¹Plankton Canada, Burlington, Ontario, Canada
²Fisheries & Oceans Canada, Canada Centre for Inland Waters, 867 Lakeshore Rd., Burlington, ON, L7R 2A6, Canada

*Corresponding author: munawarm@dfo-mpo.gc.ca

Summary

   Phytoplankton samples were collected from three depths (epilimnion, metalimnion and hypolimnion) at a single station in the central basin of Lake Erie during the summer of 2004 (July and August). This area of the central basin has a long history of hypoxia. Oxygen stress (Dissolved Oxygen < 4 µg l^-1) was observed only during August in the hypolimnion and not in July. A substantial amount of phytoplankton was recorded in the hypoxic stratum (1.3 g m^-3) dominated by Chlorophyta (34%) and other phytoflagellates. A large number of species (111) were identified in the hypoxic stratum. The most prevalent (as biomass) were a mixture of species belonging to Cyanophyta, Chlorophyta, Chrysophyceae, Cryptophyceae and Dinophyceae (Chroococcus dispersus var. minor, Tetraëdron minimum, Chrysochromulina parva, Rhodomonas minuta and Gymnodinium helveticum). These species are common in the Great Lakes and occur in diverse nutrient conditions. Importantly, these species do not characterize either eutrophic or oligotrophic conditions. Some species, including Rhodomonas minuta and Chrysochromulina parva, were recorded in all strata, while others such as Cyclotella atomus and C. glomerata were found only in the epilimnion. P/B quotients were highest for picoplankton and lowest for nanoplankton and their distribution in the water column did not seem to be affected by hypoxic conditions. The detailed phytoplankton biomass, species composition and P/B ratios presented in this study confirm that the hypoxic zone is viable and supports a large number of photosynthetically active species. The plankton identified in the oxic and hypoxic strata were similar in composition. Furthermore, the hypoxic samples did not contain dead settling algae but in fact were active and viable. Our findings support the conclusions of Meilander et al. (2008) that the oxygen stressed hypolimnion observed in the summer of 2004 harboured a healthy phytoplankton community, which is anything but dead.