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-Jûniko Lakes, northern Japan (40° N). On the basis of continuous observations, a considerable variation was confirmed in mixing patterns among the lakes, including non-stratified 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. Ô-ike) complete circulation in a vernal circulation period. It was suggested that such variations were caused in association with an inflow 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 Ô-ike, the composition and abundance of zoobenthos changed bathymetrically in accordance with dissolved oxygen concentrations. As in several other Japanese intermountain lakes, two tubificine oligochaetes, Tubifex tubifex and Limnodrilus hoffmeisteri, and four chironomids dominated by Chironomus sp., occurred at the upper profundal bottoms in which dissolved oxygen became deficient. In spite of the differences in mixing patterns, the hypoliminion in all the lakes other than two flowing ones at the head were anoxic during stratified 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.
