DEMOGRAPHIC AND EXPERIMENTAL APPROACHES TO ZOOPLANKTON POPULATION DYNAMICS (EGG RATIO, BIRTH RATE, ROTIFER, THIRD SISTER LAKE, MICHIGAN)

Quantitative analyses of zooplankton population dynamics require accurate estimates of birth and death rates. I present a general method of computing finite birth and death rates of natural zooplankton populations from changes in the age distribution of eggs and changes in population size. The method is applicable to cases in which eggs hatch periodically owing to variable rates of oviposition. When morphological criteria are used to determine the age distribution of eggs at the beginning and end of a sampling interval, egg mortality can be incorporated in estimates of population birth rate. I raised laboratory populations of Asplanchna priodonta, a common planktonic rotifer, in semicontinuous culture to evalute my method of computing finite birth rate. The Asplanchna population became synchronized to a daily addition of food but grew by the same amount each day once steady state was achieved. The steady-state rate of growth, which can be computed from the volume-specific dilution rate of the culture, was consistent with the finite birth rate predicted from the population's egg ratio and egg age distribution.; I also investigated the midsummer dynamics of rotifers in the plankton of Third Sister Lake, Michigan. The rotifers were controlled by predatory and competitive interactions with crustacean zooplankton. Exposing the natural rotifer community to various concentrations of the predaceous copepod, Mesocyclops edax, yielded independent estimates of prey-specific birth and mortality rates. When I compare these estimates with net rates of population growth obtained from changes in abundance of animals in the lake, I conclude that predation by Mesocyclops is sufficient to account for the observed dynamics of Polyarthra, Conochilus, Keratella, and copepod nauplii. Experimental manipulations of herbivorous crustacean zooplankton and of Rhodomonas minuta, a potential resource for rotifers, did affect the population growth rate and egg ratio of Polyarthra, but the magnitude of these competitive effects is small compared to predation imposed by Mesocyclops. Rotifers that escaped predation by Mesocyclops, such a Filinia, Asplanchna, Ascomorpha, Collotheca, and Conochiloides, have morphological and behavioral characteristics that may reduce their vulnerability to Mesocyclops.