Nutrient fluxes through the Humber estuary

The Humber is a large and complex estuarine system on the east coast of England fed by several rivers. Fluxes of dissolved inorganic nutrients to, through and from this estuary over 1990–1993 are estimated from point flux calculations and property salinity plots. Internal nutrient sources and sinks are quantified. Fluxes of nutrients to the system are highly seasonal; fluxes of nitrate and phosphate are dominated by winter flows in the River Trent, the ammonium flux is dominated by the Rivers Aire and Don. The tidal Trent has a large internal source of ammonium, removes about 80% of its dissolved phosphate load and functions as a sink for nitrate. The tidal Ouse has large internal sources of nitrate, phosphate and ammonium, removes over 60% of its dissolved phosphate load, is an overall source of nitrate and a large sink for ammonium. The main estuary, below the confluence of the two tidal river systems, removes about 6% of the phosphate and 50% of the ammonium entering at the confluence or about 50% and 80%, respectively, if internal sources are taken into account. Nitrate behaves conservatively within the main estuary; thus any conversion of ammonium to nitrate is balanced by a nitrate sink of comparable size. Buffering mechanisms in the outer estuary may release phosphate in similar magnitudes to the direct dissolved export. The seasonality in nitrate flux to the system is preserved throughout the system. The seasonality of the phosphate flux to the system is almost eliminated in the lower part of the tidal rivers at the early part of the salinity gradient. This, combined with phosphate buffering, may render the offshore region nitrogen-limited under conditions suitable for algal growth. Overall the Humber system removes 85% of its dissolved phosphate load, and 4% of its dissolved inorganic nitrogen load.