In the Northeast, removal of dams and addition of fish ladders have renewed the connection between marine and freshwater, facilitating passage of anadromous fish from ocean to lake to spawn. Restoration of coastal streams benefits the declining anadromous alewife (Alosa pseudoharengus) population, but management decisions regarding alewife (re)introduction may strongly impact freshwater ecosystems, where they can strongly affect zooplankton communities through predation and nutrient cycling. Here we evaluate the potential consequences of alewife restoration on seasonal changes in algal and Daphnia stoichiometry and densities, and Daphnia reproduction and life history in a lake with anadromous alewives (Bride Lake) and in a lake without alewives (Linsley Pond). We focus on Daphnia because of its central role as a grazer and because the effects of food quality and predation on Daphnia stoichiometry and life history are well-characterized. We concentrate on seasonal comparisons between early spring, when anadromous alewives are absent from lakes, and early summer, when predation by young-of-the-year anadromous alewives on Daphnia is intense.

Results/Conclusions

From early spring to early summer, we observed a larger increase in algal nitrogen and phosphorus content in Bride Lake than in Linsley Pond, likely reflecting nutrient loading and cycling by alewives. The addition of nitrogen and phosphorus from the alewives caused a decrease in algal C:N ratio and C:P ratio in Bride Lake – a trend not observed in Linsley Pond. Daphnia elemental composition often does not track seasonal changes in algal stoichiometry, indicating that other factors like alewife predation and Daphnia life history influence elemental content. In Bride Lake, Daphnia size decreased and reproductive output remained constant, indicating strong top-down selection. In Linsley Pond, Daphnia size remained constant and reproductive output declined. Our results indicate important interactions between food quality (as indicated by algal elemental composition), life history, and predation. Alewife predation appears to strongly impact Daphnia size and reproductive output, even while increased nutrient cycling increases food quality in Bride Lake. In these lakes, changes in Daphnia and algal stoichiometry influence Daphnia life history response to predation. Our field measurements provide a detailed understanding of the impact of alewife (re)introduction on Daphnia life history and highlight the importance of studying interactions between predation and nutrient stoichiometry.