Metalimnetic Gradient and Phosphorus Loss from the Epilimnion in Stratified Eutrophic Lakes

Abstract

Phosphorus loss from the epilimnion in the beginning of summer is reduced in those highly eutrophic stratified lakes in which a sharp temperature gradient is developed in the metalimnion early in the season. Due to increased water density and viscosity, the falling organic particles are trapped in the metalimnion. The easily oxidizable material is decomposed in this layer causing severe oxygen depletions. The orthophosphate phosphorus does not easily complex with metallic ions in these reducing conditions, so it is not carried down into the sediments. Remaining not far from the reach of the epilimnion it may be easily brought back to the euphotic layer (by turbulent diffusion and migrating organisms) to be incorporated, once more, into algal biomass. Thus, less phosphorus is temporarily lost to the sediments when the metalimnetic gradient is steep. This would enhance the hypertrophic situations in the epilimnia of sharply stratified lakes in early summer, even before the whole hypolimnion is depleted of oxygen and the rate of internal loading from the sediments is increased.