Individual lake characteristics modify the life cycle and diapause habitat of two neighbouring populations of the cyclopoid copepod Thermocyclops oithonoides

Abstract

Thermocyclops oithonoides (Cyclopoida, Copepoda) populations were studied in two neighbouring lakes with similar seasonal hydrographical characteristics. The two populations showed different life-cycle and diapause characteristics. The major differences were found in the number of generations per year (three versus two), the timing of induction and termination of diapause, temperature and oxygen conditions during diapause migration to the sediment, and habitat selection for wintering. Diapause initiation occurred at about 12 h light: 12 h dark in one population, but about 8 h L: 16 h D in the other. Higher habitat temperatures (approximately 5 °C) seemed to allow completion of a further generation in the warmest lake. The population descending early into the hypolimnion utilized the deep profundal sediment as a diapausing habitat. These specimens were most likely physiologically prepared for diapause, including tolerance to low-oxygen or anoxic conditions. The population with an additional generation, probably not yet in the complete diapause phase, avoided low-oxygen conditions and utilized mainly the oxygenated part of the profundal sediment as a diapausing habitat. The major part of both populations diapaused in the sediment, but a small fraction was in "active diapause" in the plankton during winter. In these populations of T. oithonoides, day length, modified by habitat temperature, seemed to influence diapause initiation, whereas habitat distribution during winter seemed to be influenced by the combined effect of diapause ontology and oxygen conditions in the potential wintering habitat.