Effects of benthivorous and planktivorous fish on phosphorus cycling, phytoplankton biomass and water transparency of a tropical shallow lake

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.