DIET SELECTION BY COPEPODS IN THE PRESENCE OF CYANOBACTERIA

Boeckella, the dominant calanoid in many Southern Hemisphere lakes, ca n survive, grow and reproduce to varying extents on monocultures of cy anobacteria. In this study, we determined the effects of algal and cya nobacterial 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 alon e and mixed with equal biomasses of Cryptomonas sp., Choricystis or a cyanobacterium. Food preferences were calculated as ratios of the rate s at which the copepods removed each food at high and low food concent rations. In high-concentration mixtures with cyanobacteria, Cryptomona s was consistently preferred by both Boeckella spp. In low-concentrati on mixtures, both Boeckella spp. preferred Anabaena and Nosier, which they removed at high rates (81-142 ml mg(-1) h(-1)), although Cryptomo nas was selected in preference to Oscillatoria and Microcystis. When f ed mixtures of filamentous cyanobacteria, both species of Boeckella sh owed invariant discrimination against Nostoc, and shifts in preference between Anabaena and Oscillatoria that were related to food concentra tion. Microcystis, the least favoured food, appeared to have a toxic e ffect on B.triarticulata.