The Cinaruco River, a lowland floodplain river in the Venezuelan llanos, has a strongly seasonal hydrology, low nutrient concentrations, and high fish diversity and abundance. Fish exclosure/enclosure experiments were conducted in the littoral zone of the river channel and connected lagoons to examine seasonal variation in the magnitude of fish effects on benthic organic matter and algal biomass. During the dry season, large-fish exclosures in the channel accrued significantly more sediment, organic material, and chlorophyll than control cages after 20 d. Grazing scars suggested the bocachico, Semaprochiloduskneri, was a major consumer of organic-rich sediments. Further experiments were conducted to test the hypothesis that the relative strength of top−down (grazer) control of organic matter in sediments varies according to species, hydrologic period, and habitat. At flooding onset (May), S. kneri migrate to the Orinoco River to reproduce and feed. Thus, their densities are extremely low in the Cinaruco during the interval when nutrient inputs from newly flooded plains should be greatest, whereas densities are highest during the low-water season. Experiments conducted during the low-water period in the river channel and floodplain lagoons revealed significant treatment (large-fish exclosure, total fish exclosure, S. kneri enclosure, control) effects for accumulation of sediment mass, organic material mass, and chlorophyll a on tiles after 8 d. Chlorophyll a concentrations were significantly greater in lagoons than river-channel sites. Mean mass of sediments and organic material matched our prediction of grazer control during the low-water season. Experiments during the early rising-water period, when S. kneri emigrate from the Cinaruco, yielded no significant habitat or treatment effects after 3 d. Overall, our results support a model predicting continuous, gradual change in the magnitude of top−down effects of benthivorous grazing fishes on organic material on sediments as a function of seasonal changes in water level.
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