| Summary: | Streams in geothermal areas may offer good experimental opportunities for investigating the potential influences of climate changes upon freshwater ecosystems. Eight adjacent streams, varying in temperature, in the Hengill geothermal area, SW Iceland were selected to test hypotheses concerning the effects of nutrient addition and temperature on the community structure of primary producers. These were that: increasing temperature would have major effects on the structure of the primary producer communities, reflected in gross composition, species representation and growth form occurrence; that nitrogen addition would increase production whilst also changing community composition; and that there would be some interaction between temperature and nutrient effects that might give indications as to how sub–Arctic streams might be expected to change with progressive global warming and the associated secondary effects of changing land use. The density of the liverwort Jungermannia exsertifolia (Steph.) was low in the coldest streams but the biovolume of the biofilms on stones was high. In slightly warmer streams the density of the bryophyte Fontinalis antipyretica (Hedw.) was high along with high algal biovolume on stones. In the warmest streams the cover of F. antipyretica was very high. Green algal biovolume was high in all streams. Cyanobacteria and diatom biovolume was high in the cool and cold streams while it was lower in the warmest streams. Variables such as conductivity, pH and discharge along with the density of invertebrates and fish were examined in a relation to the temperature and nutrient influences on primary producers. Diatom communities included 64 taxa, 47 of them identified to species level. The coldest streams contained the most diverse diatom communities along with the highest growth–form diversity. The major grazer, Radix peregra (Mollusca) influenced growth forms by decreasing the diversity but it did not influence the density of diatoms. Small diatom populations were denser in warmer streams ...
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