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Body size, food availability and seasonal rotifer community structure in Deer Lake, British Columbia Schreiber, Dorothee


The differential effects of body size on species' demographic parameters has long been hypothesized to be a powerful structuring force in zooplankton communities. The size-efficiency hypothesis predicts that large species, due to metabolic efficiency and-greater effectiveness of food collection, should displace small species when food is limiting, in the absence of predation. According to the threshold-food concentration hypothesis, small-bodied rotifers achieve r=0 at a lower food concentration than large rotifers; however, large rotifers have higher maximal reproductive rates. I attempted (1) to assess the importance of food concentration in structuring the species and size composition of a natural rotifer community in Deer Lake, Burnaby B.C., and (2) describe seasonal changes in rotifer community structure with reference to temperature, competition and predation. The threshold food hypothesis relates specifically to rotifers, and its significance has been tested in published laboratory studies. Therefore, I predicted that the y-intercepts of regression equations relating food concentration (measured as size-fractioned chlorophyll a) and reproductive output would be higher for small species than for large ones, and that the slopes of these lines would be higher for large species than for small ones. I found no patterns with respect to body size in either of these two parameters; however, I found some evidence for size-efficiency within a single species, Keratella cochlearis. The large form of K. cochlearis reproduced at a significantly lower food concentration than either of the two smaller forms. Average rotifer body size of the whole community showed no change with chlorophyll concentration. Recent research which suggests that threshold food levels change along several environmental gradients may explain the lack of support my data provided for the threshold-food hypothesis. Additionally, selective grazing may change the food requirements needed for reproduction of various species. Temperature was important in determining seasonal species abundance, likely because of physiological responses of development rate to temperature. I did not find that species with high loadings on those principal components axes that were significantly correlated with Daphnia or cyclopoid copepod abundance had attributes which conferred resistance to interference competition or predation. However, spined, small Keratella cochlearis co-occured seasonally with predatory cyclopoid copepods. Although competition and predation may not have been measured adequately, or at a scale relevant to rotifer survival and reproduction, it appears that temperature is the most important factor I measured in organizing rotifer species into communities in Deer Lake.

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