Horizontal and vertical gradients in sediment nutrients on mudflats in the Shannon estuary,Ireland

Spatial variability in nutrient concentrations in the intertidal sediments and pore waters has been studied along downshore transects and in core profiles at three locations in the Shannon estuary on the west coast of Ireland. The parameters measured were N, as Kjeldahl N, NO₃, NO₂, and NH₃, and P as total P and PO₄ along with a range of other environmental variables such as salinity and sediment organic content. The concentrations of all nutrients varied with season, but winter values were generally low in comparison with polluted mainland european estuaries. There was a great deal of variation in nutrient concentrations along the transects, and coefficients of variability of up to 153% (NH₃), 173% (NO₃), 129% (NO₂) and 117% (PO₄) were found. Overall, there was little evidence of any trends in concentration in any of the nutrients from the top to the bottom of the transects, although it was occasionally possible to link particular instances to local conditions such as the presence of the channel or a stream. Sediment core profiles showed typical patterns, with NO₃ concentrations for example being highest in surface sediments, while NH₃ and PO₄ concentrations increased with depth. Rather surprisingly perhaps, NO₃ could still be detected on occasion at depths of up to 20 cm, well below the Redox Potential Discontinuity (RPD) and the limit of oxygen penetration which oxygen microelectrodes had measured as being within a few mm of the surface. This was ascribed to the activities of the macrofauna, in that the oxidised sediment which lined the burrows could clearly be seen in some cores. This study not only shows that nutrient distributions along estuarine gradients are linked to physicochemical factors such as oxygenation and freshwater/marine influence, but also that sediment instability, through random physical events such as storms, and macrofaunal activity play an important role and that these latter factors deserve closer attention.