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

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

DNA barcoding vs. morphological identification of larval fish and embryos in Lake Huron: Advantages to a molecular approach

The Great Lakes provide habitat to over 160 species of freshwater fish, many of which are ecologically and economically important. Concern for management and conservation of declining fish populations makes it important that accurate identification techniques are used for environmental monitoring programs. DNA barcoding may be an effective alternative to morphological identification for industrial monitoring programs of larval and embryonic fish, but comparisons of the two approaches with species from the Great Lakes are limited. It may be particularly important to examine this issue in the Great Lakes because a relatively young group of post-glacial fish species are present which may be difficult to resolve using morphology or genetics. Six hundred and fifty seven larval fish were identified from Lake Huron (Ontario, Canada), using morphology and DNA barcoding. DNA barcoding was used to identify 103 embryos that morphology could not identify. Morphological identification and DNA barcoding had a percent similarity of 76.9%, 96.6% and 96.6% at the species, genus, and family levels, respectively. Thirty-seven specimens were damaged and unidentifiable using morphology; 35 of these were successfully identified using DNA barcoding. However, 23 other specimens produced no PCR product for barcoding using 2 different primer sets. Discrepancies between morphology and DNA barcoding were driven by two major factors: inability of cytochrome oxidase I to resolve members of the genus Coregonus and limited resolution of morphological features for Catostomus. Both methods have pros and cons; however, DNA barcoding is more cost-effective and efficient for industrial monitoring programs.     

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.     

Heavy metals in sediments and uptake by burrowing mayflies in western Lake Erie basin

During the past two decades, burrowing Hexagenia mayflies have returned to the western basin of Lake Erie. Because of their importance as a prey resource for higher trophic levels and their extensive residence time in potentially contaminated sediment, Hexagenia may be a source of heavy metal transfer. To better understand the distribution and transfer of heavy metals in sediment and mayflies, sediment and mayfly nymphs were collected from 24 locations across the western basin of Lake Erie in May 2007. Following USEPA protocols, samples were analyzed for 16 elements using ICP-OES or ICP-MS. Metal concentrations in the sediments exceeded the Threshold Effect Level for at least one metal at all sample sites. Sediment heavy metal distribution profiles indicate metal concentrations are correlated with organic matter content, and the highest heavy metal concentrations were found in the central deeper region of the western basin where organic content in the sediments was greatest. Hexagenia were distributed throughout the western basin, with greatest density (1350/m²) within the Detroit River plume. The Cd and Zn levels in mayflies were on average approximately 4 and 2 times greater, respectively, than sediment levels, and the Cd concentrations in the sediments exceeded the Threshold Effect Level at 27 of 28 sites and exceeded the Probable Effect Level at 9 of 28 sites. Spatial representation of heavy metal concentrations in mayflies exhibited a similar pattern to the spatial distribution of heavy metals and organic matter in the sediments with higher concentrations of metals found in mayflies residing in the central deeper region of the western basin.     

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

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

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

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

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.     

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.     

Tracking changes in nutrient delivery to western Lake Erie: Approaches to compensate for variability and trends in streamflow

Tracking changes in stream nutrient inputs to Lake Erie over multidecadal time scales depends on the use of statistical methods that can remove the influence of year-to-year variability of streamflow but also explicitly consider the influence of long-term trends in streamflow. The methods introduced in this paper include an extended version of Weighted Regressions on Time, Discharge, and Season (WRTDS) modeling that explicitly considers nonstationary streamflow by incorporating information on changes in the frequency distribution of daily measured streamflow (discharge) over time. Soluble reactive phosphorus (SRP) trends in annual flow-normalized fluxes (loads) at five long-term monitoring sites in the western Lake Erie drainage basin show increases of 109 to 322% over the period 1995 to 2015. About one-third of the increase appears attributable to increasing discharge trends, while the remaining two-thirds appears to be driven by changes in concentration versus discharge relationships reflecting higher concentrations for any given discharge during recent years. Trends in total phosphorus and three nitrogen parameters (total nitrogen, nitrate-nitrite, and total Kjeldahl nitrogen) at the 10 sites analyzed were much less pronounced, and commonly show decreases in concentration-discharge relationships accompanied by increases in discharge, resulting in little net change in total flux. Trends in monthly SRP fluxes and discharge, dissolved versus particulate fractions of nutrients, and N:P flux ratios were also evaluated. The methods described here provide tools to more clearly discern the effectiveness of nutrient-control strategies and can serve as ongoing measures of progress, or lack of progress, towards nutrient-reduction goals.     

The relative importance of anammox and denitrification to total N2 production in Lake Erie

Production of dinitrogen gas via microbially mediated anaerobic ammonium oxidation (anammox) and denitrification plays an important role in removal of fixed N from aquatic ecosystems. Here, we investigated anammox and denitrification potentials via the ¹⁵N isotope pairing technique in the helium flushed bottom water (~0.2 m above the sediment) of Sandusky Bay, Sandusky Subbasin, and Central Basin in Lake Erie in three consecutive summers (2010?2012). Potential rates of anammox (0–922 nM/day) and denitrification (1 to 355 nM/day) varied greatly among sampling sites during the 3 years we studied. The relative importance of anammox to total N₂ production potentially ranged from 0 to 100% and varied temporally and spatially. Our study represents one of the first efforts to measure potential activities of both anammox and denitrification in the water column of Lake Erie and our results indicate the Central Basin of Lake Erie is a hot spot for N removal through anammox and denitrification activities. Further, our data indicate that the water column, specifically hypolimnion, and the surface sediment of the Lake Erie Central Basin are comparatively important for microbially mediated N removal.     

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.     

Distribution and abundance of freshwater polychaetes, Manayunkia speciosa (Polychaeta), in the Great Lakes with a 70-year case history for western Lake Erie

Manayunkia speciosa has been a taxonomic curiosity for 150 years with little interest until 1977 when it was identified as an intermediate host of a fish parasite (Ceratomyxa shasta) responsible for fish mortalities (e.g., chinook salmon). Manayunkia was first reported in the Great Lakes in 1929. Since its discovery, the taxon has been reported in 50% (20 of 40 studies) of benthos studies published between 1960 and 2007. When found, Manayunkia comprised < 1% of benthos in 70% of examined studies. In one extensive study, Manayunkia occurred in only 26% of 378 sampled events (1991–2009). The taxon was found at higher densities in one area of Lake Erie (mean = 3658/m²) and Georgian Bay (1790/m²) than in five other areas (mean = 60 to 553/m²) of the lakes. A 70-year history of Manayunkia in western Lake Erie indicates it was not found in 1930, was most abundant in 1961 (mean = 8039, maximum = 67,748/m²), and decreased in successive periods of 1982 (3529, 49,639/m²), 1993 (1876, 25,332/m²), and 2003 (79, 2583/m²). It occurred at 48% of stations in 1961, 58% in 1982, 52% in 1993, and 6% of stations in 2003. In all years, Manayunkia was distributed primarily near the mouth of the Detroit River. Causes for declines in distribution and abundance are unknown, but may be related to pollution-abatement programs that began in the 1970s, and invasion of dreissenid mussels in the late-1980s which contributed to de-eutrophication of western Lake Erie. At present, importance of the long-term decline of Manayunkia in Lake Erie is unknown.     

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.     

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

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

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.     

Effects of water clarity on the length and abundance of age-0 yellow perch in the Western Basin of Lake Erie

Water clarity is an important environmental variable that may affect fish populations by altering the visual environment. Effects can change feeding ability, as well as alter predation risk. The western basin of Lake Erie provides a valuable model system for studying the effects of transparency because the two main tributaries, the Maumee and Detroit rivers, differ substantially in clarity. We used Generalized Additive Models (GAMs) to quantify the relationship between transparency and the observed abundance and length of age-0 yellow perch (Perca flavescens) in August, based on surveys from 1986 to 2006. Secchi data from June to August were included in the models that best explained the variation in yellow perch abundance and length. August values for bottom oxygen and bottom temperature also increased model fit for abundance, whereas only bottom temperature improved model fit for length. Our models indicate that transparency was positively related to the August length while abundance of age-0 yellow perch was inversely related to transparency. Highest abundance was observed in areas with the lowest transparency, with peak abundances observed in areas with less than 1 m of Secchi depth. This is in contrast to August length, which increased as transparency increased, to an asymptote at around 3 m of Secchi depth. The split nature of water clarity conditions in the western basin of Lake Erie has resulted in areas with higher growth potential, versus areas with higher apparent survival.     

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.     

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.