EFFECTS OF LAKE SIZE ON PHYTOPLANKTON PHOTOSYNTHESIS

Phytoplankton photosynthesis (PP) was measured for 6 vr in seven remote Canadian Shield lakes that stratify fully during the summer and have water renewal times >5 yr but vary from 29 to 34 700 ha; Lakes Nipigon and Superior were also studied in two years. Chlorophyll and PP at optimum light were low in the smallest and largest lakes and increased systematically to values nearly five times higher in midsized lakes (approximately 10(3) ha). Daily PP per square metre of lake surface and annual PP per cubic metre of the mixed layer also varied in this manner, but annual PP per square metre was high in large lakes (despite their low density rates) because of their long growing seasons. Additional data are needed to determine whether the photosynthesis maximum in midsized lakes is inherently size related or an accidental statistical result. Intraannually, chlorophyll-based photosynthesis parameters (P(m)B, alpha(B)) were similar in all lake sizes, but interannually they varied by two to three times; this interannual variation was significantly correlated with total rainfall during May and June. Implications for extrapolating experimental results from small to large lakes, selecting lakes for interregional comparison studies, and predicting how climatic warming would affect phytoplankton photosynthesis are discussed.