Export of Nutrients from the Catchment of the Upper Szeszupa River (Drainage Basin of the Neman River) and Its Seasonality

The article presents the dynamics of concentrations and export of nitrogen, phosphorus, TOC in the upper Szeszupa River (tributary of the River Neman) in the period from 2000 to 2014 (15 years) based on monthly analyses performed in Poland in the scope of the National Environmental Monitoring. The lakeland river with a mean discharge of 1.6 m³ s⁻¹ and catchment dominated by agricultural land exports approximately 20 kg ha⁻¹ of organic carbon compounds per year. The export of nitrogen is insignificant (3.8 kg ha⁻¹) with 55% accounting for the element in the form of organic compounds and 31% for nitrates. Phosphorus export is also relatively low (0.12 kg ha⁻¹), with 30% of the load of TP constituted by orthophosphates. During four months (February–May), 40–60% of annual export of nutrients was discharged, whereas the load of nitrates and organic nitrogen was higher than the contribution of outflowing water. From 2010, an increasing tendency has been observed in organic nitrogen export. This may be related to the intensification of animal production in NE Poland and an increase in livestock density.