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

Abstract

Benthic fluxes of dissolved inorganic nitrogen (NO 3 − and NH 4 +), dissolved organic nitrogen (DON), N 2 (denitrification), O 2 and TCO 2 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 NO 3 − were generally directed into the sediments at rates of up to −225 μmol N m −2 h −1. NH 4 + was mostly taken up by the sediments at rates of up to −52 μmol N m −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 μmol m −2 h −1) and out of (39 μmol m −2 h −1) the sediments. Average N 2 fluxes at all sampling sites were similar during summer (162 μmol N m −2 h −1) and winter (153 μmol N m −2 h −1). Denitrification was fed both by nitrification within the sediment and NO 3 − from the water column. Sediment respiration rates played an important role in the dynamics of nitrification and denitrification. NO 3 − fluxes were significantly related to TCO 2 fluxes ( p<0.01), with a release of NO 3 − from the sediment only occurring at respiration rates below 1000 μmol C m −2 h −1. Rates of denitrification increased with respiration up to TCO 2 fluxes of 1000 μmol C m −2 h −1. At sediment respiration rates above 1000 μmol C m −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 NO 3 − 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 NO 3 − denitrified dropped to about 6% of the total river NO 3 − load.