Lake Biwa is located at 34°58'-35°3' N, 135°52'-136°17' E (near Kyoto, Japan) and consists of a northern basin (the main part) and a southern basin (the smaller part). The former is oligotrophic and the latter is nearly eutrophic. Only the northern basin is considered. Data for this study were collected in 1970s. The annual adsorbed solar energy was 4153 MJ while the mean depth of the lake is 44 m. It is possible to identify two zones in the column water: a light one (<20m) and a dark one (between 20m and 24m). The average amount of suspended solid (SS) in the light zone was 1.3 [gm"3J] (National Institute for Research Advancement, 1984) while the average amount of dissolved organic carbon (DOC) was 1.6 [gC m"3] (Mitamura and Sijo, 1981). The average amount of total plankton plus zooben-thos in the whole water column was 0.16 [gC m"3] (Sakamoto, 1975).
Lake Mendota is located at 43°04' N, 89°24' W (near Madison, Wisconsin, USA) and is a eutrophic lake. Its energy budget was investigated by Dutton and Bryson (1962) and Stewart (1973). The annual adsorbed solar energy was 4494 MJ while the mean depth of the lake is 12.2 m. Two zones of the water column were identified: the euphotic one (until 9m) and the aphotic one (the last 3.2m). The average amount of SS in the light zone was 1.9 [gm 3] (National Institute for Research Advancement, 1984) while the average amount of DOC was 3.3 [gC m"3 J] (Brock, 1985). The average amount of total plankton plus zoobenthos in the whole water column was 0.62 [gC m"3] (Brock, 1985).
Lake Trasimeno is the largest lake in peninsular Italy (area 124 km2); it is shallow (mean depth 4.7 m, maximum 6.3 m), and accumulation processes are favored. The water level of the lake showed strong fluctuations with respect to meteorological conditions; hydrological crises occur after several years with annual rainfall < 700 mm. Lake Trasimeno can be considered homogeneous for chemical and physical parameters (Maru, 1994) and very sensitive to meteorological variability or human impact. According to the Vollenweider-OECD classification (Giovanardi et al., 1995), Lake Trasimeno is mesotrophic, whereas by using the annual phosphorus loading estimation method (Maru, 1994) and the Hillbrich-Ilkowska method (Hamza et al., 1995), the lake is classified as eutrophic.
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