Rates of nitrification, distribution of nitrifying bacteria and inorganic N fluxes in northern Bering-Chukchi shelf sediments

Abstract

Spatial distribution of sediment nitrification rates and fluxes of ammonium and nitrate were measured in shelf sediments of the northern Bering and Chukchi seas. The sediments could be divided into three main areas 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 input of nitrogen-rich organic material. Tube-dwelling amphipods dominated in the sandy sediments of the northern Bering Sea, while bivalves dominated in the fine textured sediments of the Chukchi Sea. Sediments underlying the low productive Alaska Coastal Water (ACS) were characterized by low macrofaunal biomass and an input of lower quality organic material. Generally nitrification rates and nutrification potentials (NP) were highest in BSAS and lower in ACS. Nitrification rates of surface sediment, calculated from NP, accounted for 90% of the measured rates in ACS, but only 35–75% in BSAS. These data together with the distribution patterns of NP and pore water nitrate profiles implied, that most 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 NH 4 + efflux was five-fold greater from BSAS compared to ACS, whereas the estimated sediment net NH 4 + production was three-fold greater. The increase in NH 4 + efflux relative to net NH 4 + production could mostly be attributed to macrofaunal excretion. The NO 3 − flux between sediment and water column was correlated with NO 3 − concentrations in the bottom water. At concentrations higher than 10 μM NO 3 −, the flux was directed into the sediment and at lower concentrations out of the sediment. Spatial distribution of high bottom water NO 3 − concentrations correlated with high NH 4 + fluxes out of the sediment. This resulted in a lower net efflux of inorganic nitrogen from these sediments.