An assessment of riverine loads of nutrients to Lake Peipsi, 1995-1998.
Stålnacke, Per; Sults, Ülo; Vassiljev, Anatoli; Skakalsky, Boris; Botina, Alla; Roll, Gulnara; Pachel, Karin; Maltsman, Tatiana
The Lake Peipsi is the fourth largest lake in Europe (3,555 km2) with a drainage basin of approximately 44,000 km2. An assessment study of the nutrient loads to the Lake Peipsi showed that the riverine transport is the most important pathway for the total input of nutrients. In particular, it was shown that the lake, from its rivers, received an average of 20,500 tonnes of nitrogen (N) and 910 tonnes of phosphorus (P) annually during the time period 1995-1998. These estimates are substantially lower than previously reported. The total nutrient loads correspond to area-specific loads of 4.7 kg ha-1 for nitrogen and 0.21 kg ha-1 for phosphorus. The nitrogen load was low in a Baltic Sea and Western European perspective and was, for example, almost two times lower than the corresponding load to the Gulf of Riga. The phosphorus load was somewhat higher than the corresponding riverine load to the Gulf of Riga but almost two times lower than the average phosphorus transport in the rivers draining to the Baltic proper and the Western Baltic. Examination of the spatial variation of nutrient loads showed that the two largest basins, i. e. those of the Velikaya and Emajogi rivers, contributed approximately 80 % (16,500 tonnes yr-1) of the nitrogen load and 84 % (760 tonnes yr-1) of the phosphorus load transported to the lake via rivers. The Velikaya River alone accounted for approximately 65 % of the total riverine load. The area-specific loads of N and P ranged from 3.0 to 8.4 kg ha-1 for N and from 0.05 to 0.24 kg ha-1 for P. However, the nutrient levels in almost all the studied rivers were relatively low or moderate given the large share of agricultural land in the drainage basin (42%). Uncertainties in the load estimates were identified and mainly devoted to lack of tot-N and tot-P analsyis and low sampling frequency in Russian rivers.