The decomposition of plant litter is one of the most important ecosystem processes in the biosphere and is particularly sensitive to climate warming. Aquatic ecosystems are well suited to studying warming effects on decomposition because the otherwise confounding influence of moisture is constant. By using a latitudinal temperature gradient in an unprecedented global experiment in streams, we found that climate warming will likely hasten microbial litter decomposition and produce an equivalent decline in detritivore-mediated decomposition rates. As a result, overall decomposition rates should remain unchanged. Nevertheless, the process would be profoundly altered, because the shift in importance from detritivores to microbes in warm climates would likely increase CO(2) production and decrease the generation and sequestration of recalcitrant organic particles. In view of recent estimates showing that inland waters are a significant component of the global carbon cycle, this implies consequences for global biogeochemistry and a possible positive climate feedback.

AimWe tested the hypothesis that shredder detritivores, a key trophic guild in stream ecosystems, are more diverse at higher latitudes, which has important ecological implications in the face of potential biodiversity losses that are expected as a result of climate change. We also explored the dependence of local shredder diversity on the regional species pool across latitudes, and examined the influence of environmental factors on shredder diversity.Location World-wide (156 sites from 17 regions located in all inhabited continents at latitudes ranging from 67°N to 41°S). MethodsWe used linear regression to examine the latitudinal variation in shredder diversity at different spatial scales: alpha (a), gamma (g) and beta (b) diversity. We also explored the effect of g-diversity on a-diversity across latitudes with regression analysis, and the possible influence of local environmental factors on shredder diversity with simple correlations.Results Alpha diversity increased with latitude, while g-and b-diversity showed no clear latitudinal pattern. Temperate sites showed a linear relationship between g-and a-diversity; in contrast, tropical sites showed evidence of local species saturation, which may explain why the latitudinal gradient in a-diversity is not accompanied by a gradient in g-diversity. Alpha diversity was related to several local habitat characteristics, but g-and b-diversity were not related to any of the environmental factors measured. Main conclusionsOur results indicate that global patterns of shredder diversity are complex and depend on spatial scale. However, we can draw several conclusions that have important ecological implications. Alpha diversity is limited at tropical sites by local factors, implying a higher risk of loss of key species or the whole shredder guild (the latter implying the loss of trophic diversity). Even if regional species pools are not particularly species poor in the tropics, colonization from adjacent sites may be limited. Moreover, many shredder species belong to cool-adapted taxa that may be close to their thermal maxima in the tropics, which makes them more vulnerable to climate warming. Our results suggest that tropical streams require specific scientific attention and conservation efforts to prevent loss of shredder biodiversity and serious alteration of ecosystem processes.

Small forest streams and their riparian vegetation are closely linked ecosystems. Stream consumers obtain most of their energy from leaf litter provided by the terrestrial vegetation. Thus, understanding the relationship between riparian vegetation, aquatic communities and litter decomposition may help explaining the variability in aquatic communities and processes among non-impacted streams, and anticipate their responses to anthropogenic-induced changes in the riparian vegetation. We surveyed 10 small non-impacted forest streams in central Portugal for riparian vegetation (species richness), benthic litter (species richness and biomass), aquatic hyphomycete conidia in transport (species richness and conidia concentration) and macroinvertebrates associated with benthic litter (taxon richness, density and biomass), during the litter fall peak. We found significant correlations between (a) aquatic communities and riparian vegetation species richness, (b) aquatic communities and benthic litter species richness and biomass and (c) within aquatic communities. Oak litter decomposition rates (from a previous experiment on the same streams) were also correlated with riparian tree species richness. This survey showed that spatial variability in riparian vegetation, benthic litter, aquatic communities and litter decomposition can be high even within a relatively small area, and allowed the identification of complex interactions between these components of the aquatic detrital food web.positive correlation between aquatic hyphomycete species richness, macroinvertebrate taxon richness, litter decomposition and riparian tree species richness suggests that anthropogenic-induced decreases in riparian species richness may affect aquatic communities and processes. Surveys over streams naturally differing in environmental conditions may allow forecasting the response of aquatic communities and processes to anthropogenic activities.

Biomonitoring programs to access the ecological integrity of freshwaters tend to rely exclusively on structural parameters. Here we evaluated stream ecological integrity using (a) benthic macroinvertebrate derived metrics and a biotic index as measures of structural integrity and (b) oak litter decomposition and associated fungal sporulation rates as measures of functional integrity. The study was done at four sites (S1, S2, S3 and S4) along a downstream increasing phosphorus and habitat degradation gradient in a small stream. The biotic index, invertebrate metrics, invertebrate and fungal communities' structure and sporulation rates discriminated upstream and downstream sites. Decomposition rates classified sites S4 and S2 as having a compromised ecosystem functioning. Although both functional and structural approaches gave the same results for the most impacted site (S4), they were complementary for moderately impacted sites (S2 and S3), and we therefore support the need for incorporating functional measures in evaluations of stream ecological integrity.