Structure and production of a benthic insect assemblage in a neotropical stream

The structure and production of a benthic insect assemblage were assessed over a 12-mo period in a neotropical stream draining the Caribbean slope of Costa Rica. Two stream habitats, riffles and pools, were sampled monthly. Benthic assemblage composition was similar to that reported for other areas in Central America and was dominated by the orders Ephemeroptera (Tricorythodes, Leptohyphes, Thraulodes, Baetis?), Coleoptera (Elmidae), and Diptera (mainly Chironomidae). Collector-gatherers were the dominant functional-feeding group and very few shredders were found. Annual habitat-weighted secondary production was highest for Tricorythodes (87.07 mg ash-free dry mass [AFDM] m(-2) y(-1)) and Chironomidae (74.47 mg AFDM m-2 y(-1)). Total habitat-weighted secondary production was 363.65 mg AFDM m(-2) y(-1). Riffles supported the highest abundance, biomass, and secondary production of benthic insects. This study provides the Ist estimates of secondary production for a neotropical stream. Abundance, biomass, and secondary production in the study stream were low in comparison to results obtained in subtropical and temperate regions. However, annual production to biomass (P/B) ratios were high (range 5-103), indicating rapid population turnover. Monthly assessment of assemblage structure showed that benthic abundance increased with the number of days since the last large rainstorm (>50 mm/24 h). In addition, habitat-weighted abundance was negatively correlated with stream discharge. The abundance of macroconsumers (e.g., fishes and benthic shrimps) that potentially reduce food resources and prey upon insects might in part be responsible for the low abundance, production, and proportion of insect shredders in the study stream. Results suggest that other organisms (i.e., macroconsumers, microbes) are filling the functional roles that are normally assumed by benthic insects in temperate regions. Therefore, energy is potentially flowing through pathways other than insects from primary producers and detritus to upper trophic levels. In addition precipitation and subsequent high discharge are major factors determining the structure of benthic communities in the study stream. Our results support the hypothesis that the structure and production of the benthic insect assemblage in this neotropical stream are the result of interactions between the physical environment (e.g., discharge and habitat type) and macroconsumers.