Denitrification in freshwater and coastal marine ecosystems: Ecological and geochemical significance

Abstract

Denitrification occurs in essentially all river, lake, and coastal marine ecosystems that have been studied. In general, the range of denitrification rates measured in coastal marine sediments is greater than that measured in lake or river sediments. In various estuarine and coastal marine sediments, rates commonly range between 50 and 250 µmol N m−2 h−1, with extremes from 0 to 1,067. Rates of denitrification in lake sediments measured at near‐ambient conditions range from 2 to 171 µmol N m−2 h−1. Denitrification rates in river and stream sediments range from 0 to 345 µmol N m−2 h−1. The higher rates are from systems that receive substantial amounts of anthropogenic nutrient input. In lakes, denitrification also occurs in low oxygen hypolimnetic waters, where rates generally range from 0.2 to 1.9 µmol N liter−1 d−1. In lakes where denitrification rates in both the water and sediments have been measured, denitrification is greater in the sediments. The major source of nitrate for denitrification in most river, lake, and coastal marine sediments underlying an aerobic water column is nitrate produced in the sediments, not nitrate diffusing into the sediments from the overlying water. During the mineralization of organic matter in sediments, a major portion of the mineralized nitrogen is lost from the ecosystem via denitrification. In freshwater sediments, denitrification appears to remove a larger percentage of the mineralized nitrogen. N2 fluxes accounted for 76–100% of the sediment‐water nitrogen flux in rivers and lakes, but only 15–70% in estuarine and coastal marine sediments. Benthic N2O fluxes were always small compared to N, fluxes. The loss of nitrogen via denitrification exceeds the input of nitrogen via N2 fixation in almost all river, lake, and coastal marine ecosystems in which both processes have been measured. Denitrification is also important relative to other inputs of fixed N in both freshwater and coastal marine ecosystems. In the two rivers where both denitrification measurements and N input data were available, denitrification removed an amount of nitrogen equivalent to 7 and 35% of the external nitrogen loading. In six lakes and six estuaries where data are available, denitrification is estimated to remove an amount of nitrogen equivalent to between 1 and 36% of the input to the lakes and between 20 and 50% of the input to the estuaries.