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Predation on nanoflagellates by metazoan zooplankton was investigated using a radioactively labeled flagellate, Poterioochromonas malhamensis, as a tracer cell in laboratory incubations of freshly collected plankton assemblages. Experiments conducted in the fall, winter and spring indicated that rotifers dominated the grazing on nanoflagellates by metazoans in the winter (68%) and spring (92%). Rotifer grazing was not determined in the autumn. It is likely that the greater impact of rotifer grazing in the spring was due to the occurrence of abundant filamentous cyanobacteria and gelatinous colonial phytoplankton which selectively depressed feeding rates of crustaceans compared to rotifers. Crustacean predation on nanoflagellates was highest in the autumn when cladocerans (primarily Daphnia spp.) were abundant. Predation by metazoan zooplankton in this lake appeared capable of removing the total standing stock of heterotrophic and phototrophic nanoplankton in < 1 d. Impacts of ciliated protozoa on nanoplankton, calculated from abundances and literature feeding rates, ranged from approximately one-third to four times that of metazoan predation depending on season and method of calculation. The relative importance of the different groups of predators appears to vary seasonally which is expected to alter the transfer of energy, carbon and nutrients from bacteria to higher trophic levels.

Selected lakes in Hillsborough County, Florida have been hydrologically augmented with groundwater to offset stage declines caused by excessive pumping of the Floridan Aquifer. Augmentation water can be relatively rich in 226 Ra (.5 decays per minute [dpm] L 21 ). We measured 226 Ra activities in shells and soft tissues of adult bivalve molluscs (Elliptio cf. buckleyi) from groundwater-augmented and nonaugmented lakes to assess bioaccumulation of 226 Ra by mussels. Mussels from augmented lakes displayed higher 226 Ra in both shells and tissues than did mussels from nonaugmented lakes. Within a sample, 226 Ra activity in Elliptio tissues was higher than the value measured in shells. Highest activities were found in a composite mussel sample (n 5 6) from an augmented lake; soft tissue activity was 619 6 33 dpm g 21 dry weight and shell activity was 147 6 7 dpm g 21 g dry weight. Large mussels displayed greater activities in soft tissues and shells than did small mussels. We transplanted animals from a nonaugmented lake into a groundwater-augmented water body. 226 Ra activity in dry tissue rose from 32 6 1 to 196 6 2 dpm g 21 within 2 months. When 226 Ra-rich mussels (232 6 2 dpm g 21 ) from the augmented lake were transferred to the nonaugmented lake, they showed no significant 226 Ra loss over the 69-d experiment. Large Elliptio mussels concentrated 226 Ra in their soft tissues to levels about 1,000 to 25,000 times concentrations in lake water. Pumping of groundwater in Florida for residential, agricultural, and industrial use contributes dissolved 226 Ra to some surface water bodies, where it can be bioaccumulated by bivalve molluscs.

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