Benthic NHJ and NO5 flux following sedimentation of a spring phytoplankton bloom in Aarhus Bight, Denmark

Abstract

A seasonal study of NH: and NO; fluxes and concentrations at the sediment-water interface was carried out at a 15 m deep station in Aarhus Bight, Denmark. In winter, NHZ and NO; were released from the sediment at comparable rates (0.20 to 0.40 mm01 N m-2 dC1). A phytoplankton bloom developed rapidly in early spring. Immediately after mass sedimentation of diatoms, the sediment transiently released NH: at a high rate (up to 1.5 mm01 N m-'d-') and a dramatic change to a NO; uptake was observed (flux ca -0.80 mm01 N m-2 d-l). Subsequently both the NH.; release and NO; uptake decreased (summer fluxes ca 0.35 and -0.15 mm01 N m-' d' , respectively). From late summer, NO? was again released from the sediment (ca 0.30 mm01 N m-' d-') and a second, weaker maximum of NH: release (ca 0.70 mm01 N m-' d-l) was observed in fall. Seasonal variation of NHJ and NO: concentrations at the sediment surface (upper 2 mm) and in the bottom water agreed well with observed flux patterns. The high NH; release and NO; uptake immediately after spring bloom sedimentation indicated rapid increases of mineralization and denitrification. Sediment nitrification seemed to be inhibited, however, probably because the O2 penetration depth was reduced after sedimentation. Uptake of bottom water NO: rather than nitrification therefore seemcd to support the denitrification maximum. Even when calculated for a whole year, about 50 % of the NO< consumed during denitrification was supplied from the bottom water. Annual nitrogen budgets also indicated that denitrification accounted for 25 O/O of the total inorganic n~trogen release from the sediment.