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Abstract

Laboratory studies utilizing natural (field-collected) bacteria and the cultured alga Chlamydomonas reinhardi showed species-dependent shifts in the clearance ratios of algae:bacteria (CR) by several size classes of five species of Cladocera. Selectivities were determined between algae and bacteria at various algal concentrations as well as in filtrates of different pH and dissolved organic carbon content, and between different bacterial size fractions. Mesh sizes were similar among grazer species, but ‘single-mode’ species with uniform, expanded gnathobase elements that allow only filter-feeding ( Pseudosida bidentata and Ceriodaphnia reticulata ) were more efficient at capturing bacteria at low algal concentrations than were ‘dual-option’ species with reduced gnathobases and second trunklimbs modified for individual particle capture ( Bosmina longirostris and Eubosmina tubicen ) or scraping { Chydorus sp.) At high algal concentrations CR values of these two species groups converged toward a similar ratio. In some experiments convergence was mediated by reduced selectivity against bacteria by the dual-option species (decline in CR); in others the cause was increased selectivity against bacteria by the single-mode species (increase in CR). Within the Bosminidae, Bosmina saturated ingestion at lower food concentrations than Eubosmina ; however, selectivity shifts with food concentration were similar. In general, dual-option species saturated ingestion at higher food concentrations, were more sensitive to bacterial cell size and less sensitive to extremes of pH and DOC than were single-mode species. Abrupt selectivity shifts observed just above the incipient limiting concentration are consistent with behavioral mechanisms for selective flexibility based on limb morphology and discount passive mechanisms such as association of bacteria with algae or filter clogging. Differences in the magnitude and trajectory of these shifts noted between replicate experiments also suggest that variations in bacterial size or shape as well as in water chemistry may influence filtration of free bacteria by cladocerans. In order to understand the utilization of free bacterial resources by filter-feeders, particulate and soluble features of the environment around bacteria need to be taken into account.

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