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Abstract:
Continuous automatic data recording and periodic water sampling were carried out in dual purpose reservoirs (fish farming and crop irrigation). The study was carried out in the Israeli coastal area and in the Jordan Valley, at two sites in each reservoir - windward (west) and leeward (east) - and at several depths per site.
Shallow reservoirs (about 3-4 m deep) developed a daily pattern of thermal and oxygen stratification with full nocturnal mixing. Deep reservoirs (5-8 m deep) developed, in addition to the daily pattern at the upper layers (2-3 m), a seasonal stratification. This led to the formation of an anoxic hypolimnion which lasted until midseason when the water level decreased to about 5 m due to irrigation. The daily wind (west sea breeze) induced movement of the upper oxygenated layer eastward. This layer downwelled in the east of the reservoir and the subsurface layer upwelled in the west, leading to a horizontal oxygen gradient in which the dissolved oxygen concentration in the east was higher and reached a greater depth than in the west. Differences in weather between the geographic locations led to stronger stratification in the Jordan Valley.
Implications for reservoir management and differences from other types of water bodies are discussed.
