Erie's Western Basin Research Articles

Seasonal Carbon Budget Succession in Lake Erie's Western Basin

AbstractLake Erie's Western Basin is a eutrophic region and likely hotspot for carbon transformation. While this basin has received much attention for its high nutrient loads from the Maumee River and recurring harmful algal blooms, carbon has gone understudied. To investigate the seasonal and spatial variability in inorganic and organic carbon budgets, we completed three surveys in spring, summer, and fall on a transect from the Maumee River to South Bass Island. In each survey, we observed higher spatial variability of all carbon species within 11 km of the Maumee River mouth relative to sites outside of Maumee Bay. This variability was driven by pulses of direct river water carbon, steep nutrient gradients, and patchy bloom conditions. Seasonal variability was also greater in Maumee Bay, with the highest river discharge in June adding large amounts of dissolved inorganic and organic carbon and pCO2 flux out of the water when productivity from the diatom bloom was smaller. In August, when and where we observed a Microcystis bloom, particulate organic carbon increased in concentration, and pCO2 flux switched directions into the water. In October, Chl‐a concentrations and oxygen saturations were lowest, indicating a seasonal slowdown in productivity, and river discharge was the lowest, resulting in the lowest total carbon observed and dissolved organic matter chemistry indicating less contribution from the terrestrial watershed. In the open water outside of Maumee Bay seasonal and spatial carbon budget dynamics were more stable, highlighting the importance of riverine inputs on lake carbon cycling.

Impacts of dreissenid mussel growth and activity on phytoplankton and nutrients in Lake Erie's western basin

Abstract To examine the potential impact of invasive dreissenid mussels on in situ populations of phytoplankton and nutrients in western Lake Erie, we combined mussel population estimates from a 2018 survey, results from mussel excretion, grazing, and in situ growth experiments, along with nutrient measurements on collected lake water. We calculated the proportion of the water column filtered per day, based on both clearance rates from grazing experiments and mussel biomass. In most cases the water column was filtered less than once per day. Based on mussel densities from nearby survey sites, we found that mussels could be expected to clear less than 5% of phytoplankton from the water column each day. We combined measurements of nitrogen and phosphorus excretion by mussels with survey densities and found that concentrations of nitrogen and phosphorus from excretion were much less than the ambient inorganic nitrogen and phosphorus measured throughout the season. Despite the modest potential impact that we measured, spatial variability in mussel density and temporal variability in nutrients and seston suggest that more substantial impact likely occurs in some conditions. Lastly, we used a mass balance approach to compare flows of nitrogen and phosphorus attributable to mussel assimilation, growth, and excretion. The proportion of assimilated nitrogen (0.01-0.21) and phosphorus (0.007-0.08) due to growth changed markedly throughout the season, but the excretion rate sometimes exceeded the apparent assimilation rate. These differences in growth:assimilation suggest changes in food quantity or quality, fluctuations in growth rates over time, or other physiological effects can lead to short-term imbalance in nutrient cycling by mussels, which could lead to locally important impacts on phytoplankton and algal blooms. Moreover, this work underscores the importance of mapping mussel densities at fine spatial scales and across interannual variation.

The Fate and Bioavailability of Resuspended Phosphorus in Lake Erie's Western Basin

The Fate and Bioavailability of Resuspended Phosphorus in Lake Erie's Western Basin

Improved red-edge chlorophyll-a detection for Sentinel 2

Chlorophyll-a (chl-a) concentration is an indicator of algal biomass. The Sentinel 2 platform offers greatly improved spatial resolution over other satellite platforms designed for water based chl-a retrievals and includes a “red-edge” band at 704 nm not present on the Landsat 8 operational land imager. This study provides validation of an improved version of a well known semi-analytical chl-a retrieval algorithm. The algorithm is provided with several free image processing utilities and the improved approach can be implemented with minimal technology skills. The improved performance is the result of replacing a fixed chl-a specific absorption coefficient (a*) with a variable model. This method was applied to three Sentinel 2 images taken over the Lake Erie western basin correlating with an in-situ dataset of 24 samples where chl-a ranged from 1.89 mg m−3 to 70.20 mg m−3. The variable a* model produced chl-a retrievals with normalised root mean squared error of prediction (NRMSEP) = 7.5%, bias = -0.47 mg m−3, coefficient of determination (R2) = 0.91 and Nash-Sutcliffe efficiency (NSE) = 0.90). This represented a 23% reduction in NRMSEP, an 85% reduction in bias and an increase in NSE of 7% over the default algorithm using a fixed a* value. Creation of chl-a retrieval algorithms that consider the variability in a* should result in algorithms that perform better against a wide range of chl-a concentrations and are less likely to require local recalibration. Obtaining accurate chl-a retrievals from a satellite platform with the spatial resolution of Sentinel 2 will allow satellite monitoring of many more inland waters than previously possible.

RAD‐Seq Refines Previous Estimates of Genetic Structure in Lake Erie Walleye

AbstractDelineating population structure helps fishery managers to maintain a diverse “portfolio” of local spawning populations (stocks), as well as facilitate stock‐specific management. In Lake Erie, commercial and recreational fisheries for Walleye Sander vitreus exploit numerous local spawning populations, which cannot be easily differentiated using traditional genetic data (e.g., microsatellites). Here, we used genomic information (12,264 polymorphic loci) generated using restriction site‐associated DNA sequencing to investigate stock structure in Lake Erie Walleye. We found low genetic divergence (genetic differentiation index FST = 0.0006–0.0019) among the four Lake Erie western basin stocks examined, which resulted in low classification accuracies for individual samples (40–60%). However, more structure existed between western and eastern Lake Erie basin stocks (FST = 0.0042–0.0064), resulting in greater than 95% classification accuracy of samples to a lake basin. Thus, our success in using genomics to identify stock structure varied with spatial scale. Based on our results, we offer suggestions to improve the efficacy of this new genetic tool for refining stock structure and eventually determining relative stock contributions in Lake Erie Walleye and other Great Lakes populations.

Climate Change and Nutrient Loading in the Western Lake Erie Basin: Warming Can Counteract a Wetter Future.

In the past 20 years, Lake Erie has experienced a resurgence of harmful algal blooms and hypoxia driven by increased nutrient loading from its agriculturally dominated watersheds. The increase in phosphorus loading, specifically the dissolved reactive portion, has been attributed to a combination of changing climate and agricultural management. While many management practices and strategies have been identified to reduce phosphorus loads, the impacts of future climate remain uncertain. This is particularly the case for the Great Lakes region because many global climate models do not accurately represent the land-lake interactions that govern regional climate. For this study, we used midcentury (2046-2065) climate projections from one global model and four regional dynamically downscaled models as drivers for the Soil and Water Assessment Tool configured for the Maumee River watershed, the source of almost 50% of Lake Erie's Western Basin phosphorus load. Our findings suggest that future warming may lead to less nutrient runoff due to increased evapotranspiration and decreased snowfall, despite projected moderate increases in intensity and overall amount of precipitation. Results highlight the benefits of considering multiple environmental drivers in determining the fate of nutrients in the environment and demonstrate a need to improve approaches for climate change assessment using watershed models.

The influence of body size and season on the feeding ecology of three freshwater fishes with different diets in Lake Erie

Isotopic niche and length-isotope relationships were quantified across the body size of three similarly-sized fish species with different feeding strategies as adults (benthivore: freshwater drum, Aplodinotus grunniens; piscivore: walleye, Sander vitreus; and omnivore: white perch, Morone americana) in Lake Erie's western basin using δ13C, δ15N and δ34S. Stable isotopes demonstrated that resource utilization changed with body size for all three species and length-isotope relationships varied with season. Isotopic niche overlap was lower when modelled with three isotopes (δ13C, δ15N, and δ34S) than with two (δ13C and δ15N), providing greater resolution of feeding ecology among the species. Based on isotopic niches, there was significant overlap in resource use among species and size classes in spring, but overlap decreased in the fall for both. In this study, freshwater fish species with different adult feeding strategies partition resources through different mechanisms that vary through body size and season. Isotopes supported the generally identified feeding ecology of each species but demonstrated that each species underwent significant changes in feeding ecology with increasing body size. Changes in isotopes across season, and body size for each species demonstrate the need for a more thorough understanding of how resource use changes with body size and season in freshwater fish.

Cyanobacterial blooms in the central basin of Lake Erie: Potentials for cyanotoxins and environmental drivers

Abstract Lake Erie western basin (WB) cyanobacterial blooms are a yearly summer occurrence; however, blooms have also been reported in the offshore waters of the central basin (CB), and very little is known about what drives these blooms or their potential for cyanobacterial toxins. Cyanobacteria Index was quantified using MODIS and MERIS data for the CB between 2003 and 2017, and water samples were collected between 2013 and 2017. The goals were to 1) quantify cyanobacteria, 2) determine environmental drivers of CB blooms, and 3) determine the potential for cyanobacterial toxins in the CB. Dolichospermum (Anabaena) occurred in the CB during July before the onset of the WB bloom, and then in August and September, the cyanobacteria community shifted towards Microcystis. The largest Dolichospermum blooms (2003, 2012, 2013, and 2015) were associated with reduced water clarity (Secchi disk depth 20 μmol/L) and high nitrogen-to‑phosphorus ratios (>100), which indicate nutrient concentrations or ratios did not select for Dolichospermum. Additionally, the sxtA gene, but not mcyE or microcystins, were detected in the CB during July 2016 and 2017. The mcyE gene and microcystins were detected in the CB during August 2016 and 2017. The results indicate the CB's potential for cyanotoxins shifts from saxitoxins to microcystins throughout the summer. Continued monitoring of cyanobacteria and multiple cyanobacterial toxins is recommended to ensure safe drinking water for CB coastal communities.

Using Landsat to extend the historical record of lacustrine phytoplankton blooms: A Lake Erie case study

Long-term records of phytoplankton blooms in freshwater lakes are necessary both for understanding basin-scale changes to watersheds and for providing a key constraint for assessing processes driving blooms. However, due to the inherent constraints of in situ sampling and the short time period covered by modern space borne sensors, few long-term records exist. Historical data from sensors such as Landsat offer strong potential for creating new records of past blooms. Here, we use a novel evaluation procedure based on multiple metrics to assess algorithm suitability and robustness for generating long-term bloom records using Landsat 5 imagery. Evaluation metrics are based on bloom presence, spatial distribution, magnitude and timing, using both in situ Microcystis biovolume and remotely-sensed Cyanobacterial Index (CI) data from MERIS for 2002–2011. Applying this procedure for a test case focusing on Lake Erie's western basin, an algorithm based on a near infrared threshold with simple atmospheric correction through subtraction of the shortwave infrared band, combined with an additional “greenness” filter based on a hue threshold, performs best. Implementing this algorithm for 1984–2001 reveals the long-term trends in peak bloom magnitude prior to the start of the MERIS and MODIS record (2002–2015), and more than doubles the period of record that can be used to understand bloom occurrence and growth for this system. More broadly, we demonstrate that Landsat observations can be used to identify macro-scale features of blooms. For Lake Erie, the performance of the final Landsat algorithm is comparable to that of the MERIS CI algorithm, despite Landsat's broad spectral bands and long revisit time.

Polychlorinated Biphenyl Bioaccumulation Patterns Among Lake Erie Lower Trophic Level Consumers Reflect Species Ecologies.

Polychlorinated biphenyl (PCB) concentrations were quantified in lower trophic level consumers of the Lake Erie western basin food web to assess the capacity of PCBs to discriminate among trophically similar species inhabiting different compartments of the same ecosystem. Zooplankton were characterized by higher proportions of less chlorinated and hydrophobic homologs relative to zebra mussel (Dreissena polymorpha), mayfly (Hexagenia limbata) and emerald shiner (Notropis atherinoides) samples. PCB biota-sediment accumulation factors (BSAF) differed significantly among species with zebra mussels and emerald shiners having the highest BSAFs. Principal components analysis of sample PCB profiles reflected the contrasting pelagic and benthic habitats occupied by filter-feeding zooplankton and zebra mussel samples. Benthic mayfly PCB profiles were characterized by increasingly hydrophobic (logK OW ≥ 6.9) congeners with more variable emerald shiner profiles reflecting the greater mobility and extent of spatial habitat integration achieved by this secondary consumer. These results contribute to growing evidence for the use of PCBs as ecological tracers in aquatic ecosystems.

Spatial distributions of external and internal phosphorus loads in Lake Erie and their impacts on phytoplankton and water quality

Re-eutrophication in Lake Erie has led to new programs to reduce external phosphorus loads, and it is important to understand the interrelated dynamics of external and internal phosphorus loads. In addition to developing phosphorus load response curves for algal biomass in the western basin and hypoxia in the central basin, we used a two-dimensional (vertical-longitudinal) hydrodynamic and ecological model to show that both external and internal phosphorus loads were distributed homogeneously in the water column in Lake Erie's western basin. In the stratified central and eastern basins phosphorus released by organic matter decay and crustacean zooplankton excretion was concentrated in the upper water column, contributing 100–119% of the phytoplankton phosphorus demand, while phosphorus released by dreissenids and from anoxic sediments was distributed primarily in the hypolimnion during the growing season. Simulated reductions in external phosphorus loads decreased individual phytoplankton groups most at times when they were normally most abundant, e.g., Microcystis decreased the most during September. Phosphorus was limiting over the simulation periods, but water temperature and light conditions also played critical roles in phytoplankton succession. While water column phosphorus responded quickly to external phosphorus reduction, pulses of phosphorus (riverine input or sediment resuspension) occurring immediately before the Microcystis bloom period could allow it to bloom despite long-term external phosphorus load reduction. Studies are warranted to assess the contribution of seasonal dynamics in phosphorus loading (including sediment resuspension) to Microcystis bloom development.

Comparison of Electrofishing Techniques and Effort Allocation across Diel Time Periods, Seasons, Sites, and Habitat in the Ohio Coastal Waters of Western Lake Erie

AbstractCoastal (<3‐m depth) and nearshore (3–15‐m depth) zones of large freshwater lakes are generally rich in complex habitats that are important for fisheries, but they are often highly degraded and understudied. We identified spatial and temporal sampling efficiencies for monitoring coastal fish communities in a large freshwater lake by use of electrofishing. During 2011 and 2012, we sampled 21 coastal sites in Lake Erie's western basin via daytime and nighttime electrofishing with multiple replicates throughout the summer sampling season. Nighttime electrofishing captured more species and more individuals with less effort than daytime electrofishing; nighttime electrofishing conducted early in the season (i.e., late spring and early summer) was more efficient than that conducted late in the season (i.e., late summer and early fall). A sampling design based on 500 m of shoreline per site required fewer sites and person‐hours to attain 65% and 75% of total species richness (6 and 11 sites, respectively) than a design that used 100 m/site. A 300‐m/site design was more efficient at targeting 90% of total species richness. Targeting of wetland habitat increased the number of species captured but missed species that were only found at other habitat types. A sampling design that targeted 11 sites (75% of species richness) sufficiently described fish community metrics (e.g., number of tolerant species) since the design captured nearly all fish species that were relevant to each metric. This study provides the foundation for a coastal monitoring program in western Lake Erie and serves as a starting point for program development in other large freshwater lakes.Received January 16, 2015; accepted October 8, 2015

Modeling the effects of climate change on water, sediment, and nutrient yields from the Maumee River watershed

Abstract Study region Harmful algal blooms (HABs) in the Western Basin (WB) of Lake Erie have been linked to nonpoint pollution from agricultural watersheds. The Maumee River watershed is the largest in the Great Lakes region and delivers the biggest sediment and nutrient load to Lake Erie. Study focus Climate change could alter the magnitude and timing of sediment and nutrient delivery to Lake Erie's WB. Data from four Coupled Model Intercomparison Project Phase 5 (CMIP5) models were inputted into a calibrated Soil and Water Assessment Tool (SWAT) model of the Maumee River watershed to determine the effects of climate change on watershed yields. Tillage practices were also altered within the model to test the effectiveness of conservation practices under climate change scenarios. New hydrological insights for the region Moderate climate change scenarios reduced annual flow (up to −24%) and sediment (up to −26%) yields, while a more extreme scenario showed smaller flow reductions (up to −10%) and an increase in sediment (up to +11%). No-till practices had a negligible effect on flow but produced 16% lower average sediment loads than scenarios using current watershed conditions. At high implementation rates, no-till practices could offset any future increases in annual sediment loads, but they may have varied seasonal success. Regardless of future climate change intensity, increased remediation efforts will likely be necessary to significantly reduce HABs in Lake Erie's WB.

Hexabromocyclododecane Flame Retardant Isomers in Sediments from Detroit River and Lake Erie of the Laurentian Great Lakes of North America.

Sediments collected in 2004 from along the Detroit River (n = 19) and across all of Lake Erie (n = 18) were analyzed for isomers of the flame retardant chemical, hexabromocyclododecane (HBCD), using liquid chromatography-tandem mass spectrometry. Sediment samples had ΣHBCD concentrations ranging from not detected to 1.6 ng/g d.w. γ-HBCD (56 %-100 % of ΣHBCDs) was the predominate isomer, observed in 7 of 19 samples from the Detroit River and 6 of 18 samples from Lake Erie (all within the western basin). α-HBCD was found in 4 Detroit River and 2 Lake Erie western basin sites, while β-HBCD was only in two Detroit River samples. High ΣHBCD concentrations (>100 ng/g d.w.) were found in two sludge samples from two Windsor, ON, wastewater treatment plants that feed into the Detroit River upstream. HBCD contamination into the Detroit River is a major input vector into Lake Erie and with an apparent sediment dilution effect moving towards the eastern basin.

Lagrangian analysis of the transport and processing of agricultural runoff in the lower Maumee River and Maumee Bay

Lagrangian analyses is used to evaluate the processing of nutrients and sediments during storm runoff events as water moved from the Maumee River loading station at Waterville, OH through the lower river, Maumee Bay and into Lake Erie's western basin. Chemical signatures of storm water at Waterville were used in combination with frequent collections of water at transects along the flow paths to evaluate processing. These signatures consisted of the contrasting chemographs of conservative parameters (chloride and sulfate), dissolved nutrients (dissolved reactive phosphorus (DRP) and nitrate) and particulate substances (suspended solids (SS) and particulate phosphorus (PP)). During low flow, sharp drops in concentrations of chloride and sulfate indicated that mixing zones between river water and bay/lake water occurred at the river mouth. During high flows, the location of the mixing zone between riverine and bay/lake water was indicated by the margin of storm-event sediment plumes, with larger storms extending further into the lake. Steep concentration gradients of DRP and nitrate between high storm water concentrations and low bay/lake water concentrations were also present at the plume margin. The large areas of storm water inside the plume margin contained high DRP and nitrate concentrations but relatively low SS and PP concentrations, due to SS and PP deposition along the flow paths. Because this deposition occurred in water with high DRP concentrations, little of the bioavailable PP was likely to have been released prior to deposition. This storm runoff water provides excellent media for algal growth.

Re-eutrophication of Lake Erie: Correlations between tributary nutrient loads and phytoplankton biomass

Both abiotic and biotic explanations have been proposed to explain recent recurrent nuisance/harmful algal blooms in the western basin and central basin of Lake Erie. We used two long-term (>10years) datasets to test (1) whether Lake Erie total phytoplankton biomass and cyanobacterial biomass changed over time and (2) whether phytoplankton abundance was influenced by soluble reactive phosphorus or nitrate loading from agriculturally-dominated tributaries (Maumee and Sandusky rivers). We found that whereas total phytoplankton biomass decreased in Lake Erie's western basin from 1970 to 1987, it increased starting in the mid-1990s. Total phytoplankton and cyanobacterial seasonal (May–October) arithmetic mean wet-weight biomasses each significantly increased with increased water-year total soluble reactive phosphorus load from the Maumee River and the sum of soluble reactive phosphorus load from the Maumee and Sandusky rivers, but not for the Sandusky River alone during 1996–2006. During this same time period, neither total phytoplankton nor cyanobacterial biomass was correlated with nitrate load. Consequently, recently increased tributary soluble reactive phosphorus loads from the Maumee River likely contributed greatly to increased western basin and (central basin) cyanobacterial biomass and more frequent occurrence of harmful algal blooms. Managers thus must incorporate the form of and source location from which nutrients are delivered to lakes into their management plans, rather than solely considering total (both in terms of form and amount) nutrient load to the whole lake. Further, future studies need to address the relative contributions of not only external loads, but also sources of internal loading.

Spatially Varying Population Demographics and Fishery Characteristics of Lake Erie Walleyes Inferred from a Long‐Term Tag Recovery Study

AbstractAlthough the Lake Erie population of Walleyes Sander vitreus exhibits complex spatial structuring, the extent to which population demographics also vary spatially is unknown. Using a spatial tag recovery model, we estimated region‐ and age‐specific mortalities and regional movement probabilities by using recoveries from a jaw tagging study initiated in 1990. The best‐performing model based on a comparison of quasi‐likelihood Akaike's information criterion values had age‐group‐specific movement probabilities, age‐ and region‐specific natural mortalities, and age‐group‐ and region‐specific annual fishing mortalities. Commercial fishing mortality varied considerably during the study, while recreational fishing mortality was more static. Natural mortality of age‐5 and older Walleyes was lower than that of younger fish in all regions, with values ranging from 0.30 to 0.40 for age‐4 and younger fish and from 0.13 to 0.27 for age‐5 and older fish. In Lake Erie's western basin, natural mortality of age‐4 fish was lower than that of age‐3 fish. Sensitivity analyses indicated that some natural mortality estimates were sensitive to (1) prior probability distributions assigned to mortality components and (2) assumed movement probabilities in regions where no tagging was conducted. The decline in natural mortality with age in Lake Erie's western basin matches what has been found for other populations, suggesting that such patterns are perhaps common in exploited Walleye populations. Movement probabilities in the western basin were greater than those in the combined central and eastern basins. The mortality rates and movement probabilities estimated in this study should assist in the parameterization and scaling of a spatially explicit Lake Erie Walleye assessment model, the development of which has been recommended for the lake's quota management system. Our study is one of the first to apply a spatial tag recovery model to a freshwater fish population for estimating mortality components. We encourage wider use of this method to improve the understanding of how mortality components and movements vary regionally within freshwater systems.

Diversity and distribution of free-living and particle-associated bacterioplankton in Sandusky Bay and adjacent waters of Lake Erie Western Basin

Studies on the bacterial communities in Lake Erie have been largely focused on cyanobacteria. To characterize the community structure of the other bacteria, we examined the 16S rRNA gene content of free-living (FL) and particle-associated (PA) bacterioplankton populations in Sandusky Bay (SD) and adjacent waters in the Western Basin (WB) of Lake Erie. A comprehensive survey of nutrients and other limnological variables was also performed in parallel. A total of 18,569 of 16S rRNA V6 pyrotag sequences were recovered, which were affiliated with 64 unique bacterial orders within 14 phyla. FL bacteria were composed differently from PA ones and contained significantly more Actinomycetales. In addition, FL bacteria were taxonomically more diverse. Despite the distinct environmental conditions, compositional variation was insignificant between bacteria in the SD and WB samples.

Evidence against fluvial seeding of recurrent toxic blooms of Microcystis spp. in Lake Erie's western basin

For almost two decades, the western basin of Lake Erie has been plagued with recurring toxic algal blooms dominated by the colonial cyanobacterium, Microcystis spp. Since the Maumee River is a major source of nutrients and sediment inputs into the lake, and Microcystis spp. has been identified as a member of the upstream river algal assemblage, the possibility exists that the river Microcystis species serve as a seed population for the toxic blooms occurring in the lake. Genetic profiling of toxic cyanobacteria using the microcystin synthesis gene, mcyA, clearly indicates that the toxic cyanobacteria of the river are distinct from the toxic Microcystis spp. of Lake Erie. Indeed, mcyA sequences are almost exclusively from toxic Planktothrix spp., similar to what has been documented previously for Sandusky Bay. UniFrac statistical analysis of cyanobacterial community composition by comparison of 16S–23S ITS sequences also show that the Maumee River and Lake Erie communities are distinct. Overall, these data show that despite the importance of nutrient inputs and sediments from the river, the toxic cyanobacterial blooms of Lake Erie do not originate from toxic species endemic to the Maumee River and instead must originate elsewhere, most likely from the lake sediments.

Predation by Coelotanypus (Diptera: Chironomidae) on Laboratory Populations of Tubificid Oligochaetes

Laboratory experiments were conducted with sediments and organisms collected from Lake Erie's western basin to examine the effect of Coelotanypus (Chironomidae: Diptera) predation on tubificid oligochaetes. Populations of predator and prey organisms were kept together in replicate laboratory microcosms at densities ranging from 2,500–12,500 predators/m 2 and 12,500–62,500 prey/m 2 for 15- to 30-day periods. Control populations of predators and prey were maintained separately for comparative purposes. A separate selection experiment showed that larvae preferred sediments containing tubificids or previously settled larvae over sediments that contained neither. Mortality in worm populations kept with predatory Coelotanypus larvae averaged 56.9% (S.D. = 3.9%) and was significantly higher (p < 0.001, t-test) than the 9.7% (S.D. = 3.1%)average mortality in control populations. Simulated effects of chironomid burrowing and burrow irrigation did not cause increased worm mortality. An average predation rate of 0.11 (S.D. = 0.04) prey·predator −1·day −1 was calculated from the data. Based on average Coelotanypus and tubificid densities in western Lake Erie and assuming that natural feeding rates are comparable to those in the laboratory, Coelotanypus may be capable of reducing tubificid populations by about 20% over a 2-month period during the late summer. Natural abundance data indicate that tubificid population declines are associated with periods of peak Coelotanypus density.