A FADING RECOVERY - THE ROLE OF ROACH (RUTILUS-RUTILUS L) IN MAINTAINING HIGH PHYTOPLANKTON PRODUCTIVITY AND BIOMASS IN LAKE VESIJARVI, SOUTHERN FINLAND

For 60 years, Lake Vesijärvi, especially the Enonselkä basin (2600 ha), received sewage water discharge from the City of Lahti. In 1976 the municipal sewage loading was diverted from the lake. Initially, the recovery of the lake proceeded well but after some years summer blooms of blue-green algae (Aphanizomenon, Microcystis) increased again. High biomass of roach (Rutilus rutilus L.) was considered to play an important role in maintaining high algal productivity and biomass in the lake water. Field experiments in mesocosms (22-24 m3) with different densities of roach suggested that fish biomass should be less than 30 g m-2, or 300 kg ha-1, to ensure a significant reduction in the current productivity and biomass of phytoplankton. Bioturbation of sediments and recycling of nutrients, rather than zooplanktivory, seemed to be the main factors linking roach to water quality in the mesocosms. The applicability of the mesocosm results to the management of Lake Vesijärvi is discussed. © 1990 Kluwer Academic Publishers.