Concentrations of dissolved nutrients in the tidal Yorkshire Ouse River and Humber Estuary

Abstract

Measurements of dissolved nitrate, silicate, phosphate, nitrite and ammonium in the tidal reaches of the Yorkshire Ouse and Humber Estuary, UK, are presented for the period March 1994 to March 1995. The data and their spatial trends are consistent with independent measurements derived from both freshwater and coastal measurements. Nitrate was typically less than about 550 μM. Concentrations were relatively low in the outer Humber and coastal zone and increased strongly up-estuary, reaching a mid-estuarine peak before decreasing to the lower concentrations that characterized the non-tidal river. The nitrate peak was well defined during summer and early autumn, when a strong maximum in turbidity occurred. Silicate distributions were similar to nitrate but with mid-estuarine maxima in the range 90–260 μM. These maxima were closely associated with the high turbidity zone during summer and autumn. Phosphate was dominated by freshwater inputs during spring through late summer, but exhibited a mid-estuarine peak between 5 and 10 μM and located several km up-estuary of the freshwater-saltwater interface (FSI) during the rest of the year. A pronounced peak in nitrite, between 9 and 12 μM and associated with the high turbidity zone, was observed up-estuary of the FSI during summer and autumn followed by a sharp drop to low concentrations. This nitrite depletion zone was located down-estuary of the high turbidity zone. Twin, mid-estuarine ammonium maxima were observed during winter. The up-estuary maxima (∼35 μM) were located about 30-km up-estuary of the FSI and the higher turbidity zone. Nutrient distributions are discussed in terms of physical and local, source-sink processes, and the various inputs from freshwater tributaries and wastewater discharges.