Benthic fluxes of oxygen and inorganic nutrients in the archipelago of Gulf of Finland, Baltic Sea – Effects of sediment resuspension measured in situ

Abstract Benthic fluxes of oxygen and dissolved inorganic nutrients; phosphate (DIP), ammonium (NH 4 ), nitrate + nitrite (NO x ), and silicate (DSi); and the effects of resuspension on these were studied in situ with the Goteborg benthic landers in the Gulf of Finland archipelago, Baltic Sea. The benthic fluxes were examined at two shallow stations at depths of 7 m and 20 m in May and August 2014. Resuspension altered benthic fluxes of oxygen and nutrients in most of the experiments in August, but not in May, which was mainly due to weaker resuspension treatments in spring. Additionally, the benthic nutrient regeneration rates were higher and redox conditions lower in August when the water was warmer. In August, resuspension increased the benthic oxygen uptake by 33–35%, which was, in addition to stronger resuspension treatment, attributed to higher amounts of dissolved reduced substances in the sediment pore water in comparison to conditions in May. Adsorption onto newly formed iron oxyhydroxides could explain the uptake of DIP by the sediment at the 20 m station and the lowering of the DSi efflux by 31% at the 7 m station during resuspension in August. In addition, resuspension promoted nitrification, as indicated by increased NO x fluxes at both stations (by 30% and 27% at the 7 m and 20 m station, respectively) and a lowered NH 4 flux (by 48%) at the 7 m station. Predicted increases in the magnitude and frequency of resuspension will thus markedly affect the transport of phosphorus and silicon and the cycling of nitrogen in the shallow areas of the Gulf of Finland.