Selective feeding of four zooplankton species on natural lake phytoplankton.

Grazing experiments evaluated by microscopical counting were conducted with different size classes of Daphnia hyalina, D. galeata, Eudiaptomus gracilis, and Cyclops sp., all from Lake Constance, using natural lake phytoplankton as food. Species-specific grazing selectivity coefficients were calculated for the dominant phytoplankton species from weekly experiments. Specific selectivities were found to be largely invariant through the growing season. All zooplankters grazed more efficiently on phytoflagellates such as Rhodomonas and Cryptomonas than on coccales such as Chlorella and Scenedesmus, regardless of their relative abundance in the phytoplankton assemblage. Filtering rates did not decrease in the presence of filamentous algae. Certain filamentous species were grazed efficiently, but only by D. hyalina: Anabaena planktonica, Oscillatoria amphigranulata, and Stephanodiscus binderanus. Large diatom colonies like Asterionella formosa and Fragilaria crotonensis were grazed well only by Cyclops sp. Some algal species were consistently selected against: Mougeotia thylespora and Dinobryon sp. The species-specific selectivity coefficients can be used as weighting factors to assess the 'effective food concentration' relative to Rhodomonas minuta, a reference species for optimal food.