Summary 1. We studied 10 first‐order Icelandic streams differing in geothermal influence in separate catchments. Summer temperature (August–September) ranged between 6 and 23 °C. 2. Macroinvertebrate evenness and species overlap decreased significantly with temperature whereas taxon richness showed no response. In total, 35 macroinvertebrate species were found with Chironomidae the dominant taxonomic group. Macroinvertebrate density increased significantly with temperature. Dominant species in the warm streams were Lymnaea peregra and Simulium vittatum. Algal biomass, macrophyte cover and richness were unrelated to temperature. Densities of trout (Salmo trutta), the only fish species present, reflected habitat conditions and to a lesser degree temperature. 3. Density of filter‐feeders increased significantly with temperature whereas scraper density, the other dominant functional feeding group, was unrelated to temperature. Stable isotope analysis revealed a positive relationship between δ15N and temperature across several trophic levels. No pattern was found with regard to δ13C and temperature. 4. Leaf litter decomposition in both fine and coarse mesh leaf bags were significantly correlated to temperature. In coarse mesh leaf packs breakdown rates were almost doubled compared with fine mesh, ranging between 0.5 and 1.3 g DW 28 days−1. Nutrient diffusion substrates showed that the streams were primarily nitrogen limited across the temperature gradient while a significant additional effect of phosphorous was found with increasing temperature. 5. Structural and functional attributes gave complementary information which all indicated a change with temperature similar to what is found in moderately polluted streams. Our results therefore suggest that lotic ecosystems could be degraded by global warming.

The Earth is experiencing historically unprecedented rates of warming, with surface temperatures projected to increase by 3-5 1C globally, and up to 7.5 1C in high latitudes, within the next century. Knowledge of how this will affect biological systems is still largely restricted to the lower levels of organization (e.g. species range shifts), rather than at the community, food web or ecosystem level, where responses cannot be predicted from studying single species in isolation. Further, many correlational studies are confounded with time and/or space, whereas experiments have been mostly confined to laboratory microcosms that cannot capture the true complexity of natural ecosystems. We used a 'natural experiment' in an attempt to circumvent these shortcomings, by characterizing community structure and trophic interactions in 15 geothermal Icelandic streams ranging in temperature from 5 1C to 45 1C. Even modest temperature increases had dramatic effects across multiple levels of organization, from changes in the mean body size of the top predators, to unimodal responses of species populations, turnover in community composition, and lengthening of food chains. Our results reveal that the rates of warming predicted for the next century have serious implications for the structure and functioning of these fragile 'sentinel' ecosystems across multiple levels of organization.

Th e present study was conducted in 47 diff erent riparian areas distributed throughout Denmark to investigate diversity and distributional patterns of plant communities along a lowland stream size gradient (fi rst to fi fth order). Th e investigated areas were representative for Danish riparian areas not in use for agricultural production. We investigated plant community richness along a stream size gradient and the infl uence of eutrophication on the abundance of diff erent plant communities. Vegetation analyses were performed in transects placed perpendicular to the stream channel, with a total of 1798 plots analyzed. Overall, we found a positive relationship between stream mean depth as a measure of stream size and the number of plant community types identifi ed in the riparian areas. We also found that the abundance of the identifi ed communities was positively correlated with their nutrient preference and negatively correlated with their moisture preference. Th e abundance of alkaline fens and Molinia meadows (protected community types) in riparian areas decreased with increasing size of the stream, whereas the abundance of humid meadows and wet herb fringes increased with increasing size of the stream. Based on our fi ndings, we recommend that wide buff er zones be established along streams with protected habitat types in the associated riparian areas to reduce the direct impact from agriculture. Furthermore, we recommend that wide buff er zones be established along middle-sized and large streams because several community types may develop.

In recent years an increasing number of streams have been restored to improve conditions for natural fen and meadow vegetation to develop in the associated riparian areas, but with modest success. Here we apply a controlled and replicated approach to investigate the role of flooding, sediment deposition, and seed addition for species recruitment in riparian areas with different types of standing vegetation. We expect that species recruitment is restricted in areas where the vegetation is dominated by fast-growing productive species because competition for light will make the establishment of new species difficult, especially for low-productive target species. We found that the naturally recruited species were few, mostly common, and widely distributed species. A majority of the recruited species, including target species added as seeds into the sediments, emerged in all areas independent of the characteristics of the standing vegetation. We observed significant temporal changes in compositional patterns throughout the experimental period (May to October). These changes were especially pronounced in areas with fen/fen-meadow vegetation and were closely associated with the emergence and growth of species originating from the deposited sediments (e.g. Equisetum pratense, Poa trivialis, Urtica dioica), including the seeded target species (Lychnis flos-cuculi and Lotus pedunculatus) and with a decline in fen-associated mosses and small sedges. Compositional changes were also associated with shifts toward more productive species in areas previously dominated by low and intermediately productive species. We infer that flooding and sediment deposition play a limited role for recruitment of target species in riparian areas and that sediment deposition may entail a risk of losing diversity in riparian areas.