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Dissolved inorganic nitrogen concentrations in an estuarine tidal flat

ABSTRACT Tay, H.W., Bryan, K.R. and Pilditch, C.A., 2013. Dissolved inorganic concentrations in an estuarine tidal flat Waikareao Estuary is a shallow tidally dominated estuary located in Tauranga Harbour, New Zealand. The primary aim of this study was to determine the seasonal variations in nitrate and ammonium concentration in sediment pore-waters on a tidal flat margin in the Waikareao Estuary. Vertical profiles of sediment pore-water were sampled 5, 10, 25, 50 and 75-cm depths to provide snapshots of nutrient concentration at different times of the year. Pore-water recycling processes were an important source of dissolved inorganic nitrogen (DIN) to the estuary during low tide with greatest concentrations observed in mid-winter and summer. Pore-water profiles showed ammonium as the dominant form of DIN in the wells. Nitrate concentration was higher in surface sediment and ammonium dominated the mid-depth regions (25–50 cm). The vertical stratification of the DIN in the wells suggested oxidation at the surface produced higher nitrate concentrations while at deeper depths, ammonification and dissimilatory nitrate reduction to ammonium produced higher ammonium concentrations. Long-term monitoring by the Regional Council of Kopurereroa Stream which discharges into Waikareao Estuary, showed higher nitrate concentrations than the tidal flat wells and other monitored sites. This further highlights that the primary source of nitrate into estuaries is through freshwater discharge and ammonium is dominated by sediment recycling processes.