Seasonal succession and vertical distribution of zooplankton in lake hayes and Lake Johnson

Abstract

Seasonal variations in the species composition, abundance, biomass, and vertical distribution of zooplankton in two eutrophic, warm‐monomictic lakes are described for the period December 1969 to February 1972. In Lake Hayes, Boeckella dilatata was the dominant crustacean and Ceriodaphnia dubia was subdominant. Lake Johnson was dominated by Ceriodaphnia and Bosmina meridionalis and Boeckella were subdominant. Daphnia were common in spring in Lake Hayes, but very rare in Lake Johnson. Chydorus and Asplanchna appeared in summer in both lakes. Zooplankton were slightly more abundant in both lakes in 1971 than in 1970; the annual mean was 1.7 times higher in Lake Johnson than in Lake Hayes in both years. Biomass was highest in spring. In Lake Hayes it ranged from 0.7 to 4.6 g.m‐2 dry weight; in Lake Johnson it fluctuated in 1971 from 0.3 g.m‐2 in March to 7.4 g.m‐2 in November. Animals occurred at all depths during holomixis, but were nearer the surface during stratification. Vertical distribution was often bimodal, with a peak in the metalimnion and a second, smaller one in the epilimnion. In Lake Johnson a stratum of anoxic water developed in the metalimnion each summer. In 1971 Boeckella were confined to the oxygenated hypolimnetic water below this zone, and almost disappeared from the lake as the hypolimnion became anoxic in late summer. In 1970, when they were largely confined to the epilimnion, the population increased over the same period. Ceriodaphnia had a similar vertical distribution to Boeckella in 1971, but did not decline in numbers, perhaps because of reproduction by the small number of animals remaining in the epilimnion. It is postulated that an increase in the cyclopoid population of the oxygenated hypolimnion was the result of recruitment, either from the benthic region or by migration from the epilimnion through the anoxic layer. Chydorus has appeared in the plankton of Lake Hayes since 1953, and cyclopoids have become more abundant. These changes are consistent with other evidence that the lake is becoming more eutrophic. © 1980 Taylor & Francis Group, LLC.