Relative nutritional value of ciliate protozoa and algae as food forDaphnia

Abstract

The relative importance of autotrophic flagellates, desmids, cyanobacteria, and ciliates as food forDaphnia magna was examined using cohort life tables. Each cohort was fed a single food type at a given concentration, and comparisons among each type were made. Algal feeding treatments included three levels of young (7 to 14 days old)Chlamydomonas reinhardi (Chlorophyta, Chlamydomonadacae), two levels of senescent (> 14 days old)C. reinhardi, two levels ofCryptomonas sp. (Chlorophyta, Cryptomonadacae), two levels ofStaurastrum sp. (Chlorophyta, Desmidacae), four levels of young (7 to 15 days old) or senescent (> 15 days old)Microcystis aeruginosa (Cyanophyta, Chlorococcacae), and a no-food treatment. The ciliatesCyclidium sp. andParamecium caudatum were also presented at concentrations of 1 or 10(2) cells/ml, as well as mixtures ofC. reinhardi (10(3)/ml) andCyclidium (1/ml) orP. caudatum (1/ml).Daphnia growth, reproduction, and survivorship were highest whenC. reinhardi orCryptomonas were the food source, while those starved or fedM. aeruginosa had shorter survivorship and lower growth and reproduction.Daphnia grew and had high survivorship when fedP. caudatum, but even though eggs were produced, most were aborted after 2 or 3 days.Staurastrum andCyclidium produced intermediate growth and survivorship, but reproduction was seen only in the 10(3) Staurastrum/ml treatment. Carbon and nitrogen content were general indicators of nutritional value. However, growth, reproduction, and survivorship were higher in some cohorts fed treatments containing relatively low levels of carbon and nitrogen. Other cohorts were short-lived and did not reproduce, despite being fed much higher levels of carbon and nitrogen. The results also suggest that green algae are nutritionally valuable forDaphnia, whereas cyanobacteria are not. As measured by life-table parameters, the nutritional value of ciliates was variable, with some being poor food sources. Thus, the potential of ciliates as a trophic link between microbial production and higher trophic levels may vary with the ciliate community structure. Our results suggest that ciliates alone were insufficient as a food source to supportDaphnia population growth.