Stable isotope studies of food webs in floodplains, large rivers, mangroves, and seagrasses have shown that, although a large proportion of the biomass may come from higher plants, microalgae provide a disproportionate amount of carbon assimilated by metazoan consumers. Evidence is building that this may also be the case for streams, especially those in the tropics. At the level of individual consumer species we also see that the apparent diet may not be reflected in the carbon assimilated. Tropical streams commonly have omnivoredetritivore species that potentially show this phenomenon. We tested these concepts in four moderately shaded sites in a stream in well-preserved Atlantic rainforest at Ilha Grande, Rio de Janeiro. We sampled aquatic insects, shrimps and fish as well as potential terrestrial and aquatic primary food sources. Carbon stocks from terrestrial sources predominated over carbon of algal origin (>99% of total). The primary sources of carbon showed distinctly different isotopic signatures: terrestrial sources had δ 13 C values close to −30‰, microalgae were −20‰ and macroalgae were −25‰. All fauna had δ 13 C values consistent with a carbon source derived from microalgae. Baetid mayflies and atyid shrimps exert a strong grazing pressure on periphyton and organic sediments but appear to assimilate predominantly microalgae. The palaemonid shrimp Macrobrachium olfersi also ingests large amounts of detritus of terrestrial origin, but apparently assimilates animal prey with algal δ 13 C signatures. These results support the growing view that tropical stream food chains are primarily algal based.

Coarse particulate organic matter is often broken down by specialist shredder invertebrates in temperate streams. In some tropical streams, larger, non-specialist, omnivorous fauna, (macroconsumers), particularly decapod shrimps and crabs, have been found to process coarse particulate matter. Larger shrimps and fish may also prey on or inhibit smaller invertebrates. Depending on the relative importance of larger and smaller fauna in leaf processing and in predatory interactions, we could expect that exclusion of larger fauna could either result in a decrease in leaf processing (if they were important in shredding or bioturbation) or increase in leaf processing if they negatively affected smaller shredders. We tested this by excluding fauna of different sizes from leaf peaks using bags with different sizes of mesh -0.2 mm (exclusion of most fauna), 2 mm (exclusion of larger fauna), and 10 mm (access to most fauna). Bag effect on leaf processing was minimized by constructing the bags of the same, fine, material, and sewing a relatively small window of the required mesh size. The experiment was conducted on two occasions in three streams of the urban forest of Parque Estadual da Pedra Branca, city of Rio de Janeiro. The three streams differed in larger fauna of shrimps (Macrobrachium potiuna), crabs, tadpoles, and fish. Packs were incubated for six time intervals and the rate of leaf processing calculated as the exponential rate of loss of leaf material. Rate of leaf processing was faster in bags with the largest mesh size; the rates in the other two mesh sizes were not statistically different. Rates varied between experiments and among streams. We could not attribute the faster leaf processing to any particular component of the larger fauna; the patterns of differences among streams and between experiments were not associated with particular taxa. There was no general trend of fewer smaller fauna in the presence of macroconsumers; the few smaller taxa that were different between mesh sizes were variously less and more abundant in the 10-mm mesh bags compared to the 2-mm. Known shredders were rare in the smaller fauna; the mining chironomid Stenochironomus was common, but was apparently not affected by larger fauna and apparently Handling editor: B. Oertli did not increase leaf processing. We conclude that macroconsumers and not smaller fauna had a positive effect on leaf processing, and this confirms a pattern observed in some other coastal Neotropical streams.

According to the prevalent paradigm, the major source of carbon and energy for food webs of small forested streams in temperate regions is allochthonous material from the surrounding forest. Tropical streams have not been as well studied and there has been some speculation that their food webs are more aligned with in‐stream, algal production (autochthonous carbon). We studied food sources and consumers in four sites in a coastal tropical stream with gradients of 36–254 m elevation, 8–73% canopy cover and 11.9–7.1 km2 watershed area, with the expectation that the food web would incorporate proportionately more allochthonous material as shading increased with smaller stream size. We analysed stable isotope ratios of carbon and nitrogen in basal resources and fauna and compared the values of consumers to the potential sources using a mixing model to estimate the proportion of allochthonous and autochthonous material in their diets. The predominant source of carbon in the food web was from algal production at all sites. There was no distinct increase in the proportion of allochthonous contribution to the diets of primary consumers and predators with increasing shading, and they did not generally change their diet with shading. Thus the food web was based more on autochthonous resources than would be expected from the paradigm for temperate streams.

Atyid shrimps are often an abundant component in undisturbed tropical streams. Studies in coastal streams in Puerto Rico and Brazil have demonstrated the importance of this group in removing periphyton and sediment from hard substrates and their effects on the composition and quantity of periphytic algae. We used experimental exclosures to investigate the influence of the small atyid Paratya australiensis on periphyton accrual on hard substrates in a coastal stream in the subtropics of Australia. We measured organic and inorganic matter, chlorophyll and algal biovolume in the presence and absence of shrimps on natural and artificial substrates. We found a 5-fold increase in the amount of organic matter on natural substrate in the absence of P. australiensis and a two to 10-fold increase in total periphyton mass on artificial substrate. The natural substrates did not show differences in biovolume of algae, however, algal biovolume on the artificial substrates was significantly higher in the exclusion treatment and diatoms were most affected. We conclude that P. australiensis can be considered a strongly-interacting element of the stream biota and an important species for monitoring and conservation.