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The opossum shrimp, Mysis diluviana , is an important member of the offshore food webs of the Laurentian Great Lakes, but its response to ecosystem changes that have occurred over the past several decades is not well understood. We combined the data of four long-term sampling programs, adding several years of data (post and prior) to previously published analyses to offer a longer-term, cross-basin analysis of M. diluviana populations in the Great Lakes from 1997 to 2019. Densities were high in lakes Superior and Ontario (summer values 100–300/m2), high and variable but declining (from 200–300/m2 in 1997–2004 to less than 100/m2 in 2017–2019) in Lake Michigan, low (∼20–50/m2 since 2005) in Lake Huron, and very low in shallower eastern Lake Erie (<1/m2). Biomass showed similar trends. Life history parameters (mortality, fecundity, and growth) were consistently highest in eastern Lake Erie, followed by lakes Ontario, Michigan, Huron, and Superior. Generation time was 1 year in Lake Erie and 2 years in the other lakes. Cross-basin relationships between annual M. diluviana areal densities and food indices (chlorophyll-a concentration and zooplankton biomass) were non-linear, increasing with food levels up to about 250 mysids/m2 and about 650 mg dry wt/m2. Annual growth rates were also positively correlated to both food indices in the four deep lakes, but fecundity and mortality rates were not. Our results suggest food availability is a primary factor predicting M. diluviana density and biomass. Density-dependent mortality and fish predation could explain some of the inter-lake differences, but these relationships could benefit from further investigations. [ABSTRACT FROM AUTHOR]

Zooplankton are a vital component of the food web as the major conduit of energy from primary producers to planktivorous fishes. Therefore, changes in the structure of the zooplankton community affect how energy moves through aquatic ecosystems. Changes in the zooplankton community structure are typically documented through detailed community analysis, which can be difficult to interpret and communicate to non-experts. Alternatively, a few key summary indicators of community structure may better disseminate this information to a broader audience. In this study we analyzed zooplankton data from 1997 to 2019 from the US Environmental Protection Agency's Great Lakes National Program Office to select indicators that summarized changes in the zooplankton community. Two indicators (percent of calanoids by biomass and areal density of herbivorous cladocerans) detected known changes in lakes Huron, Michigan, and Ontario, correlated with the crustacean zooplankton areal biomass above (herbivorous cladocerans) or in the hypolimnion (percent calanoids), and were related to the lake or basin-wide Carlson's Trophic State Index based on chlorophyll (TSI chl) (percent calanoids by biomass). Our findings suggest that employing one or both selected indicators in addition to TSI chl and total crustacean zooplankton areal biomass, an already reported indicator in the Great Lakes that captures overall food availability for fish, would provide additional key information about zooplankton community structure in simple metrics that could be effectively communicated to stakeholders. [ABSTRACT FROM AUTHOR]

As the human population of the Lake Ontario basin continues to grow, targeted research and monitoring activities to inform adaptive management are increasingly important for protecting the Lake Ontario ecosystem. As the most downstream of the Great Lakes, the Lake Ontario ecosystem is under pressure from a wide range of stressors including chemical contaminants and invasive species. This special issue highlights the broad network of binational research and monitoring efforts by federal, state, and provincial agencies and academic partners that took place during the 2018 Cooperative Science and Monitoring Initiative (CSMI) field year for Lake Ontario. The research and monitoring by creative and collaborative teams assembled under the umbrella of CSMI 2018 includes projects that investigated a wide variety of factors impacting the lake ecosystem, ranging from physics to chemistry and biology. This issue also provides examples of data sharing/synthesis and modeling tools that promote the use of these extensive datasets to explore ecosystem management options. The research and monitoring outcomes from CSMI 2018 provide managers with current information on the Lake Ontario ecosystem to inform decision making and guide restoration and protection efforts. [ABSTRACT FROM AUTHOR]

Zooplankton distributions are patchy due to multiple physical, chemical, and biological processes, including diel vertical migration (DVM) behavior. Heterogeneity in the offshore environment is difficult to study with net tows, but newer technologies measure finer-scale distributions. Here, we use laser optical plankton counter (LOPC) data, informed by net tows, to study distributions and DVM of zooplankton in offshore Lake Michigan during July and September 2015. Water column (5–60 m) zooplankton biomass varied by an order of magnitude among transects and a factor of two within individual transects (6–19 km distances); transect coefficients of variation (SD/mean) ranged from 7 to 22% (~0.5 km scale). Horizontal patterns in zooplankton biomass varied among size groups but were consistent from day to night, suggesting that processes driving heterogeneity persist for hours to days. Fine-scale LOPC data show that zooplankton often aggregate in thin layers (1–3 m) within the metalimnion, a feature undetectable by coarser net sampling. Although DVM was not consistently observed, some patterns emerged. Small zooplankton including copepodites, diaptomids (Leptodiaptomus ashlandi , L. minutus), and Diacyclops thomasi often migrated to surface waters at night, and large zooplankton (Limnocalanus macrurus) migrated upward at night in most cases. Beam attenuation coefficient (proxy for phytoplankton biomass) was a significant predictor for zooplankton mean depth (p < 0.001) although it explained more of the variation for night data (R2 = 0.72) than day data (R2 = 0.53). The heterogeneity observed in zooplankton distributions has implications for planktivorous fish feeding in the offshore zone, as prey density varies greatly with depth. [ABSTRACT FROM AUTHOR]

The zooplankton communities of several Laurentian Great Lakes have shifted toward greater biomass of calanoid copepods, particularly Limnocalanus macrurus , since the 1990s. Limnocalanus is an omnivore that feeds on large phytoplankton cells, ciliates, rotifers, and small crustacean zooplankton, especially copepod nauplii, and it may be an increasingly important zooplanktivore in these systems. Although there is previous research examining Limnocalanus predation rates on nauplii, we do not know if the presence of phytoplankton affects predation rates. Our initial experiments confirmed Limnocalanus preference for nauplii over small copepodites. Additional experiments showed that Limnocalanus feeding rates on nauplii decreased by 50% at the highest phytoplankton concentrations tested. Limnocalanus fed more on the larger algae tested (Cryptomonas , Cryptophyta, 40 µm) than on the smaller taxa (Scenedesmus , Chlorophyta, 10 µm). We used stable isotope analysis to infer Limnocalanus trophic position in the five Laurentian Great Lakes by comparing Limnocalanus with simultaneously captured Leptodiaptomus sicilis , another calanoid copepod known to feed on phytoplankton and microzooplankton. This analysis showed Limnocalanus at higher trophic positions in the more oligotrophic lakes Huron, Michigan, and Superior than in lakes Ontario and Erie. Summer Limnocalanus trophic position was inversely related to both the site-specific concentration of algae in the deep chlorophyll layer and a trophic state index based on spring chlorophyll and total phosphorus. Our results indicate that predation by Limnocalanus on zooplankton depends on lake algal abundance, and that feeding rates on nauplii by an individual Limnocalanus adult are likely higher in the more oligotrophic lakes. [ABSTRACT FROM AUTHOR]

Epilimnetic production has declined in Lake Ontario, but increased production in metalimnetic deep chlorophyll layers (DCLs) may compensate for these losses. We investigated the spatial and temporal extent of DCLs, the mechanisms driving DCL formation, and the use of physical variables for predicting the depth and concentration of the deep chlorophyll maximum (DCM) during April–September 2013. A DCL with DCM concentrations 2 to 3 times greater than those in the epilimnion was present when the euphotic depth extended below the epilimnion, which occurred primarily from late June through mid-August. In situ growth was important for DCL formation in June and July, but settling and photoadaptation likely also contributed to the later-season DCL. Supporting evidence includes: phytoplankton biovolume was 2.4 × greater in the DCL than in the epilimnion during July, the DCL phytoplankton community of July was different from that of May and the July epilimnion ( p = 0.004), and there were concurrences of DCM with maxima in fine particle concentration and dissolved oxygen saturation. Higher nutrient levels in the metalimnion may also be a necessary condition for DCL formation because July metalimnetic concentrations were 1.5 × (nitrate) and 3.5 × (silica) greater than in the epilimnion. Thermal structure variables including epilimnion depth, thermocline depth, and thermocline steepness were useful for predicting DCM depth; the inclusion of euphotic depth only marginally improved these predictions. However, euphotic depth was critical for predicting DCM concentrations. The DCL is a productive and predictable feature of the Lake Ontario ecosystem during the stratified period. [ABSTRACT FROM AUTHOR]