MEPS 151:1-10 (1997)  -  doi:10.3354/meps151001

Nutritional quality of two cyanobacteria: How rich is 'poor' food?

Schmidt K, Jónasdóttir SH

Cyanobacteria have often been described to be nutritionally inadequate and to interfere with zooplankton feeding. In laboratory experiments we offered 2 cyanobacteria, a unicellular Microcystis aeruginosa strain and the filamentous Nodularia spumigena, to the calanoid copepod Acartia tonsa as the sole diet and in food mixtures with the nutritious diatom Thalassiosira weissflogii. Egg production was used as criterion of food quality. The use of cyanobacteria alone was an insufficient diet. However, with increasing additions of M. aeruginosa and N. spumigena to the diatom, different effects were observed. Large additions of cyanobacteria resulted in lower egg production and often in elevated mortality of the females, but small additions of M. aeruginosa caused an increase of about 25% in egg production compared to a pure diatom diet. The influence of similar low concentrations of N. spumigena was weaker. We suppose that in the mixtures A. tonsa fed passively on M. aeruginosa, but not on the filaments of N. spumigena, and that ingested M. aeruginosa were used metabolically. As an additional test of the positive interactions between M. aeruginosa and T. weissflogii, different ratios of these species were offered to the copepods, while keeping the total food concentration constant. In mixtures egg production was higher than expected from the proportion of T. weissflogii. The highest egg production rates were observed at a 3:1 mixture of T. weissflogii to M. aeruginosa. We conclude that mono-specific food experiments may give a false impression of the nutritional quality of phytoplankton species. Cyanobacteria, which when fed alone prove to be poor food, may supplement the diet of A. tonsa.

Microcystis aeruginosa · Nodularia spumigena · Cyanobacteria · Nutritional quality · Calanoid copepods · Acartia tonsa · Egg production

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