Instar Succession, Vertical Distribution, and Interspecific Competition Among Four Species of Chaoborus

Abstract

Of four highly oligotrophic lakes of the Matamek River System, Quebec, Lac à la Croix contains Chaoborus punctipennis and C. trivittatus, Lac Gallienne C. americanus, Lac Randin C. americanus and C. trivittatus, and Lac Méchant C. flavicans and C. trivittatus. The instars are all planktonic except instars I–III of C. trivittatus in Lac à la Croix. All species have a 1-yr life cycle except C. americanus in Lac Gallienne which has a 2-yr cycle. The cycles of C. trivittatus in lacs Randin and Méchant are out of phase by 1 mo or more with those of C. americanus and C. flavicans, respectively. Seasonal increases in length occurred in most instars except those of C. punctipennis in Lac à la Croix. Both species in Lac à la Croix made fairly extensive diurnal migrations but in lacs Gallienne and Méchant all species remained near the surface. Intense competition for food is considered the most likely cause of seasonal or spatial separation of instars of similar size in all lakes.