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.

Abundance, biomass, production, and grazing loss rates of phytoplankton, free-living bacteria, and Protozoa were assessed during an intense spring phytoplankton bloom in prealpine Lake Constance (Bodensee). More than 50% of the primary production was channeled through the microbial loop. Bacteria and ciliates responded rapidly to increasing phytoplankton biomass and production. Maximal growth rates of bacteria and Protozoa were slightly lower than those of the dominating phytoplankton species. Averaged over the spring bloom, bacterial C amounted to 2 1% of phytoplankton C, bacterial production to 18% of particulate primary production. Increase of heterotrophic flagellate populations was prevented by grazing within the microbial loop, probably mainly by the feeding impact of ciliates. Although ciliates controlled flagellate production, they satisfied their food demand primarily by feeding on algae and consumed -14% of primary production. Metazoan microzooplankton (copepod nauplii and rotifers) removed -7% of the phytoplankton production. Herbivorous metazooplankton ingested slightly less. Therefore, the phytoplankton bloom was not terminated by excess grazing. POd3-depletion and subsequent sedimentation was most likely the prime cause for the termination of the spring bloom.In lakes and in some marine waters in temperate and boreal areas, the growing season starts with the onset of thermal stratification in spring. As a consequence of stratAcknowledgments

We found a recurrent seasonal pattern in abundance and composition of planktonic ciliates in Lake Constance, FRG, over a three-year period. Abundance peaks occurred in early spring and summer/autumn, while ciliate numbers were low in late spring (clear-water phase) and winter. Prostomatida and Oligotrichida dominated in early spring. They responded immediately to the phytoplankton spring bloom, while Haptorida, Peritrichida, and large Scuticociliatida (Histiobalantium) were delayed by 1 to 2 weeks. The spring community broke down at the onset of the clear-water phase.Pelagohalteria viridis containing symbiontic algae appeared shortly after this event. A highly diverse community was recorded in summer/autumn. Peritrichida, small Oligotrichida, and large Scuticociliatida reached their maxima during this season. Small Scuticociliatida were rare throughout the year and contributed moderately to total ciliate numbers only during the cold season. The observed seasonal sequence of pelagic ciliates in Lake Constance is discussed in relation to simultaneously collected data on potential food organisms and grazers.

No abstract