Sida crystallina, a cladoceran, is found in densities up to 45 per square centimeter underneath lilypads (Nuphar poly-sepalum) in Marion Lake, British Columbia. Floats, boats, and other artificial substrates are rapidly colonized by Sida to densities comparable to those underneath lilypads. Sida only persists underneath horizontal-lying substrates. On substrates receiving direct sunlight, filamentous algae grow and displace Sida. The high densities of Sida under lilypads attract several predators, both vertebrate and invertebrate; however, the overall Sida populations appears to be unaffected by predation. At the end of the summer, the population declines rapidly, possibly due to predation as well as to natural mortality. Rapid flushing occurs in Marion Lake after a rainstorm, and lilypads appear to serve as a refuge from the current. It is also likely that lilypads serve as feeding locations in areas of relatively high phytoplankton concentrations (compared with the open water of the lake). The observed colonization behaviour would then be selectively advantageous if feeding sites are in short supply and intra-specific competition is high. The dynamics of colonization were monitored four times over the summer, and population growth parameters were obtained concurrently. From a comparision of these, it seems that most of the observed growth on the artificial substrates is due to immigration and not to reproduction. All population drops are due to emigration. From an experiment testing the rate of colonization as a function of the distance to the nearest Nuphar bed, it appears that the colonization is by clumps of Sida at the mercy of the lake currents. During periods of persistent wind, colonization is most rapid in areas where these clumps tend to be concentrated by the wind. Otherwise, the artificial substrates nearest to the Nuphar beds are colonized first. The population statistics obtained over the summer show that after an initial period of rapid growth, the population growth rate becomes very slow (probably not much more than one birth per individual). It is likely that Sida is exploiting its environment to a maximum by maintaining its population as near as possible to its carrying capacity. / Science, Faculty of / Zoology, Department of / Graduate