Benthic fluxes and pore water distributions of dissolved free amino acids in the open Skagerrak

Abstract

Benthic fluxes of dissolved free amino acids (DFAA) and ammonium were measured at 11 stations with water depths of 112–682 m in the open Skagerrak (north-eastern North Sea). Sediment was incubated together with ambient overlying water at in-situ temperature in a darkened flux incubator on-board ship during three cruises in 1992, 1993 and 1994. The magnitude and direction of the benthic DFAA fluxes were related to parameters characteristic for the stations such as dissolved inorganic nitrogen fluxes, and abundance of meio and macrofauna . Fluxes of DFAA into the sediment correlated to very low effluxes or influxes of ammonium, influxes of nitrate and low abundance of meio and macrofauna. Effluxes of DFAA correlated to effluxes of ammonium and nitrate, and relatively high abundances of fauna. The most common DFAA in the benthic fluxes were aspartic and glutamic acid , serine , glycine, alanine and valine . Pore water concentrations of DFAA were measured at five of the 11 stations. Those DFAA for which the fluxes were large were also the most common in the pore water. Benthic fluxes were also calculated from pore water gradients. For most of the DFAA, the calculated fluxes were underestimated relative to the measured ones, which probably was due to either a too coarse vertical resolution of the pore water gradients in the uppermost part of the sediment, or because the reactions responsible for the net production of DFAA from organic matter took place predominantly on the sediment surface rather than in the sediment. However, the direction of the calculated fluxes was the same as the measured ones. Benthic fluxes of total DFAA (TDFAA; the sum of the nine most abundant individual DFAA) were between −20 and 13 μmol m −2 day −1 with influxes (i.e. negative fluxes) at the deepest stations. TDFAA fluxes across the sediment–water interface were approximately zero to 2.6% of the total dissolved nitrogen flux, 1–20% of the dissolved inorganic nitrogen flux and approximately zero to 12% of the dissolved organic nitrogen (DON) flux. TDFAA in the pore water constituted 1–13% of the pore water DON, similar to the TDFAA/DON ratio in the fluxes.