Phytoplankton response to changed nutrient level in Lake Peipsi (Estonia) in 1992–2001
2003
In the 1990s, the nutrient load from the catchment area, carried into the large shallow eutrophic Lake Peipsi (area 3555 km<sup>2</sup>), diminished in comparison with the 1980s. The aim of the investigation was to analyse statistically biotic response to alterations in nutrient level in the lake following the reduction in external loading. In-lake changes of the relationship between phosphorus and nitrogen compounds and phytoplankton were analysed for 10 years (1992–2001). The content of biogenic elements (total phosphorus, orthophosphate, total nitrogen and ammonia) indicated a decrease and thereafter a stabilization of the parameter values during the observation period. An opposite trend was observed for the nitrite and nitrate ions. The lowest N<sub>tot</sub>:P<sub>tot</sub> ratio (about 11–13) was registered in 1995–96. However, despite a definite decrease of the nutrient level, the biomass of phytoplankton (particularly cyanobacteria) displayed an increasing trend. Strong and long-lasting (up to October–November) algal blooms were noted in recent years. The structure of the phytoplankton community changed considerably: the share of cyanobacteria (particularly, heterocystous forms) demanding less nutrients increased while the contribution of chlorophytes decreased. The amount of nutrients per biomass unit of different phytoplankton groups decreased. Water level appears to be a significant factor affecting the content of dissolved inorganic nutrients. In particular, NO<sub>3</sub>N increased and PO<sub>4</sub>P decreased at higher water level. Large inter-annual water level fluctuations will decrease stability of the ecosystem.
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