Interactions Between Alewife ( Alosa Pseudoharengus), Their Food, and Phytoplankton Biomass in the Bay of Quinte, Lake Ontario

Abstract

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 (> 100 mm) 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 2.9 kg.ha −1 in 1977–1988. Values for the lower bay were 28 kg.ha −1 and 10.5 kg.ha −1, respectively. In several years alewife biomass exceeded the limit of 40 kg.har 1 that has been proposed for the continuing presence of Daphnia galeata mendotae. However, the abundance of this zooplankter has increased from less than 0.01 mg.L −1 (dry weight) when alewife biomass was high to 0.30 mg.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.