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The effects of planktivorous and benthivorous fish on benthic fauna, zooplankton, phytoplankton and water chemistry were studied experimentally in two eutrophic Swedish lakes using cylindrical enclosures . In enclosures in both lakes, dense fish populations resulted in low numbers of benthic fauna and planktonic cladocerans, high concentration of chlorophyll, blooms of bluegreen algae, high pH and low transparency . In the soft-water Lake Trummen, total phosphorus increased in the enclosure with fish, but in the hard-water Lake Bysjon total phosphorus decreased simultaneously with precipitation of calcium carbonate . Enclosures without fish had a higher abundance of benthic fauna and large planktonic cladocerans, lower phytoplankton biomass, lower pH and higher transparency .The changes in enclosures with fish can be described as eutrophication, and those in enclosures without fish as oligotrophication . The possibility of regulation of fish populations as a lake restoration method is discussed .

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During the last 20 -30 years the flagellate Gonyostomum semen has become more abundant in lakes, especially small humic lakes, in Scandinavia. Mass development of the alga has been reported from areas affected by anthropogenic acid deposition, and reports from bathers of health problems are becoming frequent. Although there is an apparent connection between the appearance of Gonyostomum and acidification, it seems not to be the low pH per se that is the cause, but rather interlinked factors. The present tendency towards a wider distribution of the alga in non-humic lakes, emphasizes the importance of a better understanding of small humic lakes. Gonyostomum semen -characteristics of the organismGonyostomum semen (Ehrb.) Diesing 1865. Syn. Monas semen Ehrenberg 1853

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