Seasonal phosphorus release from exposed, re-inundated littoral sediments of two Australian reservoirs

Water levels in many reservoirs typically fluctuate seasonally, but the eff ects of re-inundation of exposed sediments on nutrient dynamics in the wate r column are poorly known. This study concerns the seasonal differences in the potential of sediments from two Australian reservoirs, after having und ergone different degrees of in situ desiccation, to release P under aerobic conditions. Differences were determined between biotic and abiotic P relea se, and results were also examined in relation to sediment chemistry. The t wo reservoirs, Carcoar Dam and Lake Rowlands, demonstrated different patter ns of P release involving an interactive complex of P release mechanisms. S ediment chemistry at the reservoir margins was important because of the hig her concentrations of N, P, Fe and Mn in Lake Rowlands. Physical and chemic al processes influenced P uptake and release due to desiccation and oxidati on of sediments and were of greater importance in Carcoar Dam. Abiotic P re lease from sterilised sediments was greater than from unsterilised sediment s where both biotic and abiotic processes were apparent. Biotic P uptake an d release were especially marked in Lake Rowlands where large macrophyte be ds provided a rich source of organic matter. Little seasonal difference in P release was detected. The increased P release from dried sediments has ra mifications for internal P loading into reservoirs and for the calculation of P budgets. For managers of reservoirs where large expanses of sediment a re exposed during drying, it may be better to maintain high water levels, w here possible, during the summer by modifying drawdown practices.