Abstract
Cycling of water quality constituents in lakes is affected by thermal stratification and homo-thermal conditions and other factors such as oligotrophication, eutrophication, and microbial activities. In addition, hydrological variability can cause greater differences in water residence time and cycling of constituents in man-made lakes (reservoirs) than in natural lakes. Thus, investigations are needed on vertical mixing of constituents in new impounded reservoirs, especially those constructed to supply domestic water. In this study, sampling campaigns were conducted in the Sabalan reservoir, Iran, to investigate vertical changes in constituent concentrations during the year in periods with thermal stratification and homo-thermal conditions. The results revealed incomplete mixing of constituents, even during cold months when the reservoir was homo-thermal. These conditions interacted to create a bottom-up regulated reservoir with sediment that released settled pollutants, impairing water quality in the Sabalan reservoir during both thermal stratification and homo-thermal conditions. Analysis of total nitrogen and total phosphorus concentrations revealed that the reservoir was eutrophic. External pollution loads, internal cycling of pollutants diffusing out from bottom sediments, reductions in inflow to the reservoir, and reservoir operations regulated vertical mixing and concentrations of constituents in the Sabalan reservoir throughout the year.
Highlights
Lake thermal budgets strongly affect the depth profile of water quality constituents (Noori et al, 2019a)
Thermal stratification gradually developed in early April and lasted until late November, when the difference in water temperature between the surface and bottom layers reached more than 8 °C (Fig. 4)
Strong thermal stratification formed a distinct thermocline in May, and this progressed downwards until November
Summary
Lake thermal budgets strongly affect the depth profile of water quality constituents (hereafter ‘constituents’) (Noori et al, 2019a). The constituents are layered during thermal stratification and usually show decreasing/increasing trends with depth in the water body. In the absence of thermal stratification, i.e., when the thermal gradient vanishes (homo-thermal conditions), constituents are usually mixed throughout the depth of lakes. Homo-thermal conditions can thereby improve water quality in deep layers through penetration of dissolved oxygen to the depth. Homothermal conditions may further degrade water quality in upper layers by the transport of sulfide, nutrients and organic matter from sediments to the water surface. This may expose lake water to processes with direct consequences for ecosystem health (Lawson and Anderson, 2007; Yu et al, 2014a)
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