The Importance of Lake Ontario's Deep Chlorophyll Layer for Invertebrates and Fish

Recent declines in epilimnetic nutrients and zooplankton biomass have resulted in a shift towards deeper production in Lake Ontario. As a result, there has been an increase in the deep chlorophyll layer (DCL) during the stratified period. In turn, zooplankton grazing and subsequent nutrient regeneration may occur at deeper depths, possibly affecting the production of planktivorous fishes and invertebrates. Given Lake Ontario’s reliance on Alewife to sustain salmonid fisheries, understanding tropic shifts in food webs is of particular importance. In this study we used a diel approach to describe trophic interactions of planktivores and their prey within and around Lake Ontario’s DCL during 2013. During spring mysids appear to graze heavily on diatoms, but in July and September mysids switch to zooplanktivory and compete with Alewife and Rainbow Smelt for prey. The predatory impact of the invasive spiny water flea Bythotrephes was minimal in Lake Ontario during 2013. Bythotrephes abundance was relatively low compared to previous years, while consumptive impacts of Cercopagis were far greater in the epi- than metalimnion. An observed increase in Limnocalanus (a large-sized energy-rich calanoid) likely favors fish production in Lake Ontario’s DCL but does not benefit mysid production as mysids do not select for Limnocalanus.