Georgian Bay phytoplankton: Ecology and response to contaminants

The phytoplankton productivity of Georgian Bay was studied during 1974. Lakewide samples were collected at 16 stations during monthly cruises from April to December and identification and enumeration was carried out by the Utermohl technique. Contaminant bioassays with metals added singly and as a mixture were performed in 1980 and 1981. Based on an overall average, Diatomeae (36–73%) was the most prevalent component of phytoplankton biomass followed by Chrysophyceae (5–38%), and Cyanophyta (3–27%). Biomass means of all stations by cruise indicated a range of 0.35 to 0.61 g·m^–3 with a bimodal seasonal pattern. Although ultraplankton made the highest mean percent contribution to the biomass (37%), the other size assemblages such as < 5 µm (26%) and netplankton (29%) were not unimportant.Size fractionation of primary productivity during 1974 revealed that a major portion of photosynthesis (39–70%) was in the <20 µm size fraction. Algal Fractionation Bioassays conducted during 1980 and 1981 indicated a significant inhibition of ultraplankton productivity. Additional bioassays with single metals and in combination showed differential toxicity to various phytoplankton size assemblages.The phytoplankton biomass and floristic composition indicate the Georgian Bay ecosystem to be oligotrophic. This ecosystem appears to be controlled by the physical dynamics of thermal and flow regimes. Compared with other Great Lakes, the low P/B quotients recorded in oligotrophic Georgian Bay is enigmatic and may be attributable to the observed sensitivity of phytoplankton to contaminants originating from anthropogenic and natural sources.