Environmental conditions and the development of Planktonema lauterbornii Schmidle in phytoplankton of Karhijärvi, a lake in SW Finland

In the warm summer of 1999, the filamentous green alga Planktonema lauterbornii Schmidle appeared among dominants in the phytoplankton of Karhijärvi, a lake located in SW Finland. In the cool and rainy summer of 2000, the species almost disappeared from the lake again. P. lauterbornii was found in Karhijärvi for the first time in 1994 when it appeared in small numbers. The occurrence of P. lauterbornii in fresh waters in the Baltic Sea region is known only in a few cases whereas more records are from brackish waters. This paper describes the chemical and physical environment of Karhijärvi and the species composition of phytoplankton in summer 1999 in comparison with long-term monitoring data of the lake. A general increase of air temperature and especially the mild winters during the 1990s probably favoured the northward invasion of the warm water species Planktonema lauterbornii.