| Dissolved oxygen changes have a profound effect on the survival of aquatic organisms, and the distribution of dissolved oxygen directly affects the survival, growth and reproduction of organisms. The concentration of dissolved oxygen which is too high or too low will be harmful to aquatic organisms, especially to the fish. Layered small reservoir in front of hill which is situated in the hillside of Mount. Fu has a natural advantage to study the vertical distribution of dissolved oxygen, because the wind affects little; there is barely surface inflow and water is of good quality. This research started from April 2014 and ended in February 2015. In the early stage, we assessed the eutrophication of the reservoir, concluding the result that it ranks medium level. It is indicated that reservoir water quality is good. During the study we investigated the vertical distribution of dissolved oxygen, temperature, pH for 2 times a month on average,and recorded the transparency, the wind speed over 3-meters surface and the weather conditions at the same time. Besides, we investigated the same parameters in other points when the conditions is fine in order to study the horizontal distribution of dissolved oxygen in the reservoir. According to these data, we analyzed the spatial and temporal variation characteristics when temperature was stratified, the spatial and temporal variation characteristics when dissolved oxygen was stratified, the corresponding relationship between dissolved oxygen stratification and temperature stratification and the relation of the spatial and temporal distribution characteristics and other environmental factors. The main conclusions are as follows.This reservoir is a monomictic lake, whose temperature stratification has obvious seasonal variation characteristics. Seasonal stratification usually forms in spring, and the reservoir has three layers: the mixed layer, the thermocline, and the hypolimnion. The depths of three layers respectively are 3 meters, 3 meters and 2 meters. During this period,the water temperature is less than the air temperature by day, the water temperature rises on the whole, the biggest vertical temperature gradient is 3.12 ℃/m; Then, when summer is coming, lake stability will raise rapidly and thermocline is becoming stable. The reservoir also has three layers, that is, the thickness the mixed layer decreases to 2 meters,the thickness of thermocline becomes bigger, and the thickness of the hypolimnion lessens to 1 meters. However, water temperature is slightly greater than the temperature during the day, and the water temperature does not change overall any longer. The mixed layer’s temperature increases and then decreases as time goes on. But the temperature ofthe hypolimnion has been rising, whose amplitude remains small and the maximum temperature gradient is 4.2 ℃/m. In autumn, the reservoir stays at the disappearing stage.During this period, the temperature of the water falls and the thickness of the mixed layer increases gradually. The mixed layer’s depth in September reaches to 5 meters and it increases to 7 meters in October. Early November, seasonal stratification begins to disintegrate and then has disappeared by the end of December, when the temperature gradient is less than 1 ℃. The temperature of the mixed layer decreases gradually, but the temperature of the hypolimnion remains unchanged. Holomixing has been found in winter.When the temperature drops below 4℃ in January, it appears thermal inversion phenomenon. In February, the weather turns to warmer. Thermocline is formed in the surface layer, but at this stage the stratification of temperature remains still unstable.The reservoir’s stratification of dissolved oxygen has obvious seasonal characteristic,and its variation consistent with temperature change. In spring, dissolved oxygen has a maximum value which appears at the bottom of the thermocline. The range of dissolved oxygen is in 1 mg/L-13 mg/L. The concentration distribution of the first layer of dissolved oxygen stays relatively uniform; in the second layer, the dissolved oxygen concentration increases as the water level deepens, which the maximum dissolved oxygen gradient value is 2.5 mg/(L·m); in the third layer, the concentration of dissolved oxygen sharply reduces with the increase of the depth, the minimum dissolved oxygen concentration appearing at the bottom and the dissolved oxygen gradient at the bottom being 4 mg/(L·m). In summer, the dissolved oxygen distribution is still in the same form,but the maximum value of dissolved oxygen appears at the middle of the thermocline. In the bottom, dissolved oxygen concentration is less than 1 mg/L, belonging to the anoxic condition. The overall level of the dissolved oxygen concentration compared with that in spring reduces. The dissolved oxygen changes in the range of 0 mg/L- 12 mg/L. Autumn witnesses large changes which have taken place in dissolved oxygen distribution. Most time, the first layer’s thickness increases gradually, and dissolved oxygen concentration increases comparing with that in summer which leads to surface dissolved oxygen concentrations reaching 9 mg/L. The maximum concentration of dissolved oxygen appears at the top of the thermocline, and the increase of the dissolved oxygen concentration sharply reduces with depth. If it lasts for sunny days, the maximum of dissolved oxygen will appear in the middle of thermocline, but it is not stable; if it becomes cool, the dissolved oxygen in thermocline will mix with dissolved oxygen in themixed layer. The hypolimnion is still at the lower oxygen status. In winter, dissolved oxygen distribution stays in the completely mixed state. In February, the dissolved oxygen in water increases, achieving the maximum value(14 mg/L to 15 mg/L) of dissolved oxygen throughout the year. The horizontal distribution of reservoir layers is the same.Schmidt stability index calculation shows that the reservoir layer is formed when Schmidt stability index reaches 20 J/m2, so using 40 J/m2 as a criteria to determine whether it is layered is not applicable, and we find when the stratification becomes stable,Schmidt stability index will increasingly grow. Calculating the depth of mixed layer has indicated that using temperature gradient 1 ℃ / m as the standard of judgment method is suitable for the reservoir stratification, and the density gradient 0.5 Kg/(m3·m) as the standard is not applicable to the freshwater lake. Other criteria are also not universally applicable. Calculating Wedderburn number makes clear that the reservoir, under the condition of current climate, will not appear the upwelling phenomenon, which will form the horizontal gradient. Therefore, the monitoring data in the front of the dam can reflect the stratification of the reservoir. The temperature stratification of reservoir affects the dissolved oxygen stratification by the way of physics, chemistry and biology. So both have a close relation with each other.The concentration of dissolved oxygen is significantly negative to water temperature.In the mixed layer, because of the mixing influence in vertical, the distribution of the dissolved oxygen is unstable, and its correlation is small. However the concentration of dissolved oxygen is relevant also negatively to water temperature as time goes on, and its correlation is high. The concentration of dissolved oxygen in water body will increase when transparency become big. The degree of the transparency have a significantly correlated with the depth of water in vertical where the maximum concentration of dissolved oxygen appears. The valve of pH have a significantly positively correlated with the concentration of dissolved oxygen. To some extent, the value of PH can reflect the concentration of dissolved oxygen. The assessment of eutrophication indicates that the lake remains the medium level. But regarding the real condition of the reservoir, we should take the lake’s eutrophication into consideration in the research. |
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