Efeitos dos macroconsumidores (peixes e camarões) sobre a assembléia de insetos aquáticos e na taxa de degradação de folhas em riachos de floresta de terra firme, Amazônia central

Abstract

The objective of this study was to verify potential effects on the assemblage of aquatic insects and on leaf breakdown caused by exclusion of macroconsumers (fish and shrimp). A reduction in leaf breakdown and an increase in the abundance of aquatic insects were expected in the absence of macroconsumers, as observed in other studies in tropical and temperate streams. Macroconsumer effects on the leaf breakdown and on aquatic insect assemblages were evaluated by excluding the macroconsumers using electric fields in 1600-cm2 square areas in four streams. The experiment was carried out in blocks, each composed of two treatments (control and electrified) placed in areas of low flow with few accumulated leaves. Each of the four streams studied contained two blocks, resulting in eight replicates. A paired t test was used to test for differences in the abundance of different taxa, while NMDS analysis and CCA were used to evaluate the community composition among treatments. The most abundant shredders were Triplectides sp. and Phylloicus sp.; the abundance of these groups did not differ between the treatments. Abundance of Chironomidae (non-miners) was higher in exclusion treatments, while leaf miners did not differ between treatments. Macroconsumers negatively affected Chironomidae; however, it was not clear whether this effect was exerted by means of predation or by means of Chironomidae being dislodged from leaves during foraging by macroconsumers. After 17 days the leaf weight remaining was less in control treatments, suggesting that the macroconsumers had positively affected leaf breakdown. Macroconsumers did not have a detectable effect on the abundance of shredders; therefore, the abundance of these organisms did not differ between treatments. Due to the lack of an effect on shredders, the difference between treatments of 5.4% in the final weight of leaves could be attributed to macroconsumer activity. This small but significant difference indicates a substantial participation of shredders in leaf breakdown. An important future step in studies of leaf breakdown in central Amazon streams will be to determine the role of insects and macroconsumers separately.