DIET SELECTION BY COPEPODS IN THE PRESENCE OF CYANOBACTERIA

Boeckella, the dominant calanoid in many Southern Hemisphere lakes, can survive, grow and reproduce to varying extents on monocultures of cyanobacteria. In this study, we determined the effects of algal and cyanobacterial foods of different nutritional value and concentration on food preferences of adult female Boeckella triarticulata and Boeckella hamata. Four species of cyanobacteria (Anabaena flos-aquae, Nosier sp. 2, Oscillatoria tenuis and Microcystis aeruginosa) were offered alone and mixed with equal biomasses of Cryptomonas sp., Choricystis or a cyanobacterium. Food preferences were calculated as ratios of the rates at which the copepods removed each food at high and low food concentrations. In high-concentration mixtures with cyanobacteria, Cryptomonas was consistently preferred by both Boeckella spp. In low-concentration mixtures, both Boeckella spp. preferred Anabaena and Nosier, which they removed at high rates (81-142 ml mg(-1) h(-1)), although Cryptomonas was selected in preference to Oscillatoria and Microcystis. When fed mixtures of filamentous cyanobacteria, both species of Boeckella showed invariant discrimination against Nostoc, and shifts in preference between Anabaena and Oscillatoria that were related to food concentration. Microcystis, the least favoured food, appeared to have a toxic effect on B.triarticulata.