Diel vertical distribution of freshwater calanoid copepods in a shallow lake: its variability in males with and without an oil sac

Abstract

Aquatic ecosystems exhibit a high degree of instability with many physical and biological processes affecting the distribution and survival of zooplankton communities. Zooplankters have evolved mechanisms to avoid unfavourable conditions. Diel vertical movements of calanoid copepods were examined in a shallow eutrophic lake in Sierra de Los Padres, Buenos Aires province, Argentina (37°56′30″ S to 57°44′ 30″ W). Notodiaptomus incompositus is the most common herbivorous calanoid in the zooplankton of pampean shallow lakes. The mean depth of its males with an oil sac was greater during daylight than at dark. For individuals found deeper during dark hours the mean oil sac volume was bigger than that of those found in shallower waters. The present work suggests that copepods had different diel vertical distribution and that this could be influenced by body condition, with those males with larger lipid reserves not needing to swim to the surface to feed during dark hours.