Trophic structure of macroinvertebrates in Amazonian streams impacted by anthropogenic siltation SHEYLA R. M. COUCEIRO, 1 * NEUSA HAMADA, 2 BRUCE R. FORSBERG 3 AND CLAUDIA PADOVESI-FONSECA 4 1 Centro Universitário Nilton Lins, Programa de Pós-Graduação em Biologia Urbana. Av. Prof. Nilton Lins, 3259, Parque das Laranjeiras, CEP 69058-040 Manaus,Amazonas, Brasil (Email: sheylacouceiro@yahoo.com.br), 2 Instituto Nacional de Pesquisas da Amazônia/Coordenação de Pesquisas em Entomologia, 3 Instituto Nacional de Pesquisas da Amazônia/Coordenação de Pesquisas em Ecologia, Manaus, Amazonas, and 4 Universidade de Brasília/Instituto de Ciências – ICC sul, Departamento de Ecologia, Asa Norte, Brasília, DF, Brasil Abstract This study assessed the results of anthropogenic sediment input on macroinvertebrate trophic structure in streams located in an area of oil and natural gas exploitation in Brazil’s Amazon forest.The results indicate that macroinvertebrate communities both in streams impacted by anthropogenic sediments and in non-impacted streams are composed mainly of taxa in the following functional feeding groups: predators, gathering-collectors, scrapers, shredders and filtering-collectors. The highest densities were observed for collector-gatherers, followed by scrapers, predators, shredders and filtering-collectors. However, both the richness and the density of all groups were reduced in impacted streams. The reductions were significantly related to suspended inorganic sediment load and to the colour of suspended sediments. The relative proportion of shredders in streams impacted by anthropogenic sediments was significantly reduced as compared with the proportion observed in non-impacted streams. This resulted from lower availability of coarse particulate organic matter in these streams owing to burial of leaves and other plant material. These results indicate changes in the functioning and productivity of streams owing to anthropogenic siltation. This is because the benthic macroinvertebrate communities, sampled during this study, were dependent on the degradation of leaves, which are the primary energy source sustaining the benthic foodweb. Key words: Amazonas, aquatic insect, Brazil, erosion, FFG, functional feeding group, neotropical stream, petroleum. INTRODUCTION A wide variety of impacts caused by sediments on aquatic fauna has been documented throughout the world (Downes et al. 2006), the most frequent being reduction in the abundance of several taxa and changes in community structure (e.g. Waters 1995; Zweig & Rabeni 2001; Vasconcelos & Melo 2008; Couceiro et al. 2010). Although an extensive literature exists on siltation (e.g. Hamilton 1961; Downes et al. 2006; Allan & Castillo 2007), little is known about the effects of sediment inputs on the trophic structure of aquatic communities, especially in tropical forest areas, where these impacts have been poorly docu- mented (Bruijnzeel 1990). Furthermore, community structure and function do not always respond in the same way to anthropogenic stress, so evaluations of stream integrity need to con- sider trophic relations and productivity in addition to taxon diversity and density (Gessner & Chauvet 2002; Pascoal et al. 2005). Information on the trophic struc- ture of streams is therefore important because it reflects the complexity and energetics of the aquatic ecosystem (Cummins et al. 2005). Macroinvertebrate trophic interactions can have a strong effect on eco- logical processes by directly influencing flows of energy and materials (e.g. Vannote et al. 1980; Goul- ding et al. 1988; Wallace &Webster 1996). Functional feeding groups (FFGs) therefore allow inferences on the functioning and productivity of aquatic ecosys- tems, and can be used to investigate the effects of anthropogenic impacts on these processes. Merritt and Cummins (1996) also suggested that proportions of the different FFGs can be used as indi- cators of stream ecosystem attributes, instead of mea- surements that require large investments in time and equipment and provide only a ‘snap-shot’ view of eco- system function. The ratios between FFGs can indi- cate the relative importance of stream ecosystem characteristics, such as trophic state (autotrophy or heterotrophy), the relative amounts of coarse and fine *Corresponding author. Accepted for publication September 2010. Austral Ecology (2011) 36, 628–637 © 2010 The Authors doi:10.1111/j.1442-9993.2010.02198.x Journal compilation © 2010 Ecological Society of Australia