NUTRIENT TRANSPORT IN THE HUMBER RIVERS

The results of the weekly and storm sample measurement of the nutrient concentrations in ten Humber rivers over one annual cycle are present ed. The nutrients include soluble reactive phosphorus (SRP), total dis solved phosphorus (TDP), total phosphorus (TP), nitrate and silicon (s oluble reactive silicon). These data are combined with the river disch arge data to calculate the nutrient loads discharged into the intertid al zones. The loads are expressed as the nutrient exports normalised w ith respect to the catchment areas. The results clearly demonstrate th at the majority of the nutrient load is transported in autumn/winter s torms. The rivers in the south of the region, i.e. Trent, Don, Aire an d Calder, have high nutrient exports compared with the rivers in the n orth of the region such as the Wharfe, Swale, Nidd and Ure. These diff erences are attributable to the relative importance of effluent. disch arges and differences in the land use and extent of urbanisation in th e regions. With the exception of the R. Derwent, those rivers with a h igh nitrate export also have the highest SRP export. The results for t he R. Swale, the only river having two monitoring sites; highlight the importance of urban and agricultural impacts in the lowland region of the catchment. The export of silicon is least variable, although a sh arp spring minimum in concentration, caused by phytoplankton uptake, i s evident for the larger rivers such as the R. Trent. The fractionated forms of phosphorus also varied between the rivers but each of the fr actions: SRP, (TDP-SRP), (i.e. mainly organophosphorus and inorganic p olyphosphates hydrolysed in digestion procedure), and particulate phos phorus, significantly contributed to the load in all the rivers. For e xample the phosphorus load in the R. Trent is dominated by the SRP fra ction, whereas the R. Swale export is highly influenced by the transpo rt of suspended solids, most of which originates from the lowland regi on between Catterick and the confluence with the R. Ure. The results o f intensive hourly sampling through a storm event on the R. Swale demo nstrate the importance of the tributaries in the downstream 54 km sect ion of the river. Although the majority (85%) of the water at the down stream site originates from the upland region north of Catterick, the majority of the nitrate (74%) and SRP (78%) originates from the rivers Wiske and Cod Beck as well as minor tributaries in the southern regio n of the catchment. A detailed mass balance of the section indicates n et losses of nitrate, silicon, SRP and TDP from the water column. (C) 1997 Elsevier Science B.V.