Macroinvertebrates and the processing of leaf litter in a tropical stream

The processing of leaf fitter in temperate streams has been the subject of numerous studies but equivalent tropical ecosystems have received little attention. The decomposition and macroinvertebrate colonisation of leaf litter was investigated using litter bags placed in a first-order rainforest stream over a 48 day period. Changes in leaf chemistry were also studied. Decay of the fresh, mixed-species litter was rapid (2.5% day(-1)) in comparison to char typically observed in temperate streams. The high water temperatures (21 degrees C) at the study sire are considered an important factor in the decay rate observed. Initially, average nitrogen content was high (3.5%). Content decreased to 3.1 percent after six days immersion and, thereafter, remained relatively constant. The consequent lack of evidence for microbial conditioning was attributed to species-specific differences in decay rare, giving rise to an overlapping sequence of conditioning and mineralisation of each leaf species within individual litter bags. Soluble polyphenols were largely leached from litter during the initial period of immersion. Constancy in remaining polyphenol content may be due to complexing with proteins and/or lignin compounds. The invertebrate community which colonised litter bags was dominated by insect groups and species richness was lower than that typical of many temperate streams. Colonisation dynamics differed among functional feeding groups: collector abundance increased on a linear basis whereas shredder and predator groups increased logarithmically. Despite differences in decay rate and taxonomic composition of the invertebrate community, the pattern of leaf litter utilisation and decay in the tropical stream studied was similar to that typically observed in temperate streams.