Mixing pattern and bathymetric distribution of zoobenthos in an interconnected lake system in the Tsugaru-Jûniko lakes, Aomori prefecture, northern Japan

Abstract

The mixing patterns and bathymetric distributions of zoobenthos were studied in a spring-fed interconnected lake system (Koikuchi-no-ike Lakes) in the Tsugaru-Juniko Lakes, northern Japan (40° N). On the basis of continuous observations, a considerable variation was confrmed in mixing patterns among the lakes, including non-stratifed patterns at the heads (Lakes Ao-ike and Wakutsubo-no-ike), monomictic (L. Ochikuchi-no-ike) and dimictic with (L. Koikuchi-no-ike) and without (L. O-ike) complete circulation in a vernal circulation period. It was suggested that such variations were caused in association with an infow of unithermal phreatic waters, the duration and development of ice/snow coverings, and an accumulation of salts in the hypoliminions. In Lakes Ochikuchi-no-ike and O-ike, the composition and abundance of zoobenthos changed bathymetrically in accordance with dissolved oxygen concentrations. As in several other Japanese intermountain lakes, two tubifcine oligochaetes, Tubifex tubifex and Limnodrilus hoffmeisteri, and four chironomids dominated by Chironomus sp., occurred at the upper profundal bottoms in which dissolved oxygen became defcient. In spite of the differences in mixing patterns, the hypoliminion in all the lakes other than two fowing ones at the head were anoxic during stratifed seasons, with zoobenthos being absent in the lower profundal bottoms. An abundance of autochthonous and allochthonous organic matter in the lakes could cause a rapid decline of oxygen in the hypoliminions, producing anoxic conditions in the lake bottom.