Carbon and nitrogen cycling on intertidal mudflats of a temperate Australian estuary: II. Nitrogen cycling

Benthic fluxes of dissolved nitrogen, rates of denitrification, N₂ fixation and NH₄⁺ upward flux within the sediment (calculated from porewater profiles) were measured on the upper and lower mudflats at 2 study sites, 1 in the upper, river-dominated part of the estuary, and 1 in the lower, more marine part of the Huon Estuary, Tasmania, Australia. The calculated upward flux of NH₄⁺ from within the sediment based on porewater profiles was generally in excess of measured benthic fluxes, suggesting that NH₄⁺ was reassimilated at the sediment surface by microphytobenthos (MPB). The ratio of total CO₂ (TCO₂):NH₄⁺ produced within the sediment was generally in excess of 15, and in some cases in excess of 60. Significant influxes and effluxes of dissolved organic nitrogen (DON) were measured where the activity of MPB was highest. At times, DON influxes and effluxes were well in excess of dissolved inorganic nitrogen (DIN) fluxes, highlighting the importance of measuring DON fluxes where the activity of MPB is high. Rates of denitrification were very low, and represented only a small loss of N from the sediment, most probably as a consequence of the activity of MPB. Estimates of nitrogen assimilation by MPB showed that N₂ fixation was likely to be the major source of nitrogen during the summer at the study site in the upper estuary. There was also a high estimated C:N ratio (~20) of TCO₂ and nitrogen assimilated at this site, suggesting that a significant proportion of primary production was exuded as dissolved organic carbon rather than cellular production.