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

Abstract

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 C·m−2·d−1 (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 PO4‐P·m−2. 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 (10–50 mg PO4‐P·m−2). Comparison of the measured rate constants for turnover (ki) with the constants expected for photosynthesis (ke) showed P flux patterns. In Lake Washington, ki varied from <0.05·h−1 in winter to 5–10·h−1 during summer stratification, whereas ke was comparable during prebloom and early‐bloom periods and only about 0.1·h−1 in summer. In Findley Lake, ki (2.9–5.3·h−1) was always >ke (0.05·h−1). Only after phosphate depletion did the usual pattern of rapid uptake and release occur. Nutrient addition and isotope partitioning experiments suggested that the divergence of ki and ke reflected a stressed physiological condition of the algae that prevented incorporation and promoted compensatory rapid turnover of phosphate.