RELATIONS BETWEEN PLANKTIVOROUS FISH ABUNDANCE, ZOOPLANKTON AND PHYTOPLANKTON IN 3 LAKES OF DIFFERING PRODUCTIVITY

Water chemistry, phytoplankton, zooplankton and fish populations were studied over several years in three shallow, non-stratified lakes with differing nutrient loadings and fish communities in southwest Finland . Lake Pyhajarvi was weakly mesotrophic in 1980-1996, Lake Koylionjarv i was highly eutrophic in 1991-1996, and Lake Littoistenjarvi was meso trophic in 1993-1996 and eutrophic in 1992. In Lake Pyhajarvi, natural year-class fluctuations of vendace and smelt (range of combined bioma ss 5-28 kg ha(-1)) caused significant variation in planktivory. The ve ry dense fish stocks of Lake Koylionjarvi (mainly roach, bream and sme lt) were decimated from > 175 kg ha(-1) in 1991 to about 50 kg ha(-1) in 1996 by removal fishing. The roach stock of Lake Littoistenjarvi de clined from about 71 kg ha(-1) to about 28 kg ha(-1) during 1993-1996. In Lake Pyhajarvi, strong stocks of planktivorous fish were accompani ed with depressed crustacean zooplankton biomass, reduced role of cala noids and cladocerans, a low proportion of larger cladocerans (length > 0.5 mm), and a high chlorophyll level. In the lakes Littoistenjarvi and Koylionjarvi, zooplankton was dependent on both fish and phytoplan kton: in spite of dense fish stocks, a high crustacean biomass develop ed in a phytoplankton peak year, but it was dominated by very small cl adocerans. In Lake Pyhajarvi, late summer chlorophyll concentration wa s predictable from total phosphorus in water and cladoceran biomass (r (2) = 0.68), both factors explaining roughly similar fraction of total variation. In combined data from all three lakes, chlorophyll was alm ost solely dependent on total phosphorus, while the cladocerans were r egulated both from below by productivity and from above by fish. Our d ata from Pyhajarvi lend support to consumer regulation of late summer phytoplankton; low chlorophyll values prevailed when planktivorous fis h biomass was below 15 kg ha-(1) In large eutrophic lakes it may be di fficult to reduce fish stocks to such a low level: in Lake Koylionjarv i, after six years of removal fishing, fish biomass still remained hig her, and changes in plankton were accordingly small. Unexpectedly, in 1993-1996, phytoplankton biomass in Littoistenjarvi remained low in sp ite of low crustacean zooplankton biomass; submerged macrophytes proba bly regulated the water quality.