Benthic fluxes of nitrogen in the tidal reaches of a turbid, high-nitrate sub-tropical river

Benthic fluxes of dissolved inorganic nitrogen (NO3 and NH4 +), dissolved organic nitrogen (DON), N2 (denitrification), O2 and TCO2 were measured in the tidal reaches of the Bremer River, south east Queensland, Australia. Measurements were made at three sites during summer and winter. Fluxes of NO3 were generally directed into the sediments at rates of up to 225 mmolNm 2 h 1. NH4 + was mostly taken up by the sediments at rates of up to 52 mmolNm 2 h 1, its ultimate fate probably being denitrification. DON fluxes were not significant during winter. During summer, fluxes of DON were observed both into ( 105 mmolm 2 h 1) and out of (39 mmolm 2 h 1) the sediments. Average N2 fluxes at all sampling sites were similar during summer (162 mmolNm 2 h 1) and winter (153 mmolNm 2 h 1). Denitrification was fed both by nitrification within the sediment and NO3 from the water column. Sediment respiration rates played an important role in the dynamics of nitrification and denitrification. NO3 fluxes were significantly related to TCO2 fluxes (p!0:01), with a release of NO3 from the sediment only occurring at respiration rates below 1000 mmol Cm 2 h 1. Rates of denitrification increased with respiration up to TCO2 fluxes of 1000 mmol Cm 2 h 1. At sediment respiration rates above 1000 mmol Cm 2 h 1, denitrification rates increased less rapidly with respiration in winter and declined during summer. On a monthly basis denitrification removed about 9% of the total nitrogen and 16% of NO3 entering the Bremer River system from known point sources. This is a similar magnitude to that estimated in other tidal river systems and estuaries receiving similar nitrogen loads. During flood events the amount of NO3 denitrified dropped to about 6% of the total river NO3 load.