Nitrification and denitrification in marine sediments from Puget Sound1

The anomalous interstitial water chemistry in Puget Sound sediments (200-m water depth) indicates an intermediate zone at 20to 30-cm depths which consists of a mixture of 90% interstitial water and 10% bottom water. Irrigation by burrowing benthic organisms is believed responsible. The vertical distribution of interstitial ammonia, nitrate, and nitrite indicates that nitrification is occurring in this intermediate sedimentary zone, with oxygen being supplied by the irrigation process. Above and below this zone denitrification predominates. Denitrification in low oxygen areas has been identified as the major sink for fixed nitrogen from the world ocean (Emery et al. 1955; Delwiche 1970; Cline and Richards 1972; Holland 1973; Codispoti and Richards 1976), and therefore these areas are very important in the maintenance of steady state conditions for this important nutrient. The role of denitrification in the sediments beneath the low oxygen zone in the eastern tropical North Pacific has been estimated to be minimal (Codispoti 1973)) but a thorough investigation of dcnitrification in marine sediments has not been made. Barnes et al. (1975) reported an excess in interstitial N:! in Santa Barbara Basin sediments. Since the observed increases were larger than could be explained by reduction of interstitial NOs-, they proposed that Na was formed from NHd+ via an oxidized NOaintermediate (i.e. nitrification followed by denitrification ) . They suggested that enough 02 could diffuse into the upper few centimeters of sediments to support NI-14+ oxidation. In their model, however, they did not make clear whether nitrification and denitrification occurred simultaneously in the upper few centimeters or were spatially separated. Vanderborght and Billen (1975) modeled the distribution of nitrate in the sediments of an artificial marine lagoon and found that ’ Contribution 979, Department of Occanography, University of Washington. Financial support was provided by IDOE-NSF grant DES7503618. in sandy sediments nitrification occurred in the surface sediments and denitrification at greater depths. Bioturbation and irrigation can influence interstitial water chemistry and may provide additional O2 to the sediments which can be used for oxidation of NHd+ (Goldhaber et al. 1977). Although there are few data available on interstitial nitrate concentrations in marine sediments, several workers have reported anomalously high values (Rittenberg et al. 1955; Pamatmat 1973; Greenslate et al. 1974; Hartmann et al. 1974), suggesting that nitrification in marine sediments could be a widespread phenomenon. In this paper we present data indicating the operation of a nitrification’denitrification couple at a station in Puget Sound, Washington. Our data indicate vertical zonation of the nitrification/denitrification processes. WC propose that mixing of interstitial and oxygenated bottom water through irrigation by burrowing benthic organisms results in nitrification in an intermediate sedimentary zone, above and below which denitrification predominates. We thank P. Jumars for identifying the benthic organisms discussed in our study and for valuable discussions concerning benthic biology. S. Walter performed the sediment size analysis. We also thank S. Emerson, M. Bender, and C. Martens for valuable discussions and constructive criticisms.