Zooplankton


        The zooplankton in ponds are important a) as grazers reducing the abundance of phytoplankton, b) as recyclers of nutrients needed by algae, and c) as a critical food for many species of fish, particularly in early stages of development. Three major groups dominate the zooplankton – the cladocerans, copepods and rotifers. Zooplankton typical of shallow ponds in the region are shown in Figure 37a–c.
        Cladocerans are often called water fleas, so-called for their hopping behavior as they move slowly through the water column. Many species are excellent filter feeders, consuming large quantities of phytoplankton. Eggs are borne in a brood chamber under the carapace of the parent until sufficiently developed for release.
        Many copepods are also effective consumers of phytoplankton, though they often display more selectivity for particular types of algae than do the cladocerans. Copepods are tubular in body shape, and are hydrodynamically streamlined for efficient swimming.
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Fig. 37a  Cladocerans common to ponds in southeast Pennsylvania.



        Rotifers form a diverse group of multicellular organisms that are only distantly related to the crustacean cladocerans and copepods. Many rotifer species ingest algal and bacterial cells, and are capable of rapid population growth (population size may double within 1-2 days). The rotifers thus are often the first of the three zooplankton groups to respond to increases in phytoplankton food. Because of their small size (often 0.1 to 0.2 mm), rotifers often are less subject to fish predation than cladocerans and copepods, but are often consumed by predaceous zooplankton species.
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Fig. 37b  Common copepods of ponds in southeast Pennsylvania.




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Fig. 37c  Rotifers common in the zooplankton of ponds in Chester County.



Rotifer densities were highest in March and July, while abundances of the slower growing Cladocera and Copepoda generally increased during the season (Fig. 38).
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Fig. 38  Mean zooplankton densities in 13 study ponds.