Production and persistence of bacterial and labile organic matter at the hypoxic water–sediment interface of the St. Lawrence Estuary

Abstract

AbstractAmino acid (AA) l‐ and d‐enantiomers were quantified in whole seawater samples from the Lower St. Lawrence Estuary (LSLE) and the Gulf of St. Lawrence (GSL). The bottom waters of the LSLE display a recent decrease in dissolved O2 concentrations. Concentrations of AA, including bacterial d‐enantiomers of AA (d‐AA), generally decreased with depth, but sharply increased in the nepheloid layer of the LSLE (not in GSL) with values slightly exceeding those measured in surface waters. The organic matter (OM) in this nepheloid layer was also enriched in AA and d‐AA. Although the particles of the nepheloid layer contributed to less than 13% of total organic carbon (TOC) their contributions to total AA and AA concentration increase were estimated to be almost as important as the dissolved phase. Estimates indicated that bacterial OM represented 67.1–79.4% of TOC in the nepheloid layer of the LSLE, but 18.5–56.9% in the other analyzed waters. All the diagenetic indicators confirmed that the OM just above the sediment in the LSLE was on average less altered than in the rest of the water column or in the GSL. This study suggests that bacteria recycle relatively altered OM into biomass and bacterial dissolved OM near the water–sediment interface in the LSLE. The presence of heterogeneous OM and hypoxic conditions likely reduce OM degradation rate and thus allow for the accumulation of labile OM. Unfavorable conditions for OM degradation might be present in many areas and represent a negative feedback limiting O2 consumption and CO2 production.