Seasonal succession of diatoms in Lake Vesijaervi: comparison of sediment record with plankton counts

摘要

Lake Vesijaervi, and especially the Enonselkae basin, became heavily eutrophied in the 1960-1970s due to long-term discharge of municipal and industrial wastewaters (Keto 1982), and the Enonselkae basin suffered from massive cyanobacterial blooms into the late 1980s (Kairesalo et al. 1999). Intensified restoration measures and removal of coarse fish (biomanipulation) in 1989-1993 clearly improved the water quality. Since 1982, phytoplankton have been regularly monitored in Lake Vesijaervi; however, the early phase of eutrophication and the state of the lake preceding the eutrophication have remained largely uncertain. Paleolimnological methods have been applied in the Enonselkae basin, the most polluted basin of the lake, to examine the continuous diatom record in the sediment. The annually laminated and exactly dated sediment with high accumulation rate ( > 2 cm yr~(-1)) in the deep area of the Enonselkae basin offered a possibility to compare the seasonal diatom deposition in sediment with the corresponding phytoplankton data. In this study, the sediment records of two vernal diatom species, Stephanodiscus parvus Stoermer & Hakansson and Aulacoseira islandica (O. Mueller) Simonsen (hereafter Stephanodiscus and Aulacoseira) were compared with the corresponding phytoplankton counts in 1987-1993, the biomanipulation period of the lake basin. Also, the appearance (in autumn 1990) and colonization of a new diatom species Actinocyclus normanii f. subsalsa (Juhlin-Dannfelt) Hustedt (hereafter Actinocyclus) into L. Vesijaervi was studied in more detail than earlier by Liukkonen et al. (1997).