Limnetica 34

The size of streambed sediment particles is an important determinant of habitat complexity for stream-dwelling invertebrates. In tropical streams we tested if, at the patch scale, sandy substrates were poorer in organisms and taxa when compared with larger grain-size substrates in three experiments: (a) sampling patches with coarse and fine substrates and leaf packs in eight low-order streams, (b) manipulating substrate size at the patch scale in a low-order stream and (c) sampling coarse and fine sediment patches in a larger river. We also investigated the abundance of aquatic hyphomycete conidia in these streams to relate shredder abundance with the presence of fungal decomposers, given that tropical streams are frequently reported as being poor in shredders. In the three experiments, there were consistently no differences in terms of the family richness or total number of invertebrates across the substrate types, but taxa identity differed across substrates. Invertebrate numbers and taxa were positively correlated with current velocity. Macroinvertebrate densities (150 – 300 individuals / m²) were lower than those reported in the literature (500 – >1500 individuals / m²), but the number of taxa was high (37 families observed, 52 estimated). Shredders comprised 13 % of all individuals and 11 % of all taxa. The low number of shredders contrasted with the presence of aquatic hyphomycete spores in stream waters and in leaves sampled from the streams. We concluded that the high number of taxa and low number of individuals buffer changes in substrate, allowing the presence of invertebrates even in the less-favorable substrates (sand). Although litter and aquatic hyphomycetes were present in these streams, shredders were poorly represented; this raises the question as to why litter consumers are scarce when resources are abundant in forested tropical streams.