Since 2006, the known distribution of Hemimysis anomala has greatly expanded in the Great Lakes ecosystem, with, to date, 45 sites of occurrence among 91 monitored sites, located in four of the Great Lakes and the upper St. Lawrence River. By means of carbon and nitrogen stable isotopes, a first assessment of the feeding ecology of Hemimysis was completed. The d 13 C values of 18 individuals collected in Lake Erie (Port Mainland) on a single date (Sept. 23, 2008) ranged from -30.2 to -24.5%, indicating that Hemimysis could feed on multiple carbon sources including pelagic and littoral autochthonous and terrestrial carbon. In Lake Erie, variation in d 13 C was related to d 15 N, indicating the importance of food source for determining the trophic position of Hemimysis. The d 15 N signatures of individuals were strongly related to their C/N ratios, suggesting that variations in the nutritional value of Hemimysis may depend on trophic position. Isotopic variation among individuals in Lake Erie was complemented by temporal variation in Lake Ontario. Monthly changes (from June to December 2008) in carbon isotope signatures were observed and related to changes in water temperature, highlighting the variations in the baseline prey signatures that fuel Hemimysis diets. The observed variation in stable isotope signatures occurring among individuals within a localized Hemimysis assemblage and temporally should be considered as a key design feature in further studies attempting to identify the possible effects of Hemimysis on nearshore food webs in the Great Lakes.

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Secchi depth, total phosphorus, soluble reactive phosphorus, silica, chlorophyll a, and zooplankton (density, biomass, and average size) were sampled as part of a lake-wide, seasonal (spring, summer, and fall) assessment of Lake Ontario in 2003 to characterize the status of the lower aquatic food web. For each parameter, spatial comparisons were performed to test for differences between habitats (nearshore and offshore) and between regions (east and west) during each season. Significant differences between habitats were found only for silica and chlorophyll a; silica was higher nearshore in fall, and chlorophyll a was higher offshore in fall. Significant differences between regions were detected in Secchi depth, epilimnetic zooplankton mean length, total phosphorus and Cercopagis pengoi density and biomass; Secchi depth and zooplankton mean length were higher in the east in spring, total phosphorus and Cercopagis pengoi biomass were higher in the west in summer, and Cercopagis pengoi biomass was higher in the east in fall. Cercopagis pengoi was present lake-wide in summer and fall, but Bythotrephes longimanus was present only in fall in the Kingston basin. Mean spring total phosphorus, soluble reactive phosphorus, chlorophyll a, and epilimnetic zooplankton density and biomass are at or near record low levels. As we move into the future, persistent low levels of these lower food web elements will continue to stress alewife populations both through reduced food resources and food quality for zooplankton, and may force these fish to seek alternative food such as Mysis.

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Studies to examine larval Lake Whitefish (Coregonus clupeaformis) abundance, diet composition and growth, and the abundance of their zooplankton prey were conducted during eight years (1991–1993, 1995–1996 and 2003–2005) over the course of two decades that spanned a period of major ecosystem change—primarily dreissenid mussel related impacts—on the Bay of Quinte, northeastern Lake Ontario. Larval fish were captured in shallow, nearshore waters (0.2–2.0 m) from early April to mid-May each year. Larval Lake Whitefish fed primarily on cyclopoid copepods and small-bodied cladocerans. The key finding of our studies was that prey abundance declined by 89% from the earlier (1991–1993, 1995–1996) to the later (2003–2005) sampling years. Larval fish growth during spring was significantly correlated with prey availability. Recruitment to the juvenile stage in August was correlated with spring prey availability and larval fish growth. The observed decline in larval Lake Whitefish prey in the Bay of Quinte may be contributing to poor stock performance during and following a period of significant ecosystem change.