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

Water chemistry, phytoplankton, zooplankton and fish populations werestudied over several years in three shallow, non-stratified lakes withdiffering nutrient loadings and fish communities in southwest Finland. LakePyhajarvi was weakly mesotrophic in 1980–1996, LakeKoylionjarvi was highly eutrophic in 1991–1996, andLake Littoistenjarvi was mesotrophic in 1993–1996 and eutrophicin 1992. In Lake Pyhajarvi, natural year-class fluctuations ofvendace and smelt (range of combined biomass 5–28 kgha™1) caused significant variation in planktivory. The verydense fish stocks of Lake Koylionjarvi (mainly roach, breamand smelt) were decimated from >175 kg ha™1 in 1991 toabout 50 kg ha−1 in 1996 by removal fishing. The roach stockof Lake Littoistenjarvi declined from about 71 kg ha−1 toabout 28 kg ha−1 during 1993–1996. In LakePyhajarvi, strong stocks of planktivorous fish were accompaniedwith depressed crustacean zooplankton biomass, reduced role of calanoids andcladocerans, a low proportion of larger cladocerans (length > 0.5 mm),and a high chlorophyll level. In the lakes Littoistenjarvi andKoylionjarvi, zooplankton was dependent on both fish andphytoplankton: in spite of dense fish stocks, a high crustacean biomassdeveloped in a phytoplankton peak year, but it was dominated by very smallcladocerans. In Lake Pyhajarvi, late summer chlorophyllconcentration was predictable from total phosphorus in water and cladoceranbiomass (r2 = 0.68), both factors explaining roughlysimilar fraction of total variation. In combined data from all three lakes,chlorophyll was almost solely dependent on total phosphorus, while thecladocerans were regulated both from below by productivity and from above byfish. Our data from Pyhajarvi lend support to consumer regulationof late summer phytoplankton; low chlorophyll values prevailed whenplanktivorous fish biomass was below 15 kg ha−1. In largeeutrophic lakes it may be difficult to reduce fish stocks to such a lowlevel: in Lake Koylionjarvi, after six years of removalfishing, fish biomass still remained higher, and changes in plankton wereaccordingly small. Unexpectedly, in 1993–1996, phytoplankton biomassin Littoistenjarvi remained low in spite of low crustacean zooplanktonbiomass; submerged macrophytes probably regulated the water quality.