Influences of biotic and abiotic factors on seasonal succession of zooplankton in Hugo Reservoir, Oklahoma, USA

I measured water quality variables, primary productivity, biomass of phytop lankton palatable to zooplankton and zooplankton abundance in mesotrophic H ugo Reservoir, Oklahoma (U.S.A.) at weekly to biweekly intervals between Ma rch and September, 1996. Canonical Correspondence Analysis (CCA) was used t o estimate the influence of abiotic factors and resource availability in st ructuring the zooplankton assemblage over the season. Repeated Measures Ana lysis of Variance (ANOVA) was used to determine spatial and temporal variat ion in zooplankton abundances and to make comparisons between repeated meas ures ANOVA and CCA in analyzing the zooplankton assemblage. In addition, I analyzed relationships among cladoceran reproductive parameters (egg ratio, instantaneous birth rate, reproductive ratio and mean brood size) and envi ronmental factors over the season using product moment correlation. Results showed little evidence of bottom-up control of the zooplankton assemblage. CCA and correlations suggested temperature was the most important factor r egulating assemblage structure over the season. Several inverse relationshi ps were seen among cladoceran reproductive parameters and temperature. Decl ines in Holopedium gibberum Zaddach, Bosmina longirostris Muller and Ceriod aphnia lacustris Birge during June corresponded with increases in mean temp erature to above 25 degrees C. Increases in birth rates at the time of popu lation declines suggested that population declines were a result of increas ed mortality rates (likely due to high water temperature or predation). Spa tial variation in the zooplankton assemblage coincided with differences in both temperature and turbidity among sites. Lower abundances of Daphnia par vula Fordyce and C. lacustris and a higher abundance of rotifers at the up- reservoir site coincided with higher mean temperature of the water column a nd higher turbidity than down-reservoir sites. CCA and ANOVA showed similar results for the spatial and temporal variation in the zooplankton assembla ge while CCA provided a more clear approach for analyzing the effects of en vironmental variables on the assemblage.