Population dynamics of hydra in Pine Lake

Abstract

Conflicting reports on the density patterns of hydra populations in temperate lakes have been published. Some authors have found an annual unimodal pattern while others have reported a bimodal pattern. I studies hydra in Pine Lake, a shallow medium hard water Ohio Water Service Lake in southeastern Mahoning County, Ohio, in order to examine the population characteristics of hydra collected on slides and the relationship of these characteristics to environmental factors. The environmental factors examined were wind velocity and direction, temperature, transparency, zooplankton density, chloride ion concentration and conductivity.

Two species of hydra, Hydra oligactis and Hydra pseudoligactis were found to coexist in Pine Lake in the fall. The population density pattern observed was bimodal with spring and fall maxima. Bud ratios and dry weights of hydra collected on slides in open water showed they were healthy and reproducing asexually.

Temperature and zooplankton density were not significantly related to growth axis density, hydra density or bud ratio. Water transparency had a positive relationship to growth axis density, hydra density and bud ratio in the upper half of the water column and a strong correlation with zooplankton density throughout the water column. An upward migration of the spring population from lower to higher depths was observed and may be related to water turbulence generated by wind or to an increase rate of detachment of hydra from plants in the littoral zone and subsequent movement of detached hydra into open waters.