Photosynthetic response of lake plankton to nutrient enrichment: A test for nutrient limitation

Measurements of carbon uptake by lake plankton at different levels of added phosphorus reveal some phytoplankton strategies that contribute to their survival in low‐nutrient waters. Nutrient‐deficient populations seem to temporarily reduce, rather than enhance, photosynthesis when phosphorus becomes available. This contradicts the basic hypothesis of the 14C bioassay used to determine “limiting” nutrients in natural waters. The reduction of carbon uptake rate continues until either the nutrient addition is taken up or no further storage is possible; at this time carbon upake may be enhanced. Concurrent measurements of phosphorus uptake showed no light response unless the plankton were exposed to significant phosphorus concentrations in the dark for several hours. Phytoplankton seem to be adapted for and dependent upon intermittent events exposing them to nutrient‐enriched water. The ability of phosphate‐depleted plankton to take up phosphate in excess of their growth requirements was used to develop an index for phosphate deficiency. To normalize for different biomass levels, the optimum photosynthetic rate was divided by the maximum uptake velocity giving a range for the C:P maximum uptake from 1.2 to 206 for plankton samples of varying phosphate deficiency.