The effects of cyanobacteria on the growth, survival and behaviour of a tropical fish (Nile tilapia) and zooplankton (Daphnia lumholtzi)
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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<0.05). Although detected, we found no evidence of microcystin accumulation in the liver or muscle tissue. The ingestion rates and growth rates of juvenile Nile tilapia were not significantly affected by the presence of microcystins in their diets. Likewise growth and survival rates of the tropical cladoceran Daphnia lumholtzi in laboratory experiments were not significantly affected by the presence of microcystins in their diets. The cyanobacteria M. aeruginosa PCC7806 was a poor source of nutrients for juvenile fish and zooplankton, since it did not seem to stimulate growth both in the wild type and mutant type form. Though microcystins were present throughout the study period, our findings do not show that they have a strong effect on the diets of Nile tilapia. Nile tilapia in these lakes probably ingest cyanotoxins mostly through the cell-bound fraction, since cyanobacteria (especially Microcystis sp.) are an important constituent of their diet. Adult fish may have the ability to efficiently get rid of ingested cyanotoxins, but this may not be so for juvenile tilapia and zooplankton. Small amounts of cyanobacteria may actually stimulate growth in juvenile tilapia and even zooplankton, yet when provided in high proportions or as the sole source of nutrients they hinder growth, survival, and reproduction. Juvenile fish and most especially zooplankton, are unlikely to graze on cyanobacteria either due to their frequent formation of large colonies or because juvenile fish and zooplankton may lower their ingestion rates in the presence of cyanotoxins. Findings in this study do not provide any evidence for tolerance of tropical juvenile fish and zooplankton towards cyanobacteria toxins, yet adult fish show both the ability to utilise cyanobacteria and cope with levels of microcystins registered in the lakes.
Paper I: Hydrobiologia 635(1), Semyalo, R.; Nattabi, J. K.; Larsson, P., Diel Vertical Migration of zooplankton in a eutrophic bay of Lake Victoria, pp. 383-394. Copyright 2009 Springer Science+Business Media B.V. Full text not available in BORA due to publisher restrictions. The published version is available at: http://dx.doi.org/10.1007/s10750-009-9931-5Paper II: Semyalo, R.; Rohrlack, T.; Kayiira, D.; Kizito, Y. S.; Byarujali, S.; Larsson, P., 2009, On the diet of Nile tilapia in two eutrophic tropical lakes containing toxin producing cyanobacteria. Full text not available in BORA.Paper III: Journal of Plankton Research 31(8), Semyalo, R.; Rohrlack, T.; Larsson, P, Growth and survival responses of a tropical Daphnia (D. lumholtzi) to cell bound microcystins, pp. 827-835. Copyright 2009 The Authors. Published by Oxford University Press. Full text not available in BORA due to publisher restrictions. The published version is available at: http://dx.doi.org/10.1093/plankt/fbp032Paper IV: Semyalo, R. P.; Rohrlack, T.; Naggawa, C.; Nyakairu, G., 2009, Microcystin contamination in catches of Oreochromis niloticus in Murchison Bay, Lake Victoria and Lake Mburo - Uganda. Full text not available in BORA.Paper V: Semyalo, R.; Jensen, K. H.; Moberg, O., 2009, The effects of a toxic cyanobacterium Microcystis aeruginosa PCC 7806 on the ingestion and growth of juvenile Nile tilapia Oreochromis niloticus. Full text not available in BORA.