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

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Abstract
Rates of O2 uptake, denitrification, nitrate reduction to ammonia, and SO4 reduction were measured in a Danish estuary over 1.5 yr. In sedients near the sea entrance, O2 and SO42- 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 (4-10 cm), but not maximum reduction rate, was limited by low sulfate concentrations in the overlying water. The nitrate zone varied from 0.5-5 cm depth over the year. O2 uptake and sulfate reduction varied seasonally in accordance with temperature. Reduction of nitrate to N2 and to NH4+ as well as emission of N2O into 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 N2O corresponded to only 1-5% of the measured denitrification. Annual loss of combined N by denitrification in sediments corresponded to 5% of nitrate influx from the river.