EFFECT OF ANABAENA-FLOS-AQUAE ON THE ABILITIES OF DAPHNIA AND KERATELLA TO FEED AND REPRODUCE ON UNICELLULAR ALGAE

1. This study tests the hypothesis that the filamentous cyanobacterium, Anabaena flos-aquae, inhibits Daphnia's ability to feed and reproduce on unicellular algae more than it does Keratella's Double-label radiotracer experiments using whole and sonically disrupted Anabaena filaments showed that even low concentrations of Anabaena (5x10(3) cells ml-1) inhibited the abilities of Daphnia pulex and D. galeata mendotae to feed on Chlamydomonas and that this inhibition was due to both mechanical interference with feeding and increased food availability. A single-label radiotracer experiment showed that Anabaena (5x10(3) and 2x10(4) cells ml-1) reduced the abilities of neonate, juvenile, and adult D. pulex to feed on Chlamydomonas but had a much more pronounced effect on adults. 2. Single-label radiotracer experiments using screened (25-mu-m mesh) and unscreened suspensions of whole Anabaena filaments showed that only very high concentrations of Anabaena (10(5) cells ml-1) reduced the ability of Keratella cochlearis to feed on Cryptomonas and that this inhibition probably was due to mechanical interference with feeding, since Keratella ate Anabaena extremely inefficiently - especially when single cells and short filaments were removed by screening. 3. Experiments comparing the survivorship and fecundity of individuals fed Anabaena, Chlamydomonas, or no food showed that the Anabaena was not toxic to either Keratella or D. galeata mendotae, was not utilized by Keratella, and was utilized by Daphnia, although not as well as Chlamydomonas was. 4. Anabaena (5x10(3) or 10(4) cells ml-1) did not affect, or slightly increased, the population growth rate (r(m)) of K. cochlearis on Cryptomonas, while it substantially increased that of D. pulex. Daphnia's ability to utilize Anabaena probably more than offset its reduced ability to feed on Cryptomonas. The presence of cyanobacterial filaments in natural plankton communities could adversely affect Daphnia and decrease its ability to competitively suppress Keratella and other rotifers if the filaments were of little nutritional value to the Daphnia, greatly interfered with Daphnia's ability to feed, or contained compounds toxic to the Daphnia. 5. The impact of a filamentous cyanobacterium on a zooplankton species in a natural community probably is a complex function of numerous factors: the concentration, edibility, toxicity, and nutritional value of the cyanobacterium; the extent to which the cyanobacterium interferes with the ingestion of other types of food; the availabilities and nutritional values of other types of food. To understand interactions between filamentous cyanobacteria and different zooplankton species, it seems necessary to analyse the effects of the cyanobacteria on the feeding and reproduction of the species under a variety of conditions.