Impacts of hypereutrophication and aquatic invasive species on cladoceran dynamics in the Bay of Quinte Area of Concern, Lake Ontario

Aquatic food webs in the Laurentian Great Lakes are impacted by the legacy effects of twentieth-century land-use change and nutrient enrichment which co-occur with climate warming and invasive species. Zooplankton populations are currently degraded in the Bay of Quinte Area of Concern, Lake Ontario, which experienced cultural hypereutrophication by the 1950 s and the invasion of dreissenid mussels c. 1994. A drop in pelagic productivity was noted in 2013, raising concerns for fisheries. We examined Cladocera (Crustacea: Branchiopoda) remains in four dated sediment cores from the Bay of Quinte to assess food-web response to historic stressors. We characterized long-term trends in cladoceran abundance, composition, and Bosmina spp. mucro length (as a proxy for zooplanktivory). The largest assemblage change occurred with early 1900 s eutrophication: concentrations of B. longirostris and Chydorus sphaericus increased sharply while Daphnia longispina complex concentrations and littoral diversity declined. Minor declines in concentrations of eutrophication-associated taxa co-occurred with nutrient abatement actions, while dreissenid establishment was associated with increased zooplanktivory in the shallow Upper Bay but decreased zooplanktivory in the deeper Middle Bay. Modern-day assemblages are distinct from pre-eutrophication assemblages, especially as invasive Eubosmina coregoni is abundant, and inferred fish planktivory is elevated relative to historic conditions. Observed declines in pelagic productivity c. 2013 coincided with increases in littoral taxa and a continued depression of large pelagic grazer Daphnia , indicating a potential shift to littoral production, and suggesting that dreissenids and climate warming may contribute to altered food-web dynamics.