Environmental factors influencing chlorophyll v. nutrient relationships in lakes

SUMMARY. 1. A model relating log chlorophyll a concentration to log epilimnetic total phosphorus (TP) concentration was re‐examined based on: (a) comparative and temporal studies of four stratifying Wisconsin and other highly eutrophic temperate lakes; (b) comparative summer lake surveys from Iowa and Alberta. 2. Although P‐limited, deeper lakes with long hydraulic residence times and low external and internal nutrient loading in summer had summer chlorophyll a yields below model predictions based on spring and summer epilimnetic TP concentrations. 3. For lakes with summer epilimnetic TP between 30 and 80 mg m−3, chlorophyll a concentrations exceeded model predictions based on summer TP. This relationship held even for Lake Delavan, Wisconsin, where the ratio of available N to P was unfavourably low during spring turnover, and where the trans‐thermocline N:P flux ratio was sub‐optimal for algal needs in early summer. 4. With increasing summer TP concentrations and/or increasing epilimnetic circulation depth (>5m), chlorophyll a concentrations fell below model predictions—independent of the potential for N‐limitation. This plateauing in chlorophyll a response occurred at lower epilimnetic TP content ( −2) in lakes with elevated non‐algal light extinction coefficients. Using Tailing's algorithm for the‘column compensation point’ (algal photosynthesis = algal respiration over diel cycle), light limitation best explains this fall‐off in chlorophyll a yield. 5. The failure of the Dillon & Rigler (1974) spring TP v. summer chlorophyll a model for these Wisconsin lakes is unrelated to N‐limitation. Instead, it reflects internal adjustment in take TP in response to stratification and seasonal external P loading.