Eutrophication, pollution and fragmentation: effects on the parasite communities in roach and perch in four lakes in central Finland.

Parasite communities in the four study lakes reflected the influences of habitat fragmentation, pollution and eutrophication. Discriminant analysis of communities at the individual host level reveal two major axes. One, characterized by reduced numbers of digeneans and myxosporeans and increased numbers of acanthocephalans and monogeneans, contrasts communities in a lake affected by chemical pollution from a pulp mill with 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 contrasts communities in an oligotrophic, unpolluted lake with the two eutrophic lakes, and was more complex, reflecting habitat fragmentation, and pollution or eutrophication, probably mediated by the same mechanisms as above. Monitoring easily seen discriminating parasites following 8 years of reduced pollutant loading showed some, but not all, of the effects of pollution could be reserved in a relatively short time.