytes at the stage of maximum development. Correlation between Ca and Pb, Cu, Ni, and Cr concentration in sampled macrophytes was observed; however, there was no correlation between Ca and Zn in these plants.     

Aromatic Hydrocarbons in Biota from the Detroit River and Western Lake Erie

Polynuclear aromatic hydrocarbons (PAHs) and PCBs in zebra mussels were elevated to concentrations greater than 5,000 ng/g lipid and 15,000 ng/g lipid, respectively, at the Ambassador Bridge in the Detroit River and concentrations gradually declined at downstream locations, which included three stations in the western basin of Lake Erie (Middle Sister Island, East Sister Island, Pelee Island). PCB concentrations in zebra mussels collected at the stations in western Lake Erie were elevated relative to the concentrations in mussels at the upstream end of the Detroit River (Stoney Point). There is no evidence that PAH contamination in the Detroit River elevated PAH concentrations in zebra mussels in western Lake Erie relative to mussels at Stoney Point. Fluorescent aromatic compounds (FACs) representing metabolites of PAHs were analyzed in the bile of gizzard shad (Dorosoma cepedianum) and freshwater drum (Aplodinotus grunniens) collected from several sites in the Detroit River and western Lake Erie. Mean FAC concentrations were >l,000 ng BaP equivalents per mL of bile in fish from the Trenton Channel and Boblo Island in the Detroit River, but FAC data provided no evidence that fish captured at two sites in western Lake Erie (East Sister Island, Pelee Island) were exposed to elevated concentrations of PAHs through ingestion of contaminated biota or exposure to contaminated sediments.     

Non-steady state behaviour of heavy metals in contaminated freshwater sediments

The behaviour of heavy metals has been investigated in contaminated sediments of the river Meuse, The Netherlands. Due to temporal changes in temperature and degradability of organic matter, the depths of the redox boundaries fluctuate. This contributes to a non-steady state. As a result of oxidation processes, a distinct peak in heavy metal concentrations in pore water is measured at the sediment-water interface. Because the studied anoxic sediments contain low levels of sulphide, other solid phases are expected to be of importance in the binding of heavy metals. Furthermore, heterogeneity of the sediment and complexation with dissolved organic compounds may result in supersaturation of the anoxic pore waters with respect to discrete heavy metal sulphides, thus influencing heavy metal mobility. Calculations using concentration gradients of heavy metals indicate that diffusive fluxes between the sediment and the surface water contribute to concentrations in the surface water, although significant effects may be confined to specific locations.