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

Abstract

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.In the past 15-20 yr it has become evident that significant amounts of P may be released from aerobic sediment surfaces of shallow temperate lakes. The P release often constitutes a major fraction of the total P load to the trophogenic zone in summer (Andersen 1982; Ryding 198 5). This internal P loading has been identified as an important mechanism in delaying recovery of shallow lakes following reduced external P loading (Ryding 1985), although homeostasis in the biological structure may be important too (e.g. Jeppesen et al. 199 1). Several physical, chemical, and biological factors influence the P exchange across aerobic sediment surfaces. Lake water temperature, N03-, and pH have been proposed as important steering factors for the season-' Present address:

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