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 presented. The nutrients include soluble reactive phosphorus (SRP), total dissolved phosphorus (TDP), total phosphorus (TP), nitrate and silicon (soluble reactive silicon). These data are combined with the river discharge data to calculate the nutrient loads discharged into the intertidal zones. The loads are expressed as the nutrient exports normalised with respect to the catchment areas. The results clearly demonstrate that the majority of the nutrient load is transported in autumn/winter storms. The rivers in the south of the region, i.e. Trent, Don, Aire and Calder, have high nutrient exports compared with the rivers in the north of the region such as the Wharfe, Swale, Nidd and Ure. These differences are attributable to the relative importance of effluent discharges and differences in the land use and extent of urbanisation in the regions. With the exception of the R. Derwent, those rivers with a high nitrate export also have the highest SRP export. The results for the 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 sharp spring minimum in concentration, caused by phytoplankton uptake, is evident for the larger rivers such as the R. Trent. The fractionated forms of phosphorus also varied between the rivers but each of the fractions: SRP, (TDP-SRP), (i.e. mainly organophosphorus and inorganic polyphosphates hydrolysed in digestion procedure), and particulate phosphorus, significantly contributed to the load in all the rivers. For example the phosphorus load in the R. Trent is dominated by the SRP fraction, whereas the R. Swale export is highly influenced by the transport of suspended solids, most of which originates from the lowland region between Catterick and the confluence with the R. Ure. The results of intensive hourly sampling through a storm event on the R. Swale demonstrate the importance of the tributaries in the downstream 54 km section on the river. Although the majority (85%) of the water at the downstream 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 region of the catchment. A detailed mass balance of the