Relations between planktivorous fish abundance, zooplankton and phytoplankton in three lakes of differing productivity

Abstract

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 Koylionjarvi was highly eutrophic in 1991–1996, and Lake Littoistenjarvi was mesotrophic in 1993–1996 and eutrophic in 1992. In Lake Pyhajarvi, natural year-class fluctuations of vendace and smelt (range of combined biomass 5–28 kg ha−1) caused significant variation in planktivory. The very dense fish stocks of Lake Koylionjarvi (mainly roach, bream and smelt) 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 declined from about 71 kg ha−1 to about 28 kg ha−1 during 1993–1996. In Lake Pyhajarvi, strong stocks of planktivorous fish were accompanied with depressed crustacean zooplankton biomass, reduced role of calanoids 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 phytoplankton: in spite of dense fish stocks, a high crustacean biomass developed in a phytoplankton peak year, but it was dominated by very small cladocerans. In Lake Pyhajarvi, late summer chlorophyll concentration was predictable from total phosphorus in water and cladoceran biomass (r2 = 0.68), both factors explaining roughly similar fraction of total variation. In combined data from all three lakes, chlorophyll was almost solely dependent on total phosphorus, while the cladocerans were regulated both from below by productivity and from above by fish. Our data from Pyhajarvi lend support to consumer regulation of late summer phytoplankton; low chlorophyll values prevailed when planktivorous fish biomass was below 15 kg ha−1. In large eutrophic lakes it may be difficult to reduce fish stocks to such a low level: in Lake Koylionjarvi, after six years of removal fishing, fish biomass still remained higher, and changes in plankton were accordingly small. Unexpectedly, in 1993–1996, phytoplankton biomass in Littoistenjarvi remained low in spite of low crustacean zooplankton biomass; submerged macrophytes probably regulated the water quality.