Seasonal cycles of 02, N03- and S042- reduction in estuarine sediments: the significance of an N03- reduction maximum in spring

Abstract

Rates of oxygen uptake, denitrification, nitrate reduction to ammonia, and sulfate reduction were measured in a Danish estuary over 1.5 yr. In sediments near the sea entrance, O2 and SO?,- reduction dominated and the relative contributions of the 4 processes were 65, 3. 5 and 27 %. respectively, of the total electron flow. In sediments near the river outlet, sulfate became limiting while nitrate was more abundant. This shifted the relative contributions towards nitrate reduction: 44. 4, 33 and 19%, respectively. At low salinities, depth of the sulfate reduction zone (4 to 10 cm), but not maximum reduction rate, was limited by low sulfate concentrations in the overlying water. The nitrate zone varied from 0.5 to 5 cm depth over the year. Oxygen uptake and sulfate reduction varied seasonally in accordance with temperature. Reduction of nitrate to N, and to NH:, as well as emission of N,O the atmosphere, showed a short maximum in spring and were relatively constant throughout the rest of the year. The spring maximum coincided with a rapid water-temperature increase and a high influx of nitrate from the main river. Annual emission of N20 corresponded to only 1 to 5 % of the measured denitrification. Annual loss of combined nitrogen by denitrification in sediments corresponded to 5 % of nitrate influx from the river.