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Large cyclopoid copepods of the genus Cyclops are seldom collected in the Laurentian Great Lakes, with only Cyclops scutifer and Cyclops strenuus reported from the region. Rare reports of the species C. strenuus date back to 1972 within the Great Lakes basin. The first specimens reported as C. strenuus were collected from the St. Marys River, and additional specimens have been collected from western Lake Erie since 2013. We examined all available archived materials of C. strenuus from the Great Lakes and determined that specimens from the two localities belong to two separate species, neither of which refer to C. strenuus. Archived specimens collected from the St. Marys River in 1972 and 1995 were reidentified as Cyclops sibiricus, a Holarctic species known from Siberia, Russian Federation, Alaska, USA, and northern regions of Canada. The occurrences of C. sibiricus from the St. Marys River extend the known distribution of the species southward some 1,688 km in the Nearctic region. Cyclops specimens collected from the western basin of Lake Erie in 2013, 2014, and 2019 were identified as the Palearctic species Cyclops divergens using both conventional taxonomy and genetic barcoding. C. divergens is known from localities across much of Europe and eastward into Central Asia. The occurrences of the species from western Lake Erie constitute the first detection of C. divergens in the Great Lakes and the Nearctic region. Therefore, we expect C. strenuus does not occur in the Great Lakes basin and is likely restricted to the Palearctic region. [ABSTRACT FROM AUTHOR]

Zooplankton in Lake Ontario cover a broad range of size and taxonomic groups that exhibit a diversity of life history strategies and environmental preferences. Vertical gradients of light and temperature are particularly important controls on distribution. Monitoring based on traditional net sampling provide only a coarse assessment of overall biomass and community composition. However, a finer scale understanding of the vertical variation of zooplankton is important for comparisons to the distribution of phytoplankton and planktivorous fish. During 2018, a Triaxus vehicle and V-fin equipped with a laser optical plankton counter (LOPC) were towed several times along a western and eastern offshore transect in Lake Ontario, undulating from 5 to 60 m depth along a 10 km path. These continuous transects supplemented a traditional lakewide net-based zooplankton survey. Zooplankton respond to both horizontal gradients such as thermal bar fronts in spring as well as vertical gradients such as summer stratification and deep chlorophyll maxima (DCM). Deep zooplankton layers initially align closely with the DCM, but persist into the fall despite weaker stratification and the complete dissipation of the DCM. These layers were consistently represented by deep calanoid copepods, while other taxa were limited to the warmer epilimnion. [ABSTRACT FROM AUTHOR]

Stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) are valuable tools for studying food web structure, but broad-scale variations in baseline δ15N and δ13C are not well-understood for freshwaters. We use the Great Lakes as a case study to test for variation in isotopic ratios in lakes that exhibit gradients in residence time, watershed characteristics, and trophic state. We used a nested ANOVA to investigate differences in δ13C and δ15N of organisms of multiple trophic levels (zooplankton, benthos, rainbow smelt, and lake trout) across lakes and seasons, accounting for within-lake variation due to region and year. There were significant effects of lake on baseline values of δ13C and δ15N; patterns were generally consistent with expectations, with more oligotrophic lakes (Superior) being relatively depleted and more mesotrophic (Ontario) to eutrophic (Erie) lakes being enriched in both δ13C and δ15N. Season was a significant factor for δ13C in all taxa except benthos and for δ15N in zooplankton and rainbow smelt. Zooplankton and benthos δ15N values were highly variable, but this variation was dampened in fish, as expected due to integration of prey items over time. Post-hoc analysis of δ13C and δ15N versus lakewide average chlorophyll, residence time, and watershed characteristics indicate that these properties are correlated with stable isotopes values. This study suggests that baseline isotopes ratios in large lakes may be predictable based on lake and watershed characteristics, as has been demonstrated for some marine and smaller freshwater systems. [ABSTRACT FROM AUTHOR]

Stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) are valuable tools for identifying basal energy sources for fish production and describing trophic complexity, but cross-lake comparisons are often limited by the challenges associated with standardizing study design and isotopic baselines. Using studies conducted in collaboration with the bi-national Cooperative Science and Monitoring Initiative, as well as other research efforts across the Great Lakes, we assembled δ15N and δ13C data for zooplankton and fish muscle tissue across all Great Lakes from 2002 to 2018. We aimed to detect differences in food web structures by estimating trophic positions (TP) and the ratio of benthic to pelagic δ13C contributions for planktivorous Rainbow Smelt (Osmerus mordaX) and piscivorous Lake Trout (Salvelinus namaycush). To account for variable baselines in the lower food web, we used averaged bulk zooplankton δ15N values by lake, month, year. Despite high variation in the zooplankton, we identified a robust pattern in TP across lakes for both species. Consistent with predator-prey interactions, mean TPs of Lake Trout (4.1-5.9) were greater than Rainbow Smelt (3.1-4.6) in all lakes. TPs were lower in Superior, Huron and Michigan, and greater in Ontario and Erie. The proportion of benthic/pelagic contributions to fish diets also differed among lakes, with Lake Huron having the highest benthic reliance. Overall, TPs increased with the lake productivity and suggest that both food web complexity and resource use by these fish species differs across lakes. [ABSTRACT FROM AUTHOR]

Offshore particulate matter is typically planktonic in origin; changes in concentrations and elemental ratios can reflect surrounding ecological conditions and aid in assessing ecosystem health. The EPA's Great Lakes National Program Office has collected particulate nutrient data in both spring and summer from all five of the Laurentian Great Lakes since the late 1990s. Here, we present a cross-lake comparison of temporal trends in particulate phosphorus (PP), particulate nitrogen (PN), particulate organic carbon (POC), total suspended solids (TSS), and seston stoichiometry (N:P) from 1997 - 2019. Significant spring trends over the full time series include increased PP in Lake Superior, increased PN in central and eastern Lake Erie, decreased PP, PN, POC and TSS in Lake Huron, and decreased PP and TSS in southern Lake Michigan. The only significant trends in summer data were declines in epilimnion PP and TSS in Lake Michigan. After over a decade of general declines, PN concentrations across most lakes started increasing in 2012, with a significant break point in that year for lakes Superior, Huron, Michigan, and Ontario. Particulate N:P has changed in recent years, indicating possible nutrient deficiency in the lower food web, and the recent changes may hint at declining seston nutritional quality. We also compare particulate nutrient temporal patterns to the timing of significant ecological changes in the Great Lakes, including the expansion of dreissenid mussel populations, declines in the spring phytoplankton bloom, and changes to the zooplankton community. [ABSTRACT FROM AUTHOR]

Acoustic backscattering strength (Sv) depends not only on biomass but also on the size of the animals insonified. The length dependence is related to the relative length of the sound wave and the animals and is highly dependent on the frequency of sound used. This relationship is non-linear, and smaller organisms have substantially lower acoustics backscattering at lower frequencies (=i.e., shorter wavelengths) compared to larger organisms. Marine acousticians use the difference in backscattering at different frequencies to infer the identity of the organisms in the scattering layers. The taxonomically less complex zooplankton community in Great Lakes should lend itself even better to this approach than the highly diverse oceans. Here we present examples from several data sets to explore the ability of Sv differencing to identify and separate fish, mysids and larger zooplankton in the Great Lakes. Acoustic returns from both ships and unmanned vehicles were compared with data from Optical Plankton Counters and net tows. Sv differencing using 120 and 430 kHz data provided distinct signatures for zooplankton, mysid, and fish layers. Increased future use of multifrequency unmanned acoustic data collection systems will increase our ability to predict distribution and migrations of different zooplankton groups. [ABSTRACT FROM AUTHOR]