Interpond and intrapond variation in populations of periphytic cladoceran microcrustacea

Abstract

Periphytic cladoceran microcrustacea were collected from submerged parts of the sprawling emergent plant Ludwigia peploides (Onagraceae) in four ponds in Brazos County, Texas, on 52 dates from 23 February 1981 through 12 June 1982. Replicate samples of a constant size were collected from plant beds within each pond on each date. The periphytic cladoceran community in the four ponds consisted of 20 species, including 13 species in the family Chydoridae. Patterns of seasonal change in total density varied little between separate plant beds within ponds but varied greatly among different ponds. The timing of seasonal pulses of some species appeared to be synchronized across ponds and was attributed to regional temperature fluctuations. The season of maximum density for two of the numerically dominant taxa, Chydorus brevilabris and Alona setulosa, varied from pond to pond; each of these taxa may actually consist of more than one species in Brazos County ponds. Most of the cladoceran species appeared to reproduce parthenogenetically year-round. Gamogenetic (sexually reproducing) individuals were rarely observed. Low incidence of gamogenesis suggests that the production of resting eggs has a minor role in carrying the populations of most species through periods of stress. The periphytic cladoceran population in the smallest pond studied seemed to be impacted by periodic high intensity rainstorms. Some of the differences observed in seasonal density patterns among ponds were attributed to differences among ponds in the intensity of predation by small fishes. Factors which affected spatial variation in densities of Cladocera within plant beds were location on plant (leaves, vertical stems or horizontal stems), spatial variability in amount of detritus and filamentous algae coating submerged plant parts, and location relative to wind-induced wave turbulence. Responses to these factors varied with the species. Most periphytic cladoceran species did not show diel variation in their favored locations on Ludwigia on the occasions when this was investigated, although some species in some ponds did exhibit diurnal migrations within plant beds.