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This study examines the vertical and seasonal variations of temperature and dissolved oxygen in the water of Strezevo reservoir in southwestern Macedonia. The results provide insights into the reservoir's classification based on these parameters.
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The vertical and seasonal variations of temperature and dissolved oxygen in the water of Strezevo reservoir Belinda Sipkoska-Gastarova, B. Vrzovski, A. Stojanova
Content • Introduction • Aim • Research area • Measurements • Results • Conclusions
INTRODUCTION Temperature and dissolved oxygen concentrations are important parameters to classify the reservoirs. Temperature can have distinctive impact on several parameters in the water. Thermal stratification which occurs in the reservoirs as a result of water’s temperature-dependent density have a direct impact on levels of dissolved oxygen in the water column. Dissolved oxygen can provide information about the overall functioning and productivity of the reservoir. Oxygen is required for respiration by most aquatic organisms.
Aim The aim of present paper is to explain how temperature affects reservoir and concentrations of dissolved oxygen, and on the basis of provided results to classify the reservoir.
Strezevo reservoir is located in the south western part of the Republic of Macedonia and it provides water for public utilities, for irrigation, for the industry, etc. It has surface area of 450 ha, maximum depth of 70m and average depth about 45m. This reservoir is fed by Semnica River and Alimentary Canal.
All researches were carried out at 7 research sites, 3 from surface waters at sites P1, P2, and P3 and 4 from different depths (5m, 10m, 15m, and 20m) at site P2. Samples were collected at monthly intervals in all seasons.
Water samples were collected in Ruttner bottle and taken to the laboratory. Time from the moment of taking the samples to conducting the analyses did not exceed 3 hours. Measurements We measured temperature with thermometer in Ruttner bottle and after a period of equilibration we read it.
We measured concentrations of dissolved oxygen by the Winkler titration method.
Results Thermal stratification at P2 sampling site
Comparative presentation of temperature changes in the reservoir
Comparative presentation of the amounts of dissolved oxygen in reservoir
Thermal stratification, generally occurs in reservoirs because water at different temperatures has different densities. Strezevo reservoir because of its geometric characteristics and the winds from Baba Mountain, is monomicticreservoir and it stratify once per year, usually during the summer.
Thermal stratification begins in early spring when rising air temperatures warm surface water. As air temperatures increase in spring and summer, surface water warms more rapidly than deeper water. Because the warmer, surface water is lighter than the colder, deeper water, these lake separate into three distinct thermal layers.
The top, warm layer is normally called the epilimnion, and the lower, cooler layer is the hypolimnion. The epilimnion, is warmed by sunlight and mixed by wind action.
The hypolimnion, receives minimal sunlight and does not mix with the upper layers. • This stratum creates a barrier, called the metalimnion, in which the epilimnion and the hypolimnion will not mix and remain separate. A temperature difference of only a few degrees is all that is necessary to increase the thermal resistance to mixing.
We measured the maximum temperatures in the surface water in Strezevo reservoir in July when the surface temperature ranged between 24-25 ºC. When the epilimnion cools in late autumn to the same temperature as the hypolimnion, stratification is broken, and the entire amount of the water in the lake circulates freely in response to wind action.
In winter we measured minimal temperatures of 4,8 ºC in all water column, at all sample sites. Both the surface as well as the bottom temperature is approximately the same at this point. From this point, in spring the solar radiation will increase and stratification will occur again.
Oxygen is essential to all forms of aquatic life, including those organisms responsible for the processes of self-purification in waters. The amount of oxygen dissolved in a water body is affected by water temperature, salinity, altitude, water inflow, and the photosynthetic activity of algae and plants. The solubility of oxygen in the water decreases as temperature increase, because of respiration and microbiological decomposition of organic matter.
Variations in oxygen concentrations can occur not only seasonally, but even over 24 hour periods, in relation to temperature and biological activity. During the daylight photosynthesis by aquatic plants occurs increasing concentrations of dissolved oxygen, while during the night, respiratory processes of aquatic organisms release CO2 into water and concentrations of dissolved oxygen declines. Changes in levels of dissolved oxygen in lakes are often associated with change in trophic condition.
Monthly variation of concentration of dissolved oxygen ranged from 6,56 mg/l in July at 5m depth to 11, 71 mg/l in May in surface water. This high values of oxygen concentration could be correlated with an accumulation of photosynthesizing phytoplankton at the surface. Sharp drop in dissolved oxygen concentrations usually occurred at the thermocline zone (5m depth). In winter, values of dissolved oxygen ranged between 10,63 mg/l to 11,18 mg/l, so they were almost consistent, as a result of the vertical mixing of water layers.
According to the operative legislation, on the base of the concentrations of dissolved oxygen, water of Strezevo reservoir can be framed in first class in almost all examined waters, because the concentrations of dissolved oxygen were above 8 mg/l.
Conclusions • Strezevo reservoir is monomicticreservoir and it stratify once per year, usually during summer. • In the surface layer the temperature value ranged between 4,8 ºC and 24 ºC according to seasonal changes in the weather, thermal stratification occurs at 5 meters depth, and at 20 m the temperature stabilized around 5-8 °C.
3. Concentrations of dissolved oxygen ranged between 6,56 mg/l and 11,71 mg/l depending on season and depth. 4. According to the operative legislation, on the base of the concentrations of dissolved oxygen, water of Strezevo reservoir can be framed in first class in almost all examined waters, because the concentrations of dissolved oxygen were above 8 mg/l.
