Determining habitat limitations of Maumee River walleye production to western Lake Erie fish stocks: documenting a spawning ground barrier

Tributaries provide spawning habitat for three of four major sub-stocks of Lake Erie walleye (Sander vitreus). Despite anthropogenic degradation and the extirpation of other potamodromous species, the Maumee River, Ohio, USA continues to support one of the largest fish migrations in the Laurentian Great Lakes. To determine if spawning habitat availability and quality could limit production of Maumee River walleye, two habitat suitability models were created for the lower 51 km of the Maumee River and the distribution and numbers of walleye eggs deposited in a 25 km stretch of river were assessed. Walleye eggs were collected using a diaphragm pump at 7 and 10 sites from March/April to May 2014 and 2015. The habitat suitability models showed that <3% of the river yielded ‘good’ walleye spawning habitat and 11–38% yielded ‘moderate’ walleye spawning habitat, depending on the model. However, a large set of rapids at river kilometer 28 and more than five river kilometers of less suitable habitat separated areas of ‘good’ habitat. The rapids may present a migratory barrier for many spawning walleye, as modeled water velocities exceed maximum estimated walleye swim speeds 71–100% of days during pre-spawn migration and spawning during the study period. In both study years, there was a sharp decline in mean egg numbers from spawning sites downstream of the rapids (439.7 eggs/2 min tow ± 990.6 SD) to upstream sites (5.9 eggs/2 min tow ± 19.4 SD). Physical barriers like rapids may reduce spawning habitat connectivity and could limit walleye production in the Maumee River.     

Post-tournament dispersal of smallmouth bass in western Lake Erie

Catch-and-release tournaments for smallmouth bass Micropterus dolomieu transport fish from many capture locations to a centralized release site. In the Great Lakes, these fish may be transported long distances (i.e., >100 km) and subsequently become concentrated at a release site, possibly negatively affecting subpopulations at popular fishing sites if fish remain concentrated in the area. To assess timing of post-tournament dispersal, 23 smallmouth bass were implanted with acoustic transmitters and released into Sandusky Bay (Lake Erie) after a large tournament (50 boats) in September 2018. Tagged fish were tracked via a grid of acoustic receivers. All surviving fish (n = 19) left the immediate vicinity of the release site (<500 m) after an average (±SE) of 13.2 (±2.4) d, with many (58%) returning to the main lake (19.7 d ± 3.4). Eight (42%) of the surviving fish left the release site but remained in the bay after 50 d. Based on the results from this study, long-term (>1 month) concentration of smallmouth bass at release sites is unlikely to be a problem for Lake Erie bass tournaments, but temporary (up to 1 month) accumulations of fish are possible. A fraction of tournament-released smallmouth bass resided in Sandusky Bay for extended periods (>2 months), which could be considered functionally harvested from main lake subpopulations. Currently, the frequency of bass tournaments on Lake Erie is uncertain, and better documentation of tournament frequency and scale is necessary to assess potential negative impacts on bass populations.     

Effects of walleye predation on variation in the stock-recruitment relationship of Lake Erie yellow perch

Stock-recruitment relationships (SRRs) may vary over time due to temporal variation in ecological conditions, reducing confidence in projections from stock-recruitment models. We examined whether the time-varying SRRs detected for yellow perch (Perca flavescens) in the western basin of Lake Erie between 1977 and 2013 could be attributed to variation in yearling walleye (Sander vitreus) predation, indexed by variation in density. Annual variation in yearling walleye density was strongly associated with yellow perch recruitment dynamics, and positively correlated with temporal variation in density-dependence of yellow perch SSRs. However, non-stationary SRRs persisted after accounting for effects of yearling walleye density, and the extent of temporal variation in SRRs actually increased. In simulations, we showed that time-varying SRRs may result more from variation in low-frequency ecological factors on the order of decades, than from variation in high-frequency ecological factors on the order of years (e.g., yearling walleye density) and thus may not be distinguishable from noise. Hence, of these two types of factors, the systematic identification, characterization and incorporation of those of low-frequency factors into stock-recruitment models (e.g., exotic mussel invasions and eutrophication, in the case of Lake Erie) may offer greater promise to improve the reliability of long-term forecasts for sustainable harvests in this and other fisheries in dynamic ecosystems.     

Evidence of lake trout (Salvelinus namaycush) natural reproduction in Lake Erie

Native lake trout were extirpated from Lake Erie around 1965 and committed restoration efforts began in 1982. In 2021 and 2022, a total of six lake trout (Salvelinus namaycush) in the free embryo or post-embryo life stage were captured in lake trout embryo traps in Lake Erie offshore of Shorehaven Reef, NY. This represents the first conclusive evidence of successful natural reproduction since extirpation. Trapping locations were identified using the results of a fine-scale positioning acoustic telemetry array, visual observations of adult lake trout exhibiting spawning behavior, and underwater cameras to visually identify possible spawning locations. Lake trout utilized a very specific spawning habitat type—the eastern side of shallow offshore humps in 5–8 m of water. These sites were comprised of habitat typically associated with lake trout spawning with slopes of 5–14° and clean rubble-cobble sized rock with visible interstitial spaces. Genetic barcoding was used to identify the post-embryo stage salmonids to species, and microsatellite genotypes assigned strongly to the Seneca strain which comprises the majority of the adult population. These findings represent a significant milestone for lake trout rehabilitation efforts in Lake Erie, confirming that successful reproduction to the post-embryo stage is possible and supporting continued rehabilitation efforts by Lake Erie management agencies.     

Discriminating natal origin of spawning adult sea lamprey (Petromyzon marinus) in Lake Champlain using statolith elemental signatures

Sea lamprey (Petromyzon marinus) is a nuisance species in the Great Lakes and Lake Champlain. Information about tributary contributions to the spawning adult phase is critical for appropriate allocation of efforts to control this species. We examined the accuracy of statolith elemental composition to identify the natal origin (i.e., individual rivers or clusters of rivers) of 33 known-origin adults from the Lake Champlain basin. To do so, we first established natal origin chemical signatures using the statoliths of 238 larvae from the same basin. Using laser-ablation inductively coupled plasma mass spectrometry, the 238 larvae originating from 12 streams and one delta were discriminated with a classification accuracy of 57% (range: 25–80%) and 70% (range: 29–80%) when individual streams and groups of streams were considered respectively, highlighting the potential of statolith microchemistry to identify natal origins. However, the assignment of natal origin for adults was overwhelmingly incorrect. Using a maximum likelihood procedure, 88% of the adults were assigned to a cluster of three streams and one delta, while only 3% of these individuals were known to originate from this particular cluster. More research is required to understand the low classification accuracy of sea lamprey adults and validate the use of statolith microchemistry to identify sea lamprey natal origin.     

Contrasting sources and mobility of trace metals in recent sediments of western Lake Erie

Concentrations of the major and trace metals varied considerably in the western basin of Lake Erie, ranging from 0.9 to 25.3?mg/g for aluminum, from 2.9 to 36.5?mg/g for iron, from 6.4 to 74.8?mg/g for calcium, from 1.2 to 13.5?μg/g for cobalt, from 2.8 to 61.6?μg/g for copper, from 2.7 to 83.0?μg/g for lead, from 0.1 to 2.9?μg/g for cadmium, and from 7.1 to 127.3?μg/g for strontium. Distinct patterns of sediment metal variability allowed the identification of two major fluvial sources and some active in-lake biogeochemical processes. The inputs of Sr were largely from the Maumee River, the inputs of Cu, Pb, Cd, and Co were dominated by the Detroit River, and the inputs of Fe and Al were roughly evenly from the two rivers. The removal of Sr and Ca from the water column was mainly through coprecipitation with calcite. In contrast, the transfer of Cu, Pb, Cd, and Co was largely attributed to the removal of fine sediment particles from the Detroit River mouth and adjacent nearshore areas and the deposition of the metals scavenged by settling organic materials in the basin's central deeper areas. The mobility of the trace metals was different during the in-lake mass transfer, with Co being the most mobile and Cd being the least mobile. Furthermore, the trace metal mobility differences have decreased significantly during the past half-century due to a substantial increase in organic matter from eutrophication in the basin.     

Variability of sedimentary phosphorus fractions in the western and Sandusky basins of Lake Erie

Surface sediments and three sediment cores from the western basin and one sediment core from the Sandusky basin were analyzed to document spatial and temporal changes in five phosphorus fractions and total phosphorus (TP). The areal distributions of the bioavailable fractions NaCl-Pi, NaBD-Pi, and NaOH-Pi and the refractory organic fraction Res-P were broadly consistent and contrasted with those of the detrital fraction HCl-Pi which showed that high concentrations occurred mostly in high-energy littoral zones and low concentrations largely in profundal depositional areas. The contrasting distributions were induced by interactions among tributary inputs, wave action, circulation, and biogeochemical cycling and transfer in the basin. As revealed by the Sandusky basin sediment record, the detrital fraction HCl-Pi was dominant (70% of TP) during European settlement and decreased rapidly by 28.0% in the early 1910s due largely to impoundments of the Maumee and Sandusky Rivers. While HCl-Pi has ever since remained relatively constant, NaCl-Pi, NaBD-Pi, and NaOH-Pi increased significantly between 1950 and 1970 in the two basins. However, the post-regulation sediment records differed considerably among these coring sites. There was a marked increase of TP in two cores, corresponding to recent return of eutrophication and massive harmful algal blooms but contrasting with a relatively constant, low loading into the lake. This signified the role of internal loading as derived partialy from legacy pollution. Furthermore, NaCl-Pi has increased progressively throughout all the records. We conclude that the increased levels of NaCl-Pi in surface sediments may have altered the internal loading and contributed to the resurgences of harmful algal blooms in Lake Erie.     

Identification and quantification of microcystins in western Lake Erie during 2016 and 2017 harmful algal blooms

Liquid chromatography-mass spectrometry (LC-MS) and tandem mass spectrometry (MS/MS) were used to provide qualitative and quantitative information about microcystin (MC) congeners in western Lake Erie. Samples were collected at eight open-water locations on selected days during harmful algal blooms (HABs) in 2016 and 2017. Seven MCs were identified and 20 MCs were tentatively identified using high-resolution mass accuracies and a unique fragment (Adda m/z 135). The most abundant MC was MC-LR, followed by MC-RR, MC-YR, and MC-LA, and these congeners were quantified. Total (extracellular and intracellular) MC concentrations ranged from 0.068 to 14.88 µg/L in 2016, and from 0.050 to 10.15 µg/L in 2017, with averages of 2.71 and 1.86 µg/L, respectively. Near-shore sites in Lake Erie had higher MC concentrations and Microcystis biovolumes than off-shore sites. This implies that nutrient loading from the Maumee River greatly influences Maumee Bay, and this influence decreases with distance from the river. Consequently, six MCs (MC-LR, MC-RR, MC-LA, MC-YR, MC-LW, and MC-LF) were quantified in water samples collected from the Maumee River and the Maumee Bay shore of Lake Erie in 2017, and MC-RR was the most abundant. The total MC concentrations in river samples ranged from 0.17 to 305.03 µg/L. Additionally, an MC degradation product (linear MC-LR) was detected at all open-water locations, and data indicated an increase in its concentration towards the end of the bloom. The trends for 2016 and 2017 HABs are comparable in terms of spatial distribution and MC congeners produced, though the intensity and peak dates change.     

Ecology and population status of Trout-perch (Percopsis omiscomaycus) in western Lake Erie

Trout-perch Percopsis omiscomaycus is among the most abundant benthic species in Lake Erie, but comparatively little is known about its ecology. Although others have conducted extensive studies on trout-perch ecology, those efforts predated invasions of white perch Morone americana, Dreissena spp., Bythotrephes longimanus and round goby Neogobius melanostomus, suggesting the need to revisit past work. Trout-perch were sampled with bottom trawls at 56 sites during June and September 2010. We examined diets, fecundity, average annual mortality, sex ratio, and long-term population trends at sites sampled since 1961. Trout-perch abundance fluctuated periodically, with distinct shorter- (4-year) and longer-term (over period of 50 years) fluctuations. Males had higher average annual mortality than females. Both sexes were equally abundant at age 0, but females outnumbered males 4:1 by age 2. Diets of trout-perch were dominated by macroinvertebrates, particularly chironomids and Hexagenia sp. Size distributions of trout-perch eggs varied widely and exhibited multiple modes indicative of protracted batch spawning. A review of the few other studies of trout-perch revealed periodic fluctuations in sex ratio of adults, which in light of our evidence of periodicity in abundance suggests the potential for sex-ratio-mediated intrinsic population regulation. Despite the introduction of numerous invasive species in Lake Erie, trout-perch remain one of the most abundant benthic invertivores and the population is relatively stable.     

Influence of increasing populations of Double-crested Cormorants on soil nutrient characteristics of nesting islands in western Lake Erie

Animals can influence the structure of an ecosystem by changing the levels of nutrient input. This is of particular importance for the islands of western Lake Erie, which are relatively nutrient poor, but have experienced increases in nutrient input from growing double-crested cormorant (Phalacrocorax auritus) populations. The objectives of this study were to evaluate changes in soil characteristics (nutrients [nitrate (NO₃), total P], pH, and δ¹³C [as a tracer of cormorant-associated nutrients]) across a gradient of cormorant nest density on two islands (Middle and East Sister) in western Lake Erie. For both islands, soil pH decreased and P concentrations increased with nest density. On Middle Island, soil nitrate concentrations increased with cormorant nest density, and varied with breeding phenology, with highest concentrations during the early and mid nesting season (272 ± 19 μg g^− 1) and lowest concentrations late in the season (165 ± 11 μg g^− 1). Following a 3-year absence of nesting activity at sites on Middle Island, soil nitrate concentrations were similar to those at low density sites. In contrast, nitrate concentrations measured on East Sister Island did not correlate with temporal or spatial patterns of cormorant nesting and remained elevated 10 years post-cormorant use. While the results of this study confirm that chronic input of allochthonous materials alters soil properties of these islands, the unique conditions of each island must be considered when predicting ecological effects and setting long-term management objectives.     

Characterizations of individual suspended mineral particles in western Lake Erie: Implications for light scattering and water clarity

Light-scattering attributes of minerogenic particles from the water column of the western basin of Lake Erie (13 sites, plus one from the central basin and one from Sandusky Bay), collected after a wind event, were characterized by scanning electron microscopy interfaced with automated image and X-ray analyses (SAX). SAX results specified scattering attributes for individual particles, including size and chemical composition, and were used in forward Mie theory calculations of minerogenic scattering and backscattering coefficients (b_m and b_b,m). Clay mineral particles, in the size range of 1–20 μm, were the dominant form of minerogenic scattering, representing > 75% of b_m and b_b,m. Levels of b_m and b_b,m were high in the western basin, apparently in part due to wind-driven sediment resuspension, and wide spatial variability was observed. The credibility of the SAX-Mie estimates of b_m and b_b,m was supported by the extent of optical closure obtained with paired bulk measurements of particulate scattering and backscattering coefficients (b_p and b_bp), and independent estimates of organic particle contributions based on empirical bio-optical models. Minerogenic particles dominated b_p and particularly b_bp, and regulated spatial differences in the related common metrics of optical water quality, including turbidity and clarity. The b_bp:b_p ratio was found to be a good predictor of the spatial differences in the relative contributions of minerogenic particles versus phytoplankton to scattering.     

DNA barcoding to confirm morphological traits and determine relative abundance of burrowing mayfly species in western Lake Erie

Burrowing mayfly species of the genus Hexagenia are well known indicators of environmental health in lakes and rivers. Two species, H. limbata and H. rigida, are indistinguishable as nymphs and as adult females. Our objectives were to develop a genetic technique to distinguish between the two species and identify morphological features that separate cryptic nymphs and adult females. Fifty nymphs were collected before emergence from 10 sites throughout the western basin of Lake Erie in 2004 and 2005. Using known specimens of adult aerial male H. limbata and H. rigida, we used the mitochondrial cytochrome c oxidase subunit 1 (COI) gene to identify a 16 base pair (bp) difference between species. DNA sequencing confirmed correct species identification based on differences in abdominal pigmentation patterns on adult female imagos in 19 of 20 cases; the lone exception was a female with very faint pigmentation. Pigmentation patterns between species were consistent on nymphs, subimagos and imagos of both sexes. Populations of both species are panmictic across the western basin of Lake Erie, but H. limbata is the numerically dominant species, representing 70 to 100% of nymphs at sites in both years. A separate lineage of H limbata was discovered in the samples. The ability to distinguish nymphs of the two species will aid in developing more sensitive ecosystem indicators.     

Assessment of Microcystis growth rate potential and nutrient status across a trophic gradient in western Lake Erie

Plankton tow samples collected from 2002 through 2009 indicate that Microcystis biovolume in western Lake Erie is often most dense in transition zone (TZ) waters between Maumee Bay and the center of the western basin. TZ waters are generally high in nutrients and turbidity, and concentrations of each decrease with distance from Maumee Bay. High Microcystis biovolume in the TZ suggests the possibility that the conditions in these waters support a greater Microcystis growth rate relative to the open lake. To test this hypothesis, during the 2008 bloom, Microcystis was collected from western Lake Erie for measurements of total protein content (TPC) as an indicator of growth rate potential and cellular nutrient content to indicate nutrient deficiencies. TPC results indicate that Microcystis in the TZ had a higher potential growth rate compared to offshore waters. TPC values in Maumee Bay were intermediate but not significantly different from the TZ and offshore. Nitrogen content of Microcystis remained high over the summer at all sites, despite very low dissolved nitrate concentrations and low total nitrogen-to-total phosphorus ratio in late summer in the lake. Ammonium level in the lake was constant during the summer, and likely provided the nitrogen source for Microcystis. Cellular phosphorus content varied between site and sample date suggesting that Microcystis was moderately phosphorus deficient. Quotas of micronutrient indicated that Microcystis was not deficient of micronutrients. Results of this study suggest the waters in and adjacent to Maumee Bay provide more favorable growth conditions for Microcystis than offshore waters.     

Evaluating sediments as an ecosystem service in western Lake Erie via quantification of nutrient cycling pathways and selected gene abundances

Lake Erie experiences annual summer cyanobacterial harmful algal blooms (HABs), comprised mostly of non-nitrogen-fixing Microcystis, due to excess nitrogen (N) and phosphorus (P) inputs (eutrophication). Lake Erie's watershed is mostly agricultural, and fertilizers, manure, and drainage practices contribute to high nutrient loads. This study aimed to clarify the role of western Lake Erie sediments in either exacerbating or mitigating conditions that fuel HABs via recycling and/or removal, respectively, of excess N and reactive P. Sediment-water interface N and orthophosphate (ortho-P) dynamics and functional gene analyses of key N transformations were evaluated during a dry, low HAB year (2016) and a wet, high HAB year (2017). On average, western basin sediments were a net N sink and thus perform a valuable ecosystem service via N removal. However, sediments were a source of ortho-P and chemically reduced N. Western basin sediments can theoretically remove 29% of average annual watershed total N loading. Denitrification rates were lower during the high (2017) versus low bloom year (2016), suggesting that high external N loading and large HABs inhibit the capacity of sediments to perform that ecosystem service. Despite being a net N sink on average, western basin sediments released ammonium and urea, chemically reduced N forms that are energetically conducive to non-N-fixing, toxin-producing cyanobacterial HABs, especially during the critical period of low external loading and high biomass. These results support other recent work highlighting the urgent need to include N cycling and internal load dynamics in ecosystem models and mitigation efforts for eutrophic systems.     

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.     

Use of otolith chemistry to discriminate juvenile Chinook salmon (Oncorhynchus tshawytscha) from different wild populations and hatcheries in Lake Huron

Chinook salmon (Oncorhynchus tshawytscha) in Lake Huron consist of wild and hatchery-reared fish distributed among several populations. This study tested whether otolith chemistry can be used to identify the natal origin of Chinook salmon in this system. Concentrations of nine elements (Mg, K, Mn, Fe, Zn, Rb, Sr, Ba, and Pb) in the otoliths of Chinook salmon juveniles from 24 collection sites (17 streams and 7 hatcheries) around Lake Huron were analyzed using laser-ablation inductively-coupled mass spectrometry. Differences in otolith chemistry were found between rearing environments (wild and hatchery), among geological regions (Precambrian, Ordovician, Silurian, Devonian, and Carboniferous), and among collection sites. Discriminant function analysis showed high classification accuracies of juveniles to their rearing environment (wild versus hatchery, 82%), geological region (84%), and collection site (87%) of origin. With these values, there is excellent potential for otolith chemistry to be used to predict the natal origin of adults, and thus inform research and management of Chinook salmon in Lake Huron.     

Spatial and temporal variability of inherent and apparent optical properties in western Lake Erie: Implications for water quality remote sensing

Lake Erie has experienced dramatic changes in water quality over the past several decades requiring extensive monitoring to assess effectiveness of adaptive management strategies. Remote sensing offers a unique potential to provide synoptic monitoring at daily time scales complementing in-situ sampling activities occurring in Lake Erie. Bio-optical remote sensing algorithms require knowledge about the inherent optical properties (IOPs) of the water for parameterization to produce robust water quality products. This study reports new IOP and apparent optical property (AOP) datasets for western Lake Erie that encapsulate the May–October period for 2015 and 2016 at weekly sampling intervals. Previously reported IOP and AOP observations have been temporally limited and have not assessed statistical differences between IOPs over spatial and temporal gradients. The objective of this study is to assess trends in IOPs over variable spatial and temporal scales. Large spatio-temporal variability in IOPs was observed between 2015 and 2016 likely due to the difference in the extent and duration of mid-summer cyanobacteria blooms. Differences in the seasonal trends of the specific phytoplankton absorption coefficient between 2015 and 2016 suggest differing algal assemblages between the years. Other IOP variables, including chromophoric, dissolved organic matter (CDOM) and beam attenuation spectral slopes, suggest variability is influenced by river discharge and sediment re-suspension. The datasets presented in this study show how these IOPs and AOPs change over a season and between years, and are useful in advancing the applicability and robustness of remote sensing methods to retrieve water quality information in western Lake Erie.     

Trends in Lake Erie phytoplankton biomass and community structure during a 20-year period of rapid environmental change

Since the 1990s, Lake Erie has experienced resurgent eutrophication due in part to climate change-driven increases in precipitation, which have combined with increasingly intensive agricultural practices in the region to produce excessive nutrient runoff into the lake. Harmful blooms of the cyanobacterium Microcystis aeruginosa (“Microcystis”) in Lake Erie’s western and central basins (WB and CB, respectively) have been a highly visible consequence of this eutrophication, however few studies have characterized intra- or interannual trends in less abundant, though likely more edible, phytoplankton taxa over the last 25 years. Here, we used the 20-year Lake Erie Plankton Abundance Study (LEPAS) dataset to quantify intra- and interannual trends in the dynamics of six major phytoplankton groups in the WB and CB during 1995–2015. Cyanobacteria biomass in the WB increased >1000-fold during this period, while biomass of all other major taxa groups increased between 10- and 100-fold. Early summer (June–July) and spring (May) communities saw more modest directional change in the biomass of both edible and less-edible taxa as well as community structure. Around 2008, the CB also began to experience Microcystis blooms concurrent with those in the WB, with similar, though less dramatic consequences for phytoplankton community structure and edible biomass. The biomass of several phytoplankton groups exhibited intra-annual oscillations with a ∼5-year period. The mechanisms underlying changes in the phytoplankton community structure and their consequences for higher trophic levels are not well understood, however increases in edible phytoplankton may be sustaining long-term upward trends in many zooplankton taxa.     

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.     

Delayed egg hatching accounts for replacement of burrowing mayflies Hexagenia rigida by Hexagenia limbata after recolonization in western Lake Erie

Egg banking may have played a role in the recolonization of two burrowing mayfly species (Hexagenia limbata and Hexagenia rigida), which recolonized western Lake Erie after over 30 years of near extirpation. H. rigida was the first of the two to colonize successfully, but was overtaken by H. limbata (the historically dominant species). To understand mechanisms of species replacement, we compared egg hatching between the two species at 20 °C in the lab under typical hatching temperatures (no incubation) and after cold storage (8 °C) for 2, 6 and 12 months. Prolonged storage at cold temperatures simulates temperature conditions to which embryos are exposed when deposited by late-emerging female imagos. Without incubation, H. rigida (95.4 ± 2.02) exhibited significantly higher egg hatching survivorship than H. limbata (85 ± 3.2) (P = 0.01). First hatch date for H. rigida was one day earlier than H. limbata, but time for 50% to hatch (5 d) and hatching duration (5 d) was the same for both species. After 2 and 6 months of incubation, H. limbata exhibited an earlier hatch date, but there was no significant difference in mean percentage of egg survival (ca 63%) between species. After 12 months of cold storage, hatching success for H. limbata was 44.6 ± 4.17%, but H. rigida eggs did not hatch. Banking of eggs at low temperature increases the chance of successful recolonization, once stressful conditions pass. The viability H. limbata eggs after long quiescence may help to explain the shift in dominance from H. rigida to H. limbata.