IMPORTANCE OF TEMPERATURE, NITRATE, AND PH FOR PHOSPHATE RELEASE FROM AEROBIC SEDIMENTS OF 4 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-1 (average summer values). In three of the lakes, water temperature alone explained approximately 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 NO3- in all four lakes, but by pH only in one lake when these three parameters varied within the normal seasonal range (temperature: 0-degrees-21-degrees-C; NO3-: 0-200-mu-M; 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 NO3- 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, NO3- additions increased the P release from sediment, probably by stimulating the mineralization process.