RATES OF NITRIFICATION, DISTRIBUTION OF NITRIFYING BACTERIA AND INORGANIC N FLUXES IN NORTHERN BERING CHUKCHI SHELF SEDIMENTS

Spatial distribution of sediment nitrification rates and fluxes of amm onium and nitrate were measured in shelf sediments of the northern Ber ing and Chukchi seas. The sediments could be divided into three main a reas depending on macrofaunal activity and input of organic nitrogen. Sediments underlying the highly productive Bering Shelf-Anadyr water ( BSAS) were characterized by a high macrofaunal biomass and a high inpu t of nitrogen-rich organic material. Tube-dwelling amphipods dominated in the sandy sediments of the northern Bering Sea, while bivalves dom inated in the fine textured sediments of the Chukchi Sea. Sediments un derlying the low productive Alaska Coastal Water (ACS) were characteri zed by low macrofaunal biomass and an input of lower quality organic m aterial. Generally nitrification rates and nutrification potentials (N P) were highest in BSAS and lower in ACS. Nitrification rates of surfa ce sediment, calculated from NP, accounted for 90% of the measured rat es in ACS, but only 35-75% in BSAS. These data together with the distr ibution patterns of NP and pore water nitrate profiles implied, that m ost sediment nitrification was confined to the sediment surface in ACS and in BSAS bivalve sediments, while most sediment nitrification took place in the ventilated burrow walls of BSAS amphipod sediments. The NH4+ efflux was five-fold greater from BSAS compared to ACS, whereas t he estimated sediment net NH4+ production was three-fold greater. The increase in NH4+ efflux relative to net NH4+ production could mostly b e attributed to macrofaunal excretion. The NO3- flux between sediment and water column was correlated with NO3- concentrations in the bottom water. At concentrations higher than 10 muM NO3-, the flux was direct ed into the sediment and at lower concentrations out of the sediment. Spatial distribution of high bottom water NO3- concentrations correlat ed with high NH4+ fluxes out of the sediment. This resulted in a lower net efflux of inorganic nitrogen from these Sediments.