Importance of temperature, nitrate, and pH for phosphate release from aerobic sediments of four shallow, eutrophic lakes

Phosphorus released from aerobic sediment surfaces made up the major fraction of the total P-load to the trophogenic zone in four shallow Danish lakes in 1987. Gross release rates were 15, 21, 33, and 100 mg P m-2 d- l (average summer values). In three of the lakes, water temperature alone explained -70% of the seasonal variation in sediment P release. Long-term experiments with undisturbed sediment cores revealed that the P release was significantly influenced by temperature and NO,- in all four lakes, but by pH only in one lake when these three parameters varied within the normal seasonal range (temperature: O”-21°C; NO,-: O-200 PM; pH: 7.5-10.5). Q10 values for the temperature effect were between 4.1 and 6.8 in the three lakes with large proportions of Fe-bound P in the sediment, but only 3.5 in the lake in which the P pool was relatively small. The thickness of the oxidized layer at the sediment surface varied from 3 to 15 mm and decreased when temperature increased. High N03- concentrations increased the thickness throughout the season and accordingly sediment P release was reduced in winter and early summer. Meanwhile, in late summer when inorganic N was depleted in lake water, N03- additions increased the P release from sediment, probably by stimulating the mineralization process.