Summer population fluctuations, feeding, and growth of Hydra in an arctic lake1

Abstract

During 1977, the population density of Hydra in the rocky littoral community in Toolik Lake, Alaska, increased from June to a maximum in late July or early August, then remained stable or decreased slightly through late August. The summer increase of Hydra corresponded to increases in the availability of copepods and Bosmina (the preferred prey) until late July. Hydra kept at lake temperatures in the laboratory with unlimited prey reproduced fast enough asexually to account for these increases: at 19°C the intrinsic rate of increase (r, biomass) was 0.53 g·g‒1·d‒1, and at 11°C, r = 0.31·d‒1. These rates, much higher than those for other Hydra at comparable temperatures, seem to be a metabolic adaptation to arctic conditions. Failure of the Hydra population to continue to increase while food resources increased in August is attributed to predation by Lymnaea, displacement by epilithic algal growth, endogenously controlled emigration from the rocky littoral, or some combination of these.Hydra appears to be the most important predator on Bosmina. Hydra at densities of 135·m‒2 on the rocky shoals could take 1,620 Bosmina · m‒2·d−1 during late July and early August 1977, about 10% per day consumption of the estimated lake standing crop of Bosmina. This is the first evidence that Hydra plays a major role in controlling a prey population.