Nitrogen and phosphorus in Estonian rivers discharging into Lake Peipsi: estimation of loads and seasonal and spatial distribution of concentrations

Our study aims to estimate the riverine loads of nutrients to Lake Peipsi sensu stricto and to assess their effect on the lake's water quality. The data of national monitoring of 1992-2007 were used for the determination of seasonal and spatial distribution of nitrogen and phosphorus in the rivers discharging into Lake Peipsi s.s. Statistical conclusions and tests were based on a multi- parametric model. Our calculations revealed that the major Estonian rivers discharged approximately 5600 tonnes of nitrogen and 179 tonnes of phosphorus annually to Lake Peipsi s.s. Nitrogen export coefficients varied from 4 to 8 kg N ha -1 yr -1 , while the range of P losses was from 0.12 to 0.21 kg P ha -1 yr -1 . Seasonal variation in nitrogen concentrations had a sinusoidal pattern, with high concentrations during winter and spring and low values in summer, whereas phosphorus dynamics created a slightly different picture, with a much more irregular variation and a smaller amplitude over the year. Only a few rivers showed a pattern of notably higher phosphorus concentrations during summer low flows and more diluted concentrations during winter and spring. The differences in the concentration of phosphorus were mainly explained by variation in water discharge, while the availability of nitrogen in rivers was determined rather by seasonality. Nutrient concentrations in rivers discharging into Lake Peipsi are probably still influenced by nutrient stores from the period of intensive fertilization. Although a local effect of nitrogen loads on the lake's water quality was detected, the concentration of phosphorus in lake water remained insensitive to year-to-year changes in the riverine load of phosphorus.