Stable isotope analysis of the potential prey and predator can be combined with gut content analysis to quantify the diet. This dietary knowledge allows the quantitative assessment of the role of key species in energy and contaminant transfer, their impact on prey communities, and their susceptibility to perturbation. The diet of Mysis relicta was examined in Lake Ontario in spring, summer, and autumn using both techniques. Mysids fed on the bottom during the day and in the pelagia and on the bottom at night. A trophic fractionation of 2.2‰ N between mysids and their prey provided the best correspondence between the observed stable isotope signature of mysids and that estimated from their diet. Tissue turnover rate of d 13 C was slow compared with that of d 15 N. Diatoms formed 50% of the assimilated diet in May. In September, 25% of large mysids feeding on the bottom contained amphipod parts and 20% contained phytoplankton. The remainder of the diet consisted of zooplankton and rotifers. The contribution of amphipods and phytoplankton could not be quantified. Revised daily consumption estimates, based on this new diet information and clearance rate estimates of consumption, gave daily consumption estimates similar to those estimated from previous bioenergetic modelling.

Particulate organic matter (POM), zooplankton, and benthic invertebrates were sampled seasonally at a midlake and east basin site in Lake Ontario. The δ13C values of POM samples were compared with measurements of chlorophyll a concentrations, areal rates of primary productivity, concentrations of dissolved free CO2 (CO2(aq)), and the δ13C of zooplankton and benthic invertebrates. The δ13C of POM was significantly correlated with the concentration of CO2(aq) at the east basin site. No correlation was found between chlorophyll a concentrations, areal rates of primary productivity, and the δ13C of POM. The δ13C of zooplankton was not always similar to the δ13C of the POM fraction collected, indicative of the complexity of food web interactions within the lower trophic levels. The δ13C of benthic invertebrates feeding on the sediment surface was similar to that of POM produced prior to stratification, indicating the importance of the spring bloom as a food source for benthic primary consumers.

The seasonal variability in the δ15N of biota at the base of the pelagic food chain in Lake Ontario was studied during the period 1995-1997. Strong seasonal patterns were observed in the δ15N of dissolved inorganic nitrogen, different size fractions of particulate organic matter, and zooplankton split into size fractions of 110-210, 210-295, and >295 µm. Similar patterns were reflected in the δ15N of individual species: Diacyclops thomasi, Bosmina longirostris, Daphnia spp., and calanoid copepods. The seasonal fluctuation of primary producers was a function of the nitrogen enrichment of dissolved inorganic nitrogen during photosynthetic uptake and the δ15N of NH4+ and NO3- used by the algae. The large range in δ15N of primary producers suggests that recycling of nitrogen is an important determinant of δ15N at the base of the food web. The δ15N of individual species of zooplankton reflected the known feeding strategies and dietary preferences.