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We investigated the roles of a benthivorous (Prochilodus brevis, Steindachner 1875) and a planktivorous (Oreochromis niloticus, Linnaeus, 1758) fish in translocating phosphorus from the benthic to the pelagic habitat of a tropical eutrophic shallow lake and its impact on phytoplankton biomass and water transparency. We performed two field experiments in 20 mesocosms (6 m3) with a 2 × 2 factorial design. Fish presence/absence was manipulated in combination with the presence/absence of a fish cage (4 m3) that prevented fish accessing the sediment. Benthivorous fish increased total phosphorus and chlorophyll a concentrations and decreased water transparency, but only when they had access to the sediment. Planktivorous fish increased the concentration of chlorophyll a without changing total phosphorus concentrations, whether or not they had access to the sediment. Results suggest that only the benthivorous fish increased phytoplankton biomass by translocating phosphorus from benthic to pelagic habitats. However, the planktivorous fish increased phytoplankton biomass by removing zooplankton and recycling nutrients within the pelagic zone. We conclude that removal of either fish species can improve the water quality of eutrophic shallow lakes in tropical regions, but only the removal of benthivorous fish will reduce the internal loading of phosphorus.

Recent studies from temperate lakes indicate that eutrophic systems tend to emit less carbon dioxide (Co-2) and bury more organic carbon (OC) than oligotrophic ones, rendering them CO2 sinks in some cases. However, the scarcity of data from tropical systems is critical for a complete understanding of the interplay between eutrophication and aquatic carbon (C) fluxes in warm waters. We test the hypothesis that a warm eutrophic system is a source of both CO2 and CH4 to the atmosphere, and that atmospheric emissions are larger than the burial of OC in sediments. This hypothesis was based on the following assumptions: (i) OC mineralization rates are high in warm water systems, so that water column CO2 production overrides the high C uptake by primary producers, and (ii) increasing trophic status creates favorable conditions for CH4 production. We measured water-air and sediment-water CO2 fluxes, CH4 diffusion, ebullition and oxidation, net ecosystem production (NEP) and sediment OC burial during the dry season in a eutrophic reservoir in the semiarid northeastern Brazil. The reservoir was stratified during daytime and mixed during nighttime. In spite of the high rates of primary production (4858 +/- 934 mg C m(-2) d(-1)), net heterotrophy was prevalent due to high ecosystem respiration (5209 +/- 992 mg C m(-2) d(-1)). Consequently, the reservoir was a source of atmospheric CO2 (518 +/- 182 mg C m(-2) d(-1)). In addition, the reservoir was a source of ebullitive (17 +/- 10 mg C m(-2) d(-1)) and diffusive CH4 (11 +/- 6 mg C m(-2) d(-1)). OC sedimentation was high (1162 mg C m(-2) d(-1)), but our results suggest that the majority of it is mineralized to CO2 (722 +/- 182 mg C m(-2) d(-1)) rather than buried as OC (440 mg C m(-2) d(-1)). Although temporally resolved data would render our findings more conclusive, our results suggest that despite being a primary production and OC burial hotspot, the tropical eutrophic system studied here was a stronger CO2 and CH4 source than a C sink, mainly because of high rates of OC mineralization in the water column and sediments.