Elevated zooplankton production in a eutrophic Lake Ontario embayment: Hamilton Harbour 2002–2014
Hamilton Harbour is an Area of Concern in western Lake Ontario, long stressed by cultural eutrophication, urbanization and invasive species. Despite high nutrient levels leading to hypolimnetic hypoxia and contaminated sediment, it is a highly productive environment. To better understand zooplankton dynamics in Hamilton Harbour, we conducted biweekly May to October sampling of zooplankton and rotifer composition at open water harbour sites from 2002–2014. May to October zooplankton density, dry biomass and total production averaged 265 ± 16 animals l−1, 306 ± 19 mg m−3 and 4131 ± 359 mg m−3, (±SE), respectively. These values are among the highest reported in the Great Lakes, with biomass two to seven times greater than in other eutrophic embayments. Zooplankton populations and taxonomic seasonality have remained relatively stable in Hamilton Harbour since 2002. Biomass is often dominated by smaller taxa such as Bosmina, Eubosmina and juvenile copepods, suggestive of high fish planktivory, but Daphnia retrocurva and D. galeata mendotae are also dominant during the summer, indicating improvements in the zooplankton community since the 1970s when Daphnia and cyclopoids were uncommon. Conversely, rotifers have declined over the last 40 years, though while still numerically dominant, now comprise <4% of total biomass and production compared to 40% in the 1970s. Both adult Dreissenid Mussels and their veliger larvae are less abundant in Hamilton Harbour compared to nearshore Lake Ontario. Zooplankton appear to be effectively utilizing high production rates of edible algae and microorganisms in the harbour. More work is needed to explore trophic interactions in this eutrophic ecosystem and the effects of hypolimnetic hypoxia on the zooplankton community.