An index that shows chemical stratification strength [IC-i; i = water quality item such as chlorophyll-a (Chl.a) and soluble phosphorus (SP)] was proposed and compared with one of thermal stratification strength indices, Schmidt's stability index (SSI), in Shiozu Bay and Lake Biwa, Japan. The proposed indices of IC-i can be easily calculated with at least one set of each water quality data in both the epilimnion and the hypolimnion. The SSI was shown to be consistent with the traditional thermocline index of thermocline strength index (TSI), but SSI is used as the stability index of the whole lake, whereas TSI is used as the stability index near the thermocline. Analyses showed that chemical stratification strength is determined largely by thermal stratification strength. Totally different characteristics of IC-Chl.a and IC-phosphate (PO4) at high SSI in the main North Basin of Lake Biwa and in Shiozu Bay were possibly due to the difference in their volumes and hydrodynamic conditions. The proposed index and relationships are especially useful to roughly determine thermal and chemical stratification when only few water quality data are available.
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