Invertebrate Predators and Preservation Time Lags in Zooplankton Samples: Effects on the Estimates of Prey Abundance and Size

Abstract

ABSTRACT The effects of predatory phantom midge (Chaoborus flavicans) larvae on the density and size estimates of crustacean zooplankton with different time lags between sampling and sample preservation were studied. Chaoborid larvae may easily be included in zooplankton samples, especially if samples are taken with net hauls. Five larvae included in a sample had a significant negative effect on the density estimate of daphnids, cyclopoid and calanoid copepods, and cyclopoid nauplii. When chaoborids were included in the samples, a three-hour preservation lag caused a >5% underestimation of population density estimate of cyclopoids and a six hour lag resulted in a 10% error. In field studies, chaoborids usually prefer small cladocerans such as bosminids as prey, but in the restricted space of the sample containers escape ability of cyclopoids was diminished. As a result, cyclopoids were the main prey of chaoborids. Chaoborids had no effect on the mean individual size of any of the taxa, but in the reference samples the mean size of daphnids and calanoids was reduced. This, as well as the reduced density of cyclopid nauplii in the reference samples, was attributed to predation by cyclopoids. The results emphasized that zooplankton samples should be preserved immediately after the sample haul.