A comparative study on the ability of tropical micro-crustaceans to feed and grow on cyanobacterial diets

Controlling noxious phytoplankton blooms by biomanipulation largely depends on the presence of large-sized (3000 m) generalist grazers, not common in the tropics. Therefore, we compared the ability of small (2000 m) microcrustaceans Ceriodaphnia dubia, Moina macrocopa, Daphnia pulex, Simocephalus vetulus and Heterocypris incongruens to feed and grow on cyanobacterial diets. We studied their feeding preferences on phytoplankton from natural lake water with a dominance of Microcystis sp., Fragilaria sp. or Planktothrix sp. to which we added Scenedesmus acutus. Also tested were the ability of D. pulex, S. vetulus and H. incongruens to reduce cyanobacterial densities by grazing, and the rate of grazing and demographic responses of S. vetulus and H. incongruens on diets of S. acutus, Microcystis sp. and Planktothrix sp. All species fed on small colonies of Microcystis sp., but not on filamentous Planktothrix sp. or colonial Fragilaria sp. Simocephalus vetulus and H. incongruens were generalists and most capable of reducing cyanobacterial densities. Demographic variables of S. vetulus, but not H. incongruens, were significantly lower on the cyanobacterial diet; for instance, the population growth rates were between 0.070.2 day(1) on cyanobacterial diets, but 0.31 on S. acutus while for H. incongruens they ranged between 0.07 and 0.08 day(1), regardless of whether the diet was a cyanobacteria or S. acutus. Our study warrants further tests on the ostracod H. incongruens in order to test its efficacy in reducing cyanobacterial densities in shallow tropical ponds.