EFFECTS OF LAKE SIZE ON PHYTOPLANKTON NUTRIENT STATUS

Phytoplankton nutrient status measurements (C/P, C/N, C/chlorophyll, N /P, alkaline phosphatase activity, and N debt) were measured for 6 yr in seven remote Canadian Shield lakes. Lakes Nipigon and Superior were also studied for 2 yr. These lakes varied in surface area from 29 to 8.223 x 10(6) ha, they all stratified fully during the summer and had water renewal times >5 yr. All lakes were severely P deficient; howeve r, the large lakes (>2000 ha) were consistently less P deficient than small lakes. A growth-rate indicator (photosynthesis normalized to par ticulate C) agreed with nutrient status indicators, in that small lake s had lower rates than large lakes. Total P was a good predictor of ch lorophyll, but factors related to lake size (temperature and mixed dep th) were equally good or better predictors of nutrient status. Decreas ing mean water column light intensity could not explain the lower P de ficiency of large lakes. The deeper, more energetic mixed layers in la rge lakes apparently cause P to be recycled more efficiently. Extrapol ation of observations or experimental results from small to large lake s requires recognition that phytoplankton in large lakes are less nutr ient deficient and may have higher growth rates.