Nutrient internal cycling and the trophic regulation of Green Lake, Wisconsin1

Abstract

Algal nutrients (Si, N, P) and dissolved oxygen were studied in Green Lake, Wisconsin, in 1971– 1972, and in 1978–1980 following a reduction of about 35% in total external P loading in December 1976.Despite oxic conditions, there is more P in the hypolimnion than would be expected from the dissolved oxygen and from classical Redfield stoichiometry. Because of a metalimnetic dissolved oxygen minimum in late summer, the calculation and interpretation of the hypolimnetic oxygen deficit is affected by turbulent transport of dissolved oxygen.Vertical fluxes of P into the mixed layer are important in late spring and early summer, but, owing to metalimnetic P depletion, negligible between mid‐July and mid‐October. Vertical transport of Si into the mixed layer is negligible in summer and early fall compared with inputs from the land drainage basin.Although P is high at spring turnover, epilimnetic concentrations of P are low and Secchi transparency is high by mid‐to‐late summer. These seasonal changes are caused by sedimentation of P and low seasonal inputs from the drainage basin. This P is recovered in the hypolimnion and recirculated throughout the lake at fall overturn. It causes silica limitation of the diatom bloom each succeeding spring.The total P content of Green Lake and its seasonal partition among the layers were remarkably stable between 1971 and spring 1980, despite the reduction in external loading in 1976. The hypolimnetic oxygen deficit also showed no significant change between the first and eighth decades of this century.