Stable Carbon and Oxygen Isotope Records From Lake Erie Sediment Cores: Mollusc Aragonite 4600 BP–200 BP

We present the first δ¹⁸O and δ¹³C data from mollusc aragonite from Lake Erie for the 4.6–0.2 ka (4600 BP to 200 BP ¹⁴C yrs) time interval and describe single and composite species isotope trends. Composite species δ¹⁸O data show an almost 2.0%o increase from 3.3 to 3.0 ka followed by a nearly 2.5%o decrease at 2.8 ka. Oxygen isotope values then fluctuate by < l‰ until 0.2 ka. This trend in oxygen values is also evident in single species analyses of Sphaerium striatinurn. The most dramatic changes in isotope values, which occur from 3.3–2.8 ka, may reflect a pattern of water level changes in Lake Erie which occurred during the Nipissing flood and its lower water aftermath. Carbon isotope data show progressively more ¹³C enriched values from 4.6 ka (averaging–6.5‰ PDB) to the present (–0.57‰). This trend may reflect the dilution of isotopically light CO₂ from the oxidation of organic matter due to rising lake levels. The short-term increase in δ¹⁸O values is coupled with a corresponding decrease in δ¹³C values. A similar pattern for Lake Erie prior to 10.5 ka was also associated with a lowering of water levels in the lake.     

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.     

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.     

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.     

A Comparison of Photosynthate Allocation in Lakes

We compared the relationships between photosynthate allocation to protein, carbohydrate, lipid and low molecular weight (LMW) fractions and the variables daylength and water temperature in Lakes Huron, Michigan, and Ontario as well as three smaller headwater lakes in the Lake Ontario drainage. In all lakes investigated the allocation of recently produced photosynthate to carbohydrate was strongly related to daylength (% carbohydrate = −3.5 * daylength (hr) + 72.8; n = 59, r² = 0.56). The percentage of photosynthate allocated to protein was a function of water temperature in all lakes although the y-intercept for the protein-temperature relationship was much lower in the three headwater lakes and Lake Ontario (% protein = 0.50 * temperature (°C) + 6.1; n = 37, r² = 0.52) than in Lake Huron and Lake Michigan (% protein = 0.68 * temperature (°C) + 24.2; n = 23, r² = 0.49). The increase in allocation to protein was related to a decrease in allocation to low molecular weight material (% LMW = −1.1 * % protein + 57.13; n = 60, r² = 0.72). The percentages of photosynthate in lipid and LMW material were not related to any of the environmental variables measured. Assuming that photosynthate allocation is related to biochemical composition, the phytoplankton in Lakes Huron and Michigan were more protein rich for a given temperature than those in Lake Ontario and in the smaller inland lakes. The protein deficit was due to an increase in allocation to LMW material.     

Chemical and Isotopic Tracer Evaluation of Water Mixing and Evaporation in a Dammed Texas River During Drought

The interaction between drought and river regulation is monitored to better understand river flow mixing, evaporation and surface‐groundwater exchange in changing regional climates and in increasingly regulated waterways. This study compared Brazos River stable isotope (δ¹⁸O and δD) and electrical conductivity values with reservoir, creek and aquifer samples in the Brazos watershed, the largest watershed in Texas. The combination of tributaries, rainfall and the Brazos River Alluvium Aquifer, on the one hand, and the Lake Whitney reservoir, on the other hand, represent endmembers of dilute run‐off water and evaporated saline water, respectively. A simple isotope mixing model that uses monthly river discharge, Lake Whitney discharge, historical monthly precipitation δ¹⁸O and pan evaporation accurately reconstructs river δ¹⁸O (±0.5‰ on average). Data and isotope balance modelling support continued evaporation of ¹⁸O‐enriched Lake Whitney water as it flows downstream, although the most evaporation took place in Lake Whitney. The difference between river and precipitation δ¹⁸O, or Δ¹⁸O_RIV‐_PPT, here a measurement of degree of evaporation, ranged from ?0.1‰ for a small creek, to 1.7‰ for the Brazos River, to at least 2.7‰ in Lake Whitney. This study indicates that drought in regulated rivers may enhance reservoir discharge dominance in river flows during peak drought conditions when combined run‐off and baseflow dominance would be expected in a similar undammed river. Copyright © 2016 John Wiley & Sons, Ltd.     

Values of Sediment Oxygen Demand Measured in the Central Basin of Lake Erie, 1979

The results of sediment oxygen demand (SOD) measurements for the central basin of Lake Erie, 1979, are presented. Two chambers were used. One, a triangular chamber, has a mixing velocity of 5 cm/sand gives values for SOD of 0.86 ± 0.42 gm²/d (n = 52). The second chamber, a hemispherical dome with gentle mixing, gives values of 0.32 ± 0.11 g/m²/d (n = 13). There are no significant differences in measured values between two stations, located 50 km apart, when measurements from the same chamber design are compared. There are no measurable effects of photosynthesis when daytime values are compared with nighttime values or when light and dark chambers are compared. A comparison of these two SOD values with rates observed for hypolimnetic oxygen decline in the water column shown that the value measured by the dome (0.32 gm/m²/d) is the most plausible value. It is hypothesized that the fluid mechanics of the triangular chamber do not properly emulate the hydrodynamics of the lake, causing the higher values.     

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.     

Kinetics of Red Clay Bluff Dissolution in Western Lake Superior

The dissolution of red clay bluff samples from the southwestern Lake Superior shoreline area in Lake Superior water or deionized water is studied by following the aqueous concentrations of reactive silica over a three month period. The dissolution process is initially rapid, followed by a first-order dissolution process (k = 9.4 × 10^?7 sec^?1) up to about thirty days. After about 30 days, the rate of dissolution of the bluff material follows linear kinetics (k = 5.4 × 10^?8 mg SiO₂/gram of bluff per second).     

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.     

新疆鄯善县水环境氢氧同位素特征及其指示作用

根据鄯善县氢氧稳定同位素资料及氘盈余（d）值,分析了地表与地下水体的δD、δ18O和d值的分布规律,并得到地下水的主要补给来源及其与地表水的相互作用关系;地下水的δD在-87.88‰～-65.97‰间,δ18O在-9.4‰～-12.37‰间;地表水的δ18O在-10.08‰～-13.63‰间,d在14.12‰～32.19‰间。结果表明：鄯善县地下水和地表水同源于大气降水,且经历了较强的蒸发作用;地下水与地表水之间的水力联系较弱,深层地下水主要接受河水在洪积扇区的入渗补给,浅层地下水主要接受河流引水灌溉入渗;不同深度地下水之间的水力联系较为密切,为统一的地下水系统。     

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.     

Sensitivity of Ancient Lake Ohrid to Local Anthropogenic Impacts and Global Warming

Human impacts on the few ancient lakes of the world must be assessed, as any change can lead to an irreversible loss of endemic communities. In such an assessment, the sensitivity of Lake Ohrid (Macedonia/Albania; surface area A = 358 km², volume V = 55 km³, > 200 endemic species) to three major human impacts—water abstraction, eutrophication, and global warming—is evaluated.It is shown that ongoing eutrophication presents the major threat to this unique lake system, even under the conservative assumption of an increase in phosphorus (P) concentration from the current 4.5 to a potential future 9 mg P m⁻³. Eutrophication would lead to a significant reduction in light penetration, which is a prerequisite for endemic, deep living plankton communities. Moreover, a P increase to 9 mg P m⁻³ would create deep water anoxia through elevated oxygen consumption and increase in the water column stability due to more mineralization of organic material. Such anoxic conditions would severely threaten the endemic bottom fauna. The trend toward anoxia is further amplified by the predicted global warming of 0.04°C yr^–1, which significantly reduces the frequency of complete seasonal deep convective mixing compared to the current warming of 0.006°C yr⁻¹. This reduction in deep water exchange is triggered by the warming process rather than by overall higher temperatures in the lake. In contrast, deep convective mixing would be even more frequent than today under a higher temperature equilibrium, as a result of the temperature dependence of the thermal expansivity of water. Although water abstraction may change local habitats, e.g., karst spring areas, its effects on overall lake properties was shown to be of minor importance.     

Development of a Bioenergetics Model for the Round Goby (Neogobius melanostomus)

The round goby (Neogobius melanostomus) was introduced into the Great Lakes in the 1990s through ballast water transfer. Gobies are potential vectors for material transfer between the benthic and pelagic zones. A bioenergetics model was developed for round gobies to enable us to quantify the flow of energy, contaminants, and nutrients from the benthos to pelagic fishes. Weight- and temperature-dependent coefficients for metabolism and consumption were derived. Food consumption increased with temperature up to 26°C before sharply decreasing, and weight-specific consumption decreased with increasing fish weight (allometric coefficient = −0.256, SE = 0.160). Specific oxygen consumption was inversely related to body mass (allometric coefficient = −0.157, SE = 0.025) and increased exponentially with temperature. Estimated Q₁₀ for respiration was 1.84. Additional parameters were obtained from the literature to describe specific dynamic action, egestion, excretion, and reproductive losses. The bioenergetics model explained growth of round gobies in Lake Erie and in their native range.     

Evidence supporting the importance of nutrient regeneration by nano- and micrograzers for phytoplankton photosynthesis in Lake Malawi/Nyasa

Photosynthesis and nutrient status measurements were compared in size fractioned water samples collected in southern Lake Malawi in three different seasons. The size fractions analyzed were 0–2.0 μm, 0–20 μm, and 0–200 μm. Total chlorophyll concentrations were relatively invariant at about 1 μg L^?1 in all seasons (wet stratified, deep mixing, dry stratified). Over 90% of total chlorophyll was < 20 μm and 40–50% < 2 μm. Stoichiometric ratios of carbon (C):nitrogen (N) and C:phosphorus (P) for the < 2-μm seston were similar to < 20-μm and < 200-μm seston samples and indicated that all size fractions were moderately N and P deficient in all seasons. N and P uptake experiments demonstrated that when the < 2-μm picoplankton were isolated from the larger sizes, they showed high N and P nutrient debts, apparently in response to the removal of nano- and micrograzers. This effect was strongest for N debt in the dry stratified season, but was apparent for P debt in all seasons. The chlorophyll-normalized light saturated rate of photosynthesis, P^b_m, was always higher in whole water samples compared to the rates of the isolated picophytoplankton which were lowest in the dry season. We infer that nano- and micrograzers were an especially important source of regenerated nutrients when the lake was stratified although their removal as a source of nutrient to the picophytoplankton may affect P^b_m in all seasons. Nitrogen regeneration was especially critical to the picophytoplankton in the dry stratified season but less important in the windy deep mixing season.     

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.     

Sediment geochemistry and contributions to carbon and nutrient cycling in a deep meromictic tropical lake: Lake Malawi (East Africa)

The biogeochemical functioning of large tropical lakes differs substantially from temperate lakes, yet remains poorly understood. We characterized the carbon, nitrogen, and phosphorus cycling in the water column and sediments of a deep meromictic tropical Lake Malawi (East Africa) by measuring geochemical distributions and compiling whole-lake geochemical budgets. Four locations (100 to 650 m water depth) were characterized. The results reveal that sediments contribute significantly to lake-wide biogeochemical budgets. Sedimentation rates have significantly increased in recent decades. While the export efficiency of organic matter from photic zone to deep sediments is low (14%), organic carbon is buried in the anoxic sediments with high efficiency (27–46%). Area-specific rates of carbon mineralization (4.1 mmol m^? 2 d^? 1) are similar to those in temperate well-oxygenated large lakes and marine sediments in similar water depths. Ammonium effluxes from sediments (0.44 mmol m^? 2 d^? 1) contribute 29% to the total nitrogen inputs into the water column, while sediment denitrification (0.035 mmol m^? 2 d^? 1) and burial of organic nitrogen (0.27 mmol m^? 2 d^? 1) remove 28% of total inputs in the lake. The recycling efficiency of phosphorus in anoxic sediments is high (73%). P effluxes average 0.037 mmol m^? 2 d^? 1, suggesting a large and previously unquantified contribution (42%) to water column P inputs. The results underscore the importance of sediments in the geochemical budgets of even large lakes and suggest trends in lacustrine carbon cycling that hold across a wide range of environments.     

Algal Organic Carbon Excretion in Lake Michigan

Algal external metabolites have been claimed to be an important carbon source for aquatic heterotrophic bacteria. We measured the release by algae of recently-fixed carbon by following the accumulation of¹⁴C-organics infiltrates of Lake Michigan samples incubated under natural light for 8 to 26 hours. Pretreatment of samples with an antibiotic and a suite of nonradioactive amino acids, to prevent microbial uptake of excreted products, did not affect the apparent release rates which ranged between 2% and 21% (X±SD = 11.34±9.32 mg-C m⁻³day ⁻¹) of short-term autotrophic production. Comparison of our release rates of 0.42 to 1.54 mg-C m⁻³ d⁻¹ (X±SD = 0.84±.40 mg-C m⁻³ day⁻¹) to estimates of bacterial demand made during a simultaneous study suggests that organic carbon released from recently-fixed internal pools may not alone support bacterial production.     

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.     

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.