Pollution of the River Venta within the Territory of Kuršėnai Town
The impact of Kuršėnai town on the pollution of the river Venta and the effect of biogenic substances on the mass of phytoplankton are dealt with in the paper. Samples were taken and registration of the indicatory aquatic plants was carried out in three places of the river, i.e. upstream Kuršėnai, in the centre of the town and downstream Kuršėnai. Concentrations of biogenic substances (compounds of total soluble phosphorus and non-organic nitrogen) and organic substances were measured. Their effect on phytocenosis was estimated by measuring the concentration of chlorophyll a and by identifying indicator plants in the river.
The research has shown that large amounts of phosphorus enter the river in this territory. The average concentration of total soluble phosphorus upstream Kuršėnai (0.069 mg l-1) does not exceed the maximum permissible concentration (MPC), whereas downstream Kuršėnai it is twice higher (0.135 mg l-1) and thus exceeds the permissible limits.
The increased concentration of chlorophyll (10.49 µg l-1 downstream in comparison to 9.11 µg l-1 upstream Kuršėnai), plenty of water plants, their varietal diversity and indicatory features downstream Kuršėnai indicate more intensive water eutrophication in comparison with the state of the Venta river upstream Kuršėnai. Chlorophyll a concentration in the places of measurement correlates with the concentration of the total soluble phosphorus.
Nitrate concentrations are higher downstream Kuršėnai than upstream (3.44 mg l-1 in comparison with 2.27 mg l-1)and exceed the MPC (2.3 mg l-1). Concentrations of nitrites and organic compounds are not subject to seasons and in most cases exceed permissible limits within the entire examined section of the river (app. 15 km).The excess amount of nitric compounds has been measured in the examined section of the river: the ratio of total soluble nitrogen and phosphorus is 45:1 upstream Kuršėnai and 27:1 downstream Kuršėnai, while optimum conditions to breed and develop the biota are when the ratio is 16:1. Relatively higher concentrations of phosphorus after the river passes the town are the reason of the increased eutrophication.