Factors controlling spatial variability in ammonium release within an estuarine bay (Alfacs Bay,Ebro Delta,NW Mediterranean)

Sediment-water ammonium fluxes, oxygen uptake and sediment characteristics were studied in an estuarine bay influenced by temporal freshwater discharges. Sediment at seven stations representing a gradient imposed by freshwater inputs was sampled for sediment-water ammonium and oxygen fluxes, chlorophyll a derivative pigments, organic content, porosity and elemental composition (Fe, Mn, Si, Al). Oxygen uptake decreased along the gradient and correlated with total chlorophyll a derivatives indicating the close coupling between aerobic metabolism and short-time sedimentation events. Ammonium release showed a discontinuous pattern of decrease along the gradient and only correlated with the Fe:Mn atomic ratio. Correlation between the structural properties of the sediment (Si:Al atomic ratio, porosity and organic content) and ammonium release was also found (excluding data from the station with the highest ammonium flux). The extent of the influence of metabolism and sediment structure on ammonium release is discussed.