Effects of near-bottom water oxygen concentration on biogeochemical cycling of C, N and S in sediments of the Gulf of Gdansk (southern Baltic)

Abstract

Sediments from four sampling sites in the Gulf of Gdansk were sampled to test how different oxygen concentrations in near-bottom water affects biogeochemical cycling of C, N and S. Vertical distributions of content of organic carbon (OC), total nitrogen (TN) and total sulfur (TS) and number of sulfate-reducing bacteria (SRB) in sediments were determined. Pore water total alkalinity (TA), dissolved inorganic carbon (DIC), sulfate, hydrogen sulfide, ammonium and phosphate were analyzed and benthic fluxes of DIC, hydrogen sulfide and ammonium were calculated.Concentrations of OC and TN decreased and concentration of TS increased with sediment depth. Highest concentrations of OC, TN and TS were observed in silty clay sediments from hypoxic and anoxic sediments below the permanent halocline. Organic matter (OM) accumulation in sediments and oxygen deficiency in near-bottom water stimulate preservation of OC and burial of TS in this area.Concentrations of TA, DIC, hydrogen sulfide, ammonium and phosphate in pore water increased, while concentration of sulfate decreased with sediment depth. Hydrogen sulfide, ammonium and phosphate was a significant additional source of TA in pore water under hypoxic and anoxic conditions.Mineralization of OM at oxygen concentrations <2 ml l−1 occurred mainly via bacterial sulfate reduction. Diurnal hydrogen sulfide fluxes under hypoxic conditions ranged from 400 to 1240 µmol m−2 d−1. Ammonium fluxes were estimated on 534 – 924 µmol m−2 d−1. Corresponding fluxes measured under anoxic conditions were 266 µmol m−2 d−1 and 106 µmol m−2 d−1.Sediments under oxic conditions became a place of the intensive regeneration of carbon - DIC flux from sediment reached 2775 µmol m−2 day−1. Sediment-water DIC fluxes under hypoxic and anoxic conditions were much lower and ranged from 1015 to 1208 µmol m−2 d−1.