INDIVIDUAL LAKE CHARACTERISTICS MODIFY THE LIFE-CYCLE AND DIAPAUSE HABITAT OF 2 NEIGHBORING POPULATIONS OF THE CYCLOPOID COPEPOD THERMOCYCLOPS-OITHONOIDES

Thermocyclops oithonoides (Cyclopoida, Copepoda) populations were stud ied in two neighbouring lakes with similar seasonal hydrographical cha racteristics. The two populations showed different life-cycle and diap ause characteristics. The major differences were found in the number o f generations per year (three versus two), the timing of induction and termination of diapause, temperature and oxygen conditions during dia pause migration to the sediment, and habitat selection for wintering. Diapause initiation occurred at about 12 h light : 12 h dark in one po pulation, but about 8 h L : 16 h D in the other. Higher habitat temper atures (approximately 5-degrees-C) seemed to allow completion of a fur ther generation in the warmest lake. The population descending early i nto the hypolimnion utilized the deep profundal sediment as a diapausi ng 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 ma inly the oxygenated part of the profundal sediment as a diapausing hab itat. The major part of both populations diapaused in the sediment, bu t a small fraction was in ''active diapause'' in the plankton during w inter. 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 com bined effect of diapause ontology and oxygen conditions in the potenti al wintering habitat.