Cyanobacteria and detritus as food for Daphni

Cyanobacteria and detritus are typically regarded as poor energy sources for zooplankton. Despite this, Daphnia occur in systems dominated by these potential food sources. Two such systems were investigated: the Manchester Ship Canal which is dominated by detritus mostly derived from sewage input, and Salford Quays a eutrophic site dominated by the cyanobacterium Oscillatoria agardhii. Experiments were performed to determine the feeding and selectivity response of Daphnia pulex to detritus and Oscillatoria agardhii compared with Chlorella vulgaris. This study found that Daphnia pulex selects for detritus over phytoplankton under experimental conditions. This study also found indirect evidence, from the examination of gut contents, that Daphnia longispina ingests detritus. This may explain why Daphnia are the dominant zooplankton in the Manchester Ship Canal which contains high levels of detritus. Field investigation found that high densities of Daphnia were only found when densities of phytoplankton (not cyanobacteria) were high. This study also found that survival of Daphnia pulex on detritus was lower than that on phytoplankton. These findings suggest that detritus (from sewage) alone is not sufficient for maintaining high densities of Daphnia. This study found that Daphnia pulex fed selectively in mixtures of phytoplankton and detritus. Filaments of Oscillatoria agardhii and Oedogonium reduced the filtration rate of Daphnia pulex on Chlorella vulgaris suggesting mechanical interference of the feeding process. Results of the survival experiment from this study suggest that Oscillatoria agardhii is toxic to Daphnia pulex. These findings may explain why Daphnia pulex is absent from Salford Quays which has high densities of Oscillatoria agardhii all year.