The filtering apparatus of eleven Cladoceran species was studied. The distances between the setulae, which act as filters, were measured. Among adult individuals, they vary from 0.2 μm in Diaphanosoma brachyurum to 4.7 μm in Sida crystallina. Species can be grouped according to the mesh-sizes, as "fine mesh filter-feeders": Diaphanosoma brachyurum, Ceriodaphnia quadrangula, Chydorus sphaericus, Daphnia cucullata and Daphnia magna; "medium mesh filter-feeders": Daphnia galeata, D. hyalina. D. pulicaria, Bosmina coregoni, and "coarse mesh filter-feeders": Holopedium gibberum and Sida crystallina. In Daphnia hyalina, the distances between setulae increase from 0.3-0.4 μm in small juveniles, to 0.8-2.0 μm in adults. In Daphnia magna, the mesh-size of the filter does not increase significantly with growth. There is good evidence that the relative abundance of the filter-feeding types varies with the trophic state of the lake. In oligotrophic lakes the "coarse mesh filter-feeders" usually dominate throughout the year. The seasonal succession of zooplankton species in eutrophic lakes can be interpreted as a succession of feeding types; during winter "coarse mesh filter-feeders" dominate, while "fine mesh filter-feeders" are most abundant during summer phytoplankton blooms. Our results support the hypothesis that the species composition of filter-feeding zooplankton is strongly influenced by the amount of suspended bacteria which are available as food only for filter-feeding species with fine meshes.
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.
Food size selection of four Daphnia, species (D. magna, D. hyalina, D. galeata, D. pulicaria) was investigated using spherical plastic beads as artificial food and with small bacteria. The size of the particles ranged from 0.1 to 35 μm with special emphasis to the particle diameters between 0.1 and 1 μm. In one set of experiments a mixture of differently sized particles was offered as food suspension and the selectivity of filtering was determined by comparing the size spectrum of the particles found in the gut contents with the spectrum in the food suspension. In a second series of experiments suspensions of uniformly sized particles were offered to single animals and their feeding activity was observed directly. In both types of experiments the mesh sizes of the filtering apparatus of the respective animals studied were measured after the experiments by, scanning electron microscopy. The mean sizes of the filter meshes were about 0.4-0.7 μm. In all experiments the size of the particles found in the gut or those which caused high feeding activities were larger than the smallest mesh sizes of the filters. As a consequence simple mechanical sieving provides a sufficient explanation for the mechanism of particle retention of the filtering process in Daphnia. D. magna was found to feed with high efficiency on suspended freshwater bacteria, the residual species investigated showed low filtering efficiencies when bacteria were offered as food.
1. Large mixotrophic ciliates (Stentor araucanus, S. amethystinus and Ophrydium naumanni) were a characteristic component of a temperate, oligotrophic lake in North Patagonia. During a 1‐year study, the abundance, biomass and primary production of these large Chlorella‐bearing ciliates were compared with those of the total plankton community. 2. Mixotrophic ciliates peaked in spring and from late summer to autumn, accounting for 1.6–43% (annual average: 16.3%) and 67–99% (annual average: 92%) of total ciliate abundance and biomass, respectively. Their contribution to total zooplankton biomass, including flagellates, rotifers, ciliates and crustaceans, was 14–76%, or 47% as an annual average. Endosymbiotic algae accounted for up to 25% of total autotrophic biomass (annual mean: 3.9%). 3. Maximum cell‐specific photosynthetic rates of S. araucanus and S. amethystinus at light saturation varied between 80 and 4400 pg C ciliate–1 h–1 with high values during autumn and winter, and low values during summer. The depth‐integrated rates of photosynthesis (0–40 m) of algal endosymbionts contributed 1–25% to total photosynthesis (annual mean: 6.5%). 4. A comparison of calculated ingestion rates with photosynthetic rates of Stentor indicates that photosynthate produced by endosymbionts generally exceeded heterotrophic food supply of Stentor during autumn and winter, but was much lower during summer, when food supply was high. 5. The mixotrophic ciliates represent an important ‘link’ between nanoplankton and higher trophic levels within the plankton community because of their high heterotrophic biomass and considerable contribution to total photosynthesis.
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