CHANGES IN THE DIATOM COMMUNITY, INCLUDING THE APPEARANCE OF ACTINOCYCLUS-NORMANII F SUBSALSA, DURING THE BIOMANIPULATION OF LAKE-VESIJARVI, FINLAND

After 60 years of receiving treated sewage effluent from the city of L ahti, Lake Vesijarvi had become very eutrophic. Diversion of the efflu ent in 1976 resulted in a slow recovery of the water quality. In 1989- 93, biomanipulation removed 380 kg of coarse fish per hectare from Eno nselka, the most polluted basin of the lake. Since then cyanobacterial blooms have decreased and water transparency has increased. Paleolimn ological analysis of deep water sediments was used to reconstruct the changes in the diatom community, with varved sediment structure provid ing a year-by-year chronology. During the biomanipulation period the f ollowing diatom species increased in Enonselka: Asterionella formosa, Fragilaria crotonensis, Stephanodiscus heterostylus and Tabellaria spp . In contrast, diatom species commonly considered to be indicators of eutrophication, such as Aulacoseira islandica, Diatoma elongatum and S tephanodiscus porous, have decreased since the end of the 1980s reflec ting the recovery of the Enonselka basin. The changes in the diatom pl ankton recorded in the less polluted basin of Laitialanselka were much less marked than those recorded from the sediments of the Enonselka b asin. In 1990 a diatom species, Actinocyclus normanii f. subsalsa, app eared for the first time in Lake Vesijarvi and since then it has been one of the dominant diatoms in the plankton of the Enonselka basin. Tw o years later it was also present in small numbers in the plankton of Laitialanselka, the least polluted basin of the lake. The appearance o f A. normanii f. subsalsa was concomitant with the dredging of the boa t harbour of Lahti city in the late summer of 1990. It was therefore a pparently not directly affected by biomanipulation but benefited indir ectly from the collapse of cyanobacterial populations that led to impr oved light and nutrient availability in the water column.