The present study shows that the depth of the mixed layer at the height of summer stagnation can be predicted from lake area and water colour. Lakes larger than 1 km~2 obtained significant hypolimnetic heating in spring and were cooled beyond 4℃ in the deep waters during the autumn circulation. Lakes ranging from 0.4 to 1 km~2 were well mixed during the spring circulation and obtained deep water temperatures up to 6℃. After the spring circulation in 1998, lakes with A 10 m had well-developed hy-polimnia with temperatures < 5℃. It has been shown (Hongve 1980, 1997) that lakes of this size and depth obtain poor aeration of the water column during the spring circulation. Accordingly, these lakes are typically spring meromictic. The hypolimnetic temperatures remained low throughout the summer stagnation. Shallower lakes (z_(max) = 9-10 m) with surface area < 0.3 km~2 and without visible water colour (< 10 CU) were mixed and heated to 5-6℃ in their deepest parts during the spring circulation, and the water column after that consisted of two layers, corresponding to an epilimnion and a metalimnion. Lakes with similar size and depth (z_(max) = 6-9 m) containing strongly coloured water (63-146 CU), were spring meromictic and obtained summer stratification with a shallow epilimnion and a well-developed hy-polimnion. 1. By the findings above, poor spring circulation, spring meromixis, looks to be the normal situation in small and relatively deep lakes in this area. 2. Further, the practical value of assessments of "direct work" (Birge 1916) for comparison between lakes is questioned.
展开▼
