PHOSPHORUS GEOCHEMISTRY IN THE SEDIMENT-WATER COLUMN OF A HYPEREUTROPHIC LAKE

The role of sediment biogeochemistry on diffusive flux of inorganic P was examined under laboratory conditions using intact sediment cores obtained from a subtropical hypereutrophic lake. The effect of light and dark conditions in the water column on soluble P flux from sediments was measured over a period of 1 yr. Soluble reactive P (SRP) level in the overlying water column was low (< 0.01 mg P L-1) under light conditions, while values under dark conditions steadily increased. This suggests algal uptake under light conditions. Diffusive flux of soluble P from sediments as calculated by measuring the increase in concentration of the overlying water in the dark was 2.71 Mg P m-2 d-1. Flux of this magnitude could increase lake water SRP levels by approximately 0.5 mg P L-1 yr-1. Soluble reactive P concentrations were roughly equivalent to dissolved P concentrations, both for the water column and for the sediment porewater. Porewater SRP increased with depth to a maximum of 6.0 mg P L-1 at 40 cm. Phosphorus flux as calculated from prewater SRP gradients averaged 1.69 mg P m-2 d-1. Sediment P fractionation indicated that water soluble P, KCl extractable P, and Fe and Al-bound P decreased with depth, whereas the amount of Ca-bound P increased. Calcium-bound P was the dominant fraction, comprising over 50% of the total P content. Ion activity products (IAP) calculated using GEOCHEM indicated that the sediments were supersaturated with respect to apatite, beta tricalcium phosphate and whitlockite, with the latter expected to be the phase controlling PO4-3 activities.