Effects of turbidity and biotic factors on the rotifer community in an Ohio reservoir

Abstract

In reservoirs physical horizontal gradients may affect zooplankton distributions as well as the biotic interactions that potentially regulate zooplankton abundance and species composition. We examined patterns of rotifer abundance and population dynamics along a turbidity gradient over a 4-year period in an Ohio reservoir. To analyze the effect of turbidity on rotifer populations we compared rotifer abundance patterns, species composition, birth and death rates at two sites with high turbidity (river site) and low turbidity (dam site) conditions. Because of the potentially important biotic interaction between rotifers and cladocerans, we also compared cladoceran abundance patterns and species composition. Our results suggest no effect of turbidity on rotifers in Acton Lake. Rotifer and cladoceran abundance patterns were similar at low and high turbidity sites. Similarity indices revealed few differences in rotifer and cladoceran species composition between sites. Rotifer birth and death rates were also similar at low and high turbidity sites. In contrast to these homogeneous spatial patterns, among year comparisons indicate high temporal variability in all parameters measured. Mean rotifer densities were similar from 1993 to 1995, but in 1996 density increased 4-fold. Rotifer species assemblages were dominated by Brachionus spp. from 1993 to 1995, while Keratella cochlearis and Polyarthra spp. were numerically dominant in 1996. Mean cladoceran density also increased in 1996 compared to previous years. Cladoceran species composition was dominated by Diaphanosoma birgei from 1993 to 1995, while Daphnia parvula and Bosmina longirostris dominated the 1996 cladoceran community. Comparison of rotifer population parameters in years of contrasting D. parvula abundance suggests that exploitative competition may be an important mechanism regulating rotifer communities in Acton Lake. Interannual variation in Daphnia abundance may in turn be controlled by variation in fish biomass.