Macroinvertebrates as bioindicators in acid sulfate soil affected streams : Case study of river Perhonjoki catchment area

Acid sulfate soils (a.s. soils) are responsible for deteriorating water quality and invertebrate communities in many rivers and small streams across Western Finland. Drainage of a.s. soils for both agriculture and forestry are the main reason to why many of Finnish streams fail to achieve good status stated in the European Water Framework Directive (WFD). Many of the streams show signs of chronical acidity with low pH-values, high metal concentrations and low species richness. Especially sensitive species from the orders Ephemeroptera, Plecoptera and Trichoptera (EPT) are absent in acidic and highly metal polluted streams. Individuals from the genus Hydropsychidae (Trichoptera) often show stress symptoms of gill-, anal papillae- and catch-net anomalies when subjected to circumstances with sublethal concentrations of metals and acidity. These responses make invertebrates good bioindicators for pollution derived from a.s. soils. Eleven streams close by or within river Perhonjoki basin in Central Western Finland were studied. These streams were mainly low order (1st to 3rd order) small sized streams. Aim of this study was to identify a.s. soil impact on small streams across river Perhonjoki catchment area, which included identification of chemical receptors and the use of bioindicators in recipient waters. The hypothesis was (1) streams below the highest Littorina Sea level (8000 – 4000 BP) shows physiochemical symptoms of a.s. soil pollution (2) which results in species composition differences between polluted and non-polluted streams and (3) frequency of Hydropsychidae abnormalities is higher in polluted than non-polluted streams. Lowland (coastal) streams differed from highland (above highest sea level) streams having higher concentrations of sulfate and aluminum, higher electric conductivity and often lower pH-values. This proved that streams below the highest Littorina Sea level show symptoms of pollution derived from a.s. soils. Species composition differed significantly between a.s. soil affected and non affected streams, proving the hypothesis that a.s. soils alter benthic invertebrate communities. The difference in species composition between lowland and highland streams was apparent especially within Ephemeroptera species composition. Low- and highland streams did not differ in Hydropsyche individual gill abnormality frequencies nor Hydropsychidae species composition. Individuals of the genus Hydropsychidae showed symptoms of gill abnormalities in both a.s. soil polluted and non-polluted streams. This result was unexpected and rejected the hypothesis that gill abnormalities occur more often in a.s. soil polluted streams