Alewife (Alosa pseudoharengus) diets were compared with the extant zooplankton community in two sections of the Bay of Quinte, Lake Ontario. In the shallow, eutrophic upper bay, cladocerans predominated both in the community and in the diet. Bosmina longirostris was avoided by alewife, while cyclopoid copepods were preferred. The sphaerical, pigmented, but small sized Chydorus sphaericus was more common in the diet than the larger B. longirostris. In the less eutrophic, deeper lower bay, cyclopoid copepods were more common in both the zooplankton community and in alewife diets than in the upper bay. Diets of both small (<100-mmfork length) and large (> 100mm) alewife showed a high degree of overlap indicating general similarity in diet composition. Alewife abundance declined significantly in the upper bay from 30 kg.ha−1 in 1972–1976 to 2.9kg.ha−1 in 1977–1988. Values for the lower bay were 28kg.ha−1 and 10.5kg.ha−1, respectively. In several years alewife biomass exceeded the limit of 40 kg.har1 that has been proposed for the continuing presence of Daphnia galeata mendotae. However, the abundance of this zooplankter has increased from less than 0.01mg.L−1 (dry weight) when alewife biomass was high to 0.30mg.L−1 recently when alewife biomass was low. There was no significant correlation between the abundance of D. galeata mendotae and chlorophyll a concentration possibly because D. galeata mendotae could not utilize the filamentous diatoms and blue-green algae that formed the bulk of the phytoplankton. The zooplankton-phytoplankton portion of the top-down predation model continues to be elusive.

Alewife (Alosa pseudoharengus) abundance in the Bay of Quinte, Lake Ontario, was monitored with a bottom trawl survey from 1972 to 1987. Prior to 1977, alewife catch-per-effort (CPE) was high in all areas of the bay but declined drastically following unusually cold winters in 1976–77 and 1977–78. Beginning in 1980, alewife CPE recovered in the lower bay, adjacent to Lake Ontario. This recovery was not observed in the upper bay where predation by a resurgent walleye (Stizostedion vitreum) population maintained the reduced numbers of alewife. We conclude that walleye predation has played an important role in generating differences in alewife abundance between the upper and lower bays since 1980. There is a negative correlation between adult alewife CPE and young-of-year (YOY) alewife CPE suggesting that intraspecific predation is an important factor in the production of YOY alewife in the Bay of Quinte. Recruitment mechanisms for alewife in the Bay of Quinte may depend more on intraspecific predation, in contrast to Lake Ontario where intraspecific competition for zooplankton has been hypothesized as a limiting factor.

A 50% reduction in phosphorus loading to the upper Bay of Quinte (Lake Ontario) from municipal sources in 1977 was followed by a major decline in phytoplankton biomass in 1978. However, by 1984–85, biomasses again approached those of the pre-phosphorus control period, despite continued low phosphorus loadings. No major shifts in phytoplankton composition occurred; domination by the diatoms Melosira and Stephanodiscus spp. and the blue-green algae Anabaena and Aphanizomenon spp. has continued. Highly significant positive correlation coefficients (r = 0.92–0.98) were found for phytoplankton — fish relationships during both the pre- and postphosphorus removal periods which coincided with pre- and postdie-off periods of white perch (Morone americana) and alewife (Alosa pseudoharengus). For the entire 16-yr period of data collection, a multiple regression model fitting upper bay phytoplankton biomass (with an adjusted R2 of 0.83) was developed with five input variables. White perch biomass alone explained more than 50% of the variance in the model. It is hypothesized that trophic interactions among other biotic components in the Bay of Quinte may be very important in controlling phytoplankton biomass.

From the 1940s on, the Bay of Quinte was subjected to increased point-source phosphorus loading. This and other stresses (exploitation, fish species invasion, and perhaps climate fluctuation) altered the productivity of the ecosystem and community structure. In the late 1970s, point-source loading was greatly reduced. At the same time, walleye (Stizostedion vitreum vitreurn) produced the largest year class ever recorded and surged to abundance greater than those seen in the 1950s before its collapse. Also, the dominant invader white perch (Morone americana), which had flourished in the 1960s and 1970s, was almost eliminated by a low-temperature kill. Studies of the bay ecosystem before and after these events indicated that while nutrients and algae responded to changes in nutrient loadings, changes in secondary producers hinged on the changes in the fish community. Use of Borgmann's biomass size spectrum model provides a focus for the interpretation of nutrient- and predator-related changes in biomass distribution. Deviations of the biomass spectrum are interpreted. The biomass and production spectra are linked to both phosphorus concentrations, observed and predicted, and to fishery exploitation rates. The results suggest that while nutrients control the overall biomass of the ecosystem and the slope of the biomass spectrum, trophic interactions and perturbations (e.g. exploitation) determine the degree to which a smooth biomass spectrum is achieved.

The sequence of fish species shifts since the 1930s in the Bay of Quinte is interpreted in terms of climatic changes and the influences of man. Among the latter, eutrophication is judged most important. The marked decrease in large piscivores in both eastern Lake Ontario and the Bay of Quinte as a consequence of these changes resulted in instability, exemplified by the explosions in populations of alewife (Alosa pseudoharengus) and white perch (Morone americana). Nutrients are imported to the bay, from Lake Ontario by way of alewife. This source has increased in the 1970s with the further decline of piscivores, and it brought about a greater fish production in the bay. We conclude that the Bay of Quinte and Lake Ontario interact significantly and should be studied as a single system.Because of its specialized feeding habits, the post-World War II rise of the walleye (Stizostedion vitreum vitreum) was related to events outside the Bay of Quinte, but its later decline was attributable to direct and indirect effects of eutrophication. Yellow perch (Perca flavescens), by contrast, were apparently not affected adversely by severe habitat changes induced by cultural eutrophication. Key words: Percidae, community ecology, species shifts, eutrophication, habitat changes