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few corixids caught in control areas were early instars as well. Whether increased densities of <br />corixidae in caged areas is a result of increased survivorship or immigration is unclear. On <br />the other hand, while the zooplankton could also fit through the mesh, they are known to avoid <br />unnatural structures, such as walls (Zaret 1980). This leads us to believe that higher densities <br />of adult copepoda and, in particular, E. s eratus, during the first weeks of the experiment are <br />a result of increased survivorship or of an increased proportion of the copepods entering the <br />water column from the benthos. <br />The fact that adult copepods and E. e ratus were significantly lower in density in the <br />plankton, but not in the benthos, suggests that they are more susceptible to predation while in <br />the water column than in the benthos. This is also supported by the observation that the <br />smaller copepod life stages (nauplii and copepodites) were the most numerous planktonic <br />groups, yet showed no significant differences in density in the controls and the treatments. <br />Size-selective predation could generate this pattern (Brooks and Dodson 1965). Since the <br />water was quite turbid (secchi depth averaged 22 cm on a calm day) the "reactive distance" for <br />a visual predator (such as fish) would be quite short, especially for something as small as a <br />nauplius or copepodite (O'Brien 1979). <br />Indirect effects <br />The benthic densities of the naupliar and copepodite stages of copepods show a <br />significant decrease in full exclosure treatment. This is probably due to increased densities of <br />corixidae and T us. Both Tanypus and the corixid genus Tricorixa are known to feed on <br />benthic organisms (e.g. chironomidae and oligochaeta, see Merritt and Cummins 1984). <br />While the density of E. s eratus (an herbivore) significantly decreased in the open and <br />10