Long‐Term Experiments on Calanoid‐Cyclopoid Interactions

Zooplankton species composition was manipulated in an 8—mo long series of experiments in outdoor 450—L tanks. The first experimental stage evaluated the impact of cyclopoids (Mesocyclops, Microcyclops, Cyclops) and calanoids (Diaptomus) on each other. The second and third stages evaluated the effects of Daphnia and mosquitofish (Gambusia affinis) on the calanoid—cyclopoid interaction. Cyclopoid predation initially caused a 50—70% reduction in calanoid abundance. After a few months, however, cyclopoids caused >300% increases in calanoid abundance, apparently by preventing calanoid overexploitation of the phytoplankton. The presence of calanoids depressed cyclopoid abundance by up to 70—90%, presumably by diminishing the availability of edible phytoplankton to cyclopoid nauplii and of prey, such as rotifers, to cyclopoid adults. The presence of Daphnia depressed calanoid abundance, probably by reducing the phytoplankton abundance, but did not affect cyclopoid abundance, perhaps because cyclopoid adults could feed on items other than phytoplankters. Coequality (i.e., similarity in abundance) of closely related taxa was in 4 of 5 cases favored by the addition of a more distantly related taxon. Thus the addition of cyclopoids favored the coequality of two Diaptomus species (D. clavipes, D. siciloides), the addition of these Diaptomus species favored the coequality of two cyclopoid genera (Mesocyclops, Microcyclops), and the addition of Daphnia favored coequality of cyclopoids and calanoids and coequality of the two Diaptomus species, but the dominance of Microcyclops over Mesocyclops. Effects of Gambusia included a drastic reduction of Daphnia populations, moderate reduction of calanoid populations, and negligible effects on cyclopoid populations.