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.

Plant litter breakdown is a key ecological process in terrestrial and freshwater ecosystems. Streams and rivers, in particular, contribute substantially to global carbon fluxes. However, there is little information available on the relative roles of different drivers of plant litter breakdown in fresh waters, particularly at large scales. We present a global-scale study of litter breakdown in streams to compare the roles of biotic, climatic and other environmental factors on breakdown rates. We conducted an experiment in 24 streams encompassing latitudes from 47.88 N to 42.88 S, using litter mixtures of local species differing in quality and phylogenetic diversity (PD), and alder (Alnus glutinosa) to control for variation in litter traits. Our models revealed that breakdown of alder was driven by climate, with some influence of pH, whereas variation in breakdown of litter mixtures was explained mainly by litter quality and PD. Effects of litter quality and PD and stream pH were more positive at higher temperatures, indicating that different mechanisms may operate at different latitudes. These results reflect global variability caused by multiple factors, but unexplained variance points to the need for expanded global-scale comparisons.

Urbanization is occurring rapidly in southeastern Asia where streams are increasingly pressured. We assessed the ecological integrity of Ampang River in Kuala Lumpur by comparing structural and functional measures between forested and urban sites. We assessed 4 forested, 1 intermediate (deforested, not channelized), and 5 urban, channelized sites. Urbanization altered substrate (concrete at urban sites), riparian vegetation, light, temperature, O 2 , conductivity, nutrients, and major ion levels, and simplified food webs. Invertebrate composition shifted from pollution-intolerant taxa at forested sites to tolerant taxa at urban sites. Richness in Surber samples was 56 species at forested sites and 27 taxa at urban sites. Basal food sources at forested sites were leaf litter and biofilm, whereas at urban sites they were a sediment mat comprising organic matter, cyanobacteria, iron bacteria, and algae. Organic matter biomass on ceramic tiles was greater at urban than forested sites, but chlorophyll a was not. Faunal abundance was greater in litter bags at urban (mean = 72.6 animals/bag, 68% chironomids) than at forested sites (27.9 animals/bag; 33% chironomids), but species richness was higher in forested sites. Food webs at forested sites were more complex and differed from those at urban sites, which lacked shredders, and grazers differed in taxonomic composition and diet between urban and forested sites. At forested sites, insect larvae grazed biofilm, whereas at urban sites, snails consumed the organic mat. Leaf decomposition was fastest at the most disturbed site indicating rapid microbial decomposition caused by increased temperatures (+4.5°C) and nutrients, and possibly increased physical breakdown caused by rapid flow carrying abrasive sediment and garbage. Urbanization has had severe impacts on ecosystem function in Ampang River that could be alleviated by preventing pollution, restoring riparian vegetation, and providing natural substrate.

Many forested headwater streams are heterotrophic ecosystems in which allochthonous inputs of plant litter are a major source of energy. Leaves of riparian vegetation entering the stream are broken down by a combination of biotic and abiotic processes and, in most temperate and boreal streams, provide food and habitat for

Breakdown of litter in streams is an important terrestrial-aquatic C pathway in forests. Macroinvertebrate detritivore shredders that feed on litter may be more abundant in temperate than tropical streams because the feeding guild is generally restricted to cool waters and because temperate streams may have more high-quality resources. However, rich shredder communities have been found in some high-altitude tropical streams. We investigated litter breakdown and the variation in resource quality of native riparian vegetation in tropical streams at different altitudes (86-3393 m) in eastern Malaysia (Borneo). At each stream, we collected 3 native local leaf species, analyzed their initial resource quality, and incubated them for 56 d in coarse-(shredder and microbial colonization) and fine-(only microbial colonization) mesh bags. We incubated Macaranga tanarius litter in all streams to compare breakdown rates directly. Shredder-mediated breakdown of native litter was detected in only 5 of 24 cases and was not accompanied by altitudinal trends, whereas Macaranga shreddermediated breakdown rates increased with elevation when rates were normalized for temperature (/degree-day).Regressions of resource quality and breakdown rates revealed consistent effects of initial N concentration in predicting breakdown rates that transcended decomposer and temperature effects. Shredder abundance, but not diversity, increased with altitude. The shredder guild may be important in tropical streams, particularly streams at higher altitudes and when litter N is not limiting.