Parasite communities in four study lakes in 1986 reflected the influences of eutrophication, pollution, and habitat fragmentation. Discriminant analyses of communities at the individual host level revealed two major axes. One contrasted communities in a lake affected by chemical pollution from a pulp mill with those from two eutrophic, less polluted lakes. Changes in the density of intermediate hosts, direct effects on ectoparasites, and impaired immune systems were regarded as important mechanisms. The second contrasted communities in an oligotrophic, unpolluted lake with those from the two eutrophic lakes and was more complex, reflecting habitat fragmentation (the absence of glochidia and some digeneans) and pollution or eutrophication, probably mediated by the same mechanisms as above. Changes in some index parasites in Lake Vatia monitored in 1994, following 8 years of reduced pollutant loading, supported our conclusion that parasite faunas in Lake Vatia in 1986 involved the effects of pollution.

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S U M M A R YThe Dactylogyrus fauna was studied from the gills of 293 roach between February 1988 and April 1989. Roach were caught from three interconnected lakes in Central Finland. Nine Dactylogyrus species were found. Of these the seven most abundant were used for studying the abundance of species on different gill-arches, niche breadth and niche overlap between the species. It was found that all species had a species-specific preference for certain gill-arches. The total abundances of all the species went through distinct seasonal changes, peaking during the spring and early summer. Some seasonal variation was also exhibited in gill-arch preferences. Niche breadth and species overlap were mostly higher during the period of peak occurrence of the various species, but in the case of some species they appeared to be unrelated to abundance, indicating possible niche restriction by other species. Most significant was the restriction of D. micracanthus to the fourth gill-arch during the period of peak occurrence of D. similis. Experimental work is needed to verify the reasons for variations in the microhabitat distribution of certain Dactylogyrus species.