The effects of cyanobacteria on the growth, survival and behaviour of a tropical fish (Nile tilapia) and zooplankton (Daphnia lumholtzi)

Recent studies on the ecological effects of cyanobacteria in lakes have focused on the harmful effects of blooms to other aquatic organisms including fish and zooplankton. Much of this work has been conducted on temperate and subtropical species whilst few studies have involved tropical species, despite the almost chronic occurrence of cyanobacteria blooms in eutrophic tropical lakes. The main purpose of this study was to fill some of this knowledge gap on the ecological effects of cyanobacteria on fish and zooplankton in tropical lakes and to discover in what ways tropical species may be better adapted to deal with cyanobacteria than temperate species. The study tested fish (Nile tilapia, Oreochromis niloticus L.) and zooplankton growth, survival, and behaviour in relation to different variants (toxic and non toxic) and species of cyanobacteria at varying quantities as well as environmental conditions. Field studies on the diet and presence of cyanobacteria toxins (microcystins) in the guts and tissues (muscle and liver) of Nile tilapia were carried out in two eutrophic tropical aquatic ecosystems (Lake Mburo and Murchison Bay, Lake Victoria) in Uganda, East Africa. The zooplankton behaviour of diel vertical migration (DVM) was investigated in the Murchison Bay in relation to environmental conditions. Laboratory studies investigated the effect of the cyanotoxins, microcystins, on Nile tilapia and zooplankton (Daphnia lumholtzi) growth, survival, and behaviour, using the toxic cyanobacterium Microcystis aeruginosa PCC 7806. Cyanobacteria are the most abundant phytoplankton group in the two study areas contributing to over 60% of the total phytoplankton biovolume. The most dominant cyanobacteria species is Microcystis sp. which makes up approximately 70% of the total cyanobacteria biovolume. Low water transparencies were well associated with high levels of chlorophyll a in the lakes, suggesting that the abundance of cyanobacteria in these lakes may have a significant effect on the light environment. Water transparency is observed to have a close association with the DVM behaviour of at least one zooplankton group in the Murchison Bay. Phytoplankton especially of the cyanobacteria is an important diet item (> 30%) to the Nile tilapia in both lakes. Microcystins (RR, LR &YR) were detected in water samples from both study lakes and in the Murchison bay the concentration of microcystins in water was well associated with the microcystins concentration in the gut of Nile tilapia (p