| Summary: | The water of lake Valkea-Kotinen has become acidified to pH ca. 5. In agreement with the decreased deposition of acidifying compounds, the results for 1990–96 indicated no further acidification. However, coverage of the bottom surface by underwater Sphagnum moss still increased markedly between 1990 and 1994, possibly due to its slow colonisation and slow recovery of the lake. Phytoplankton primary production, chlorophyll and phytoplankton biomass seemed to respond to factors linked with varying weather conditions. Water inflow from the catchment area and the extent of spring circulation were identified as major factors causing the observed changes. The high dominance of Gonyostomum semen (Raphidophyceae) in late summer suggested that, in addition to environmental variables, the behavioural strategies of individual species may also have a strong influence on the diversity and biomass of phytoplankton. Among zooplankton there was a dramatic change in 1993 when a rotifer, Kellicottia bostoniensis, suddenly became the most abundant species. This cannot be attributable to the other measured variables and may have been a random occurrence. Overall, the results suggest that 7-year time series may be too short to draw definite conclusions about possible trends. On the other hand the results demonstrated that weekly observations are, in most cases, necessary for finding interrelationships between hydrological and biological factors in small headwater lakes.
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