Variation in diatom and bryophyte communities along a temperature gradient in sub-Arctic streams: model surrogates for trends in larger ecosystems?

Abstract

Streams within geothermal areas in Iceland that vary in a syndrome of temperature-linked variables, including discharge and potential grazing pressure (snails vs. chironomid larvae), provided a test of how microvegetation structure might change systematically with such drivers. We examined if such streams could form a parallel with biome sequences, for example with similar change in overall structure and organism traits found from tundra to Boreal forest. The warmer streams had an over-story of bryophytes with more open patches of epilithic algae and Cyanobacteria, and a grazer community of snails and blackfly larvae; the vegetation of the colder streams consisted mainly of epilithic algae and Cyanobacteria, dominated by grazing chironomid larvae. Diatom species comprising the epilithic assemblages were generally smaller in the warmer streams compared with the colder streams. Temperature and discharge did not significantly influence the diversity of diatoms (Shannon Index, Sørensen Index, Evenness, Species richness), although more species were observed in the colder streams compared with the warmer streams. Trait diversity was greatest in the coldest streams, but despite high grazing pressure only one growth form (attachment by a mucilage stalk) was predominant in the warmer regimes. Grazers may have influenced diatom species traits but did not significantly suppress the diatom biomass. Analogy with the sequence from tundra, through taiga to boreal forest, over a comparable temperature gradient, showed some parallel features but important differences. Microorganism systems may thus not always be the useful surrogates for larger systems that some ecologists have suggested.