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areas is a result of increased survivorship or immigration is unclear. On the other <br />hand, while the zooplankton could also fit through the mesh, research indicates that <br />they avoid unnatural structures such as walls (Zaret 1980). This leads us to believe <br />that higher densities of adult copepoda and E. speratus are a result of increased <br />survivorship. <br />The fact that adult copepods and E. era s were significant in the plankton, <br />but not in the benthos, suggests that they are more susceptible to fish predation while <br />in the water column than while in the benthos. This is supported by the observation <br />that the smaller copepod life stages (nauplii and copepodites) were the most <br />numerous planktonic groups, yet showed no density effects of fish predation, <br />suggesting the possibility of size-selective predation (Brook and Dodson 1965). Since <br />the water was quite turbid (secchi depth averaged 22 cm on a calm day) the "reactive <br />distance" of a visual predator (such as fish) would have to be quite close for <br />something as small as a nauplius or copepodite (O'Brien 1979). <br />Indirect effects <br />The benthic densities of the naupliar and copepodite stages of copepods show <br />a negative effect of diminished fish numbers, which is probably due to increased level <br />of corixidae and Tanypus. The fact that they show significance in the benthos but not <br />in the plankton supports this since Tanypus feed in the benthos and the corixid genus <br />Tricorixa is known to feed on benthic organisms (e.g. chironomidae and oligochaeta, <br />see Merritt and Cummins 1984). While there was a direct effect of fish predation on <br />the density of E. s er u (an herbivore) chlorophyll a levels indicate there is no <br />12