Cumulative stressors shape elevated zooplankton populations in eutrophic Hamilton Harbour, Ontario.

Hamilton Harbour at the west end of Lake Ontario has historically been regarded as one of the most polluted areas of the Laurentian Great Lakes due to its intense industrial past and urbanization. This eutrophic Area of Concern experiences regular harmful algal blooms and severe hypolimnetic hypoxia due to high nutrient loadings and internal phosphorus cycling. Despite these impacts, lower food web monitoring by Fisheries and Oceans Canada (DFO) since 2002 has found some of the highest levels of zooplankton production the Great Lakes, typically an order of magnitude higher than nearshore Lake Ontario. The most dominant taxa include the cladoceans Daphnia retrocurva, D. galeata, Bosmina and the cyclopoids Diacyclops and Mesocyclops. Unlike planktivorous fishes, zooplankton appear to be largely unaffected by hypoxia, and their excessively high biomass levels are largely due to the lack of top-down control by forage fishes. However, in recent years, cladocerans and cyclopoids periodically show dramatic late summer population declines, coupled with increases in diaptomid copepods. Cumulative stressors appear to be responsible, including blooms of unpalatable filamentous algae and/or the dinoflagellate Ceratium, combined with predation by the spiny water flea Bythotrephes. This invader was virtually absent in the harbour prior to 2016. Proposed remedial actions to improve trophic transfer of zooplankton to fishes include further reductions in nutrient loadings, leading to reduced hypoxia, improvements in pelagic fish habitat and fewer undesirable algal blooms. 

Presentation Preference: Oral

Primary Presenter: Kelly Bowen, Fisheries and Oceans Canada (kelly.bowen@dfo-mpo.gc.ca)

Authors:

Kelly Bowen, Fisheries and Oceans Canada  (kelly.bowen@dfo-mpo.gc.ca)

Warren Currie, Fisheries and Oceans Canada  (warren.currie@dfo-mpo.gc.ca)

Mark Fitzpatrick, Fisheries and Oceans Canada  (mark.fitzpatrick@dfo-mpo.gc.ca)

Heather Niblock, Fisheries and Oceans Canada  (heather.niblock@dfo-mpo.gc.ca)