Comparison of phosphorus release from littoral and profundal sediments in a shallow, eutrophic lake

The release of phosphorus (P) and iron (Fe) was studied in undisturbed sediment cores from littoral and profundal sediments of shallow, eutrophic Lake Arreskov (Denmark). The cores were incubated for 100 days in darkness under oxic and anoxic conditions at 12 and 19 °C and cumulative fluxes of P and Fe were determined. The littoral sediment showed higher P release than the profundal sediment under both oxic and anoxic conditions, however, the difference was only significant during the first 35 days under anoxic conditions. The release rates of P and Fe were significantly higher under anoxic than under oxic conditions. Sequential extractions of P-fractions in sediment from different depths (0–15 cm) were conducted before and after the experiment. Under anoxic conditions, iron-bound P was the most important fraction for the loss of P from both sediments. The decrease in the iron-bound P pool contributed 54% and 86% of the loss of total particulate P (TPSed) from littoral and profundal sediments under anoxic conditions, respectively, whereas it under oxic conditions only constituted 25% of the TPSed lost from the littoral sediment. Organic P accounted for 31 and 55% of the TPSed lost from the littoral sediment under anoxic and oxic conditions, respectively, while organic P was unimportant (< 1%) for the loss of TPSed from the profundal sediment under anoxic conditions. This indicates that littoral sediments may contribute significantly to the internal P loading, especially if low O2 concentrations occur at the sediment-water interface.