Seasonal Variability of Dissolved Nutrients in the Humber–Ouse Estuary, UK

Measurements of dissolved nitrate, silicate, phosphate, nitrite and ammonium in the Humber–Ouse Estuary, UK, are presented for the period March 1994 to December 1996. The nutrient data are consistent with independent measurements made in the freshwater, non-tidal River Ouse and with earlier measurements made in the coastal zone. Nitrate, nitrite and silicate always exhibited mid-estuarine maxima, whereas this was not the case for phosphate during summer and early autumn. The mean and standard deviations (SD) of the nitrate and nitrite maxima were 515±90 μM and 9±3 μM, respectively, taken over the whole record. The mean and SDs of the silicate and phosphate maxima were 120±50 μM and 11±4 μM, respectively. The nitrate peak, which was well defined during summer and early autumn, correlated closely with a strong turbidity maximum (TM). The nutrient distributions were greatly influenced by physical transport (strong correlation between the location of the nutrient maxima and the freshwater–saltwater interface, FSI) and wastewater and freshwater nutrient inflows (spatial ‘groupingʹ of maxima locations). Mid-estuarine nutrient inputs were derived from river sources in the middle-to-upper estuary and were subsequently advected and mixed over a large area. A description and interpretation of the seasonal variability of the dissolved nutrients is given, although it is recognized that the various physical and biochemical influences can only be separated quantitatively through the application of numerical modelling techniques.