Nutrient enhancement, food quality and community dynamics in a tropical rainforest stream

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Abstract
SUMMARY: A mixed nutrient supplement was added to stream‐side experimental channels in a closed canopy rainforest in tropical Queensland over a 7‐month period, raising nitrogen and phosphorus concentrations well above ambient. Nutrient supplements enhanced decomposition of some leaf species, but levels of chlorophyll a and fine particulate organic matter did not differ between treatment and control channels. Shredding by Anisocentropus kirramus (Trichoptera) had a significant effect on leaf litter breakdown, but nutrient supplements did not enable A.kirramus to shred more material; however, biochemical analysis of A. kirramus showed that nutrient supplements enhanced the nutritional quality of the litter for this species. Treatment channels contained 75% more invertebrates than control channels, but significant differences in abundance (all positive) were detected in only five of the 109 invertebrate species present. There was no change in species richness or evenness between treatment and control channels. The limited community response indicates that (a) variables other than nutrients (e.g. light) restricted primary productivity, (b) low nutrient concentrations limit invertebrate production, (c) invertebrate community composition in this closed canopy, forest stream is resistant to nutrient enhancement, and (d) there was no direct relationship between productivity and diversity in this tropical stream.

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