Changes in water quality and the phytoplankton community associated with tilapia cage farming in tropical lakes

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We examine phytoplankton community structure and how it is influenced by commercial tilapia (Oreochromis niloticus L.) cage farming in three tropical lakes in eastern Brazil. Sampling occurred during both the wet and dry seasons for two treatments – within tilapia cages and outside the cages in the lake. Total ammonia, total phosphorus, chlorophyll a , phycocyanin and phytoplankton were measured in all water samples. In the phytoplankton community, we estimated species richness and total abundance. Thirty-three genera of algae distributed in 8 classes were identified. The most abundant classes were Chlorophyceae and Cyanophyceae (the latter of which was the most species-rich class). In Palmas Lake, chlorophyll a was constant over time (summer and winter) and location (within and outside cage farms), which may reflect the fact that fish farming is more recent in this lake. The richness and abundance of species were similar among the lakes, but Palminhas Lake tended to have more extreme values, apparently associated with the longer time fish have been farmed there. Fish farming clearly influences water quality as measured by algae species richness and abundance but is also moderated by the unique conditions in each lake and the amount of time over which the fish have been farmed. Understanding and preventing the impacts and detrimental consequences of tilapia cage farming on both the farming yield and the water quality in tropical lakes require continual monitoring.