Patterns of phosphorus supply and utilization in Lake Washington and Findley Lake1,2

Patterns of phosphorus supply and utilization were investigated for 2 years. In Lake Washington, the phytoplankton bloom began in February or early March, reaching peaks of 2,165 ( 1974) and 1,885 mg Cm rnd2 * d-l (1975). Most of the P utilized was from phosphate accumulated in the water, resulting in depletions of soluble reactive phosphorus in the euphotic zone of 260-12 mg POrP .rne2. Zooplankton excretion supplied phosphate during the later stages of the bloom and in summer. In Findley Lake, the blooms were of shorter duration and reached maxima of 400-711 mg C *m-2*d-1. Phosphate was supplied from fluvial inputs and zooplankton excretion, with little accumulation (lo-50 mg P0,-P*m-2). Comparison of the measured rate constants for turnover (k,) with the constants expected for photosynthesis (k,) showed P flux patterns. In Lake Washington, k, varied from k, (O.OS.h-l). 0 1 ft n y a er phosphate depletion did the usual pattern of rapid uptake and release occur. Nutrient addition and isotope partitioning experiments suggested that the divergence of kt and k, reflected a stressed physiological condition of the algae that prevented incorporation and promoted compensatory rapid turnover of phosphate.