In this study we quantified invertebrate drift and related it to the structure of the benthic community, over a 6-8 month period, in a 4th-order tropical stream in Costa Rica. Relative to reports from similar-sized temperate and tropical streams, drift densities were high (2-fold greater: mean 11.2 m(-3); range 2.5-25 m(-3)), and benthic insect densities were relatively low (>3-fold lower: mean 890 m(-2); range 228-1504 m(-2)). Drift was dominated by larval shrimps that represented more than 70% of total drift on any given date; the remaining 30% was composed of 54, insect taxa. Among insects, Simuliidae and Chironomidae (Diptera) and Baetidae, Leptohyphes and Tricorythodes (Ephemeroptera) comprised 24% of total drift. Drift periodicity was strongly nocturnal, with peaks at 18:00 h (sunset) and 03:00 h. Our results, and those of previous experiments in the study stream, suggest that nighttime drift is driven by the presence of predatory diurnal drift-feeding fishes and nocturnal adult shrimps. There were no clear seasonal patterns over both 'dry' and wet seasons, suggesting that benthic communities are subject to similar stresses throughout the year, and that populations grow and reproduce continuously.
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