Metalimnetic gradients and the vertical distribution of phosphorus in a eutrophic lake

Abstract

Analyses of the vertical distribution of phosphorus in a eutrophic lake showed constant accumulation of soluble reactive phosphorus and total P in the hypolimnion following spring diatom blooming. The downward flux of phosphorus was realised mostly in spring due to high rates of sedimentation and aerobic decomposition of algal biomass. Nearly two thirds of hypolimnetic phosphate-P accumulation had occurred before the hypolimnion became deoxygenated. An increase in soluble reactive P in the hypolimnion by a further 100 mg P m -3 took place during summer, most probably due to internal P loading from bottom sediments. Distinct gradients of phosphorus, chlorophyll, alkaline phosphatase activity and seston across the thermocline coupled with a low sedimentation rate in the summer presented evidence for the barrier role of the metalimnetic zone in vertical fluxes of matter in a lake. Possible consequences of this phenomenon for phosphorus cycling in a lake are discussed.