Limnological Studies on the Daily Variation and Vertical Distribution of Aquatic Environment in the Tsengwen Reservoir

Abstract

In this paper, the author deals with the vertical and daily variations on water temperature, dissolved oxygen (DO), pH and conductivity in the Tsengwen Reservoir. The Tsengwen Reservoir, 17 square kilometer, is large and deep. It has a strong turbulent motion owing to eddy development. DO is above 6 ppm in the epilimnion constantly. Water temperature is stable and is only 0.5℃-l℃difference at every water stratum within a day. In a clear bright day, water temperature at epilimnion is sometimes between 4℃ and 7℃ higher than air temperature in the evening. Cooling of surface water and evaporation developed nocturally is considered to cause the surface water sinking and convection. These are the reasons why the daily fluctuations of water temperature and DO at every stratum are small and DO increases but not decrease slightly in the evening. Concerning the vertical distribution, water body has thermocline obviously from April to October. Water temperature, DO and pH start to decrease greatly from the lower layer of the epilimnion. Under this layer, temperature decreases sharply according to increasing depth. DO and pH decrease too, but the former decreases to the lowest at 10 m, the latter at 10 m-20 m. Under this layer, DO starts to recover slightly and then to a certain depth, it decreases again. On the other hand, conductivity being constant and higher at the epilimnion starts to increase from the lower layer of the epilimnion to the 10 m where DO is the lowest, it shows the opposite curve with DO. Both conductivity and DO do not fluctuate much at different eight times within a day. This fact suggests that vertical distribution of DO has no relation to plankton migration. The fact of high conductivity, low DO and high bacteria count at the metalimnion suggests that organic solid such as debris of plankton decomposed aerobically suspended on the upper layer whereas DO depletion and high conductivity occur. In February, thermocline disappears. Both convection and turbulent motion are more developed. Water temperature is 21℃ and DO is abundant. Both water temperature and conductivity almost keep constant, but it still can be observed that DO in 10 m-15 m and pH in 10 m-20 m are the lowest. In April, thermocline begins to appear. Water temperature in the hypolimnion is 20℃ which is the lowest in all strata in a year. The fact that after decreasing of DO to the lowest level at 10 m, DO increases to a certain depth then decreases again suggests hypolimnion remaining an original phenomenon.本篇乃研究曾文水庫基礎水族環境因子之日變化及垂直分佈。曾文水庫為一大型水庫，因湖流之影響，本身擾動大，表水層溶氧量高，約有6 ppm以上，又因氣溫之日溫差相當大，高達8°~10°，水溫之日溫差極小，僅約0.5°~1℃而已，到了晚上，由於表水層水溫大於氣溫約4°~7℃，引起表水層的蒸發和冷卻，造成對流，使得水域各層水溫、溶氧量日變化不但很小，而且表水層溶氧量於晚間反而有稍增的現象。一日中不同時問之調查結果顯示水溫、溶氧量、pH值與導電度之垂直分佈變化小。10月變溫層中10~15m以之溶氧量為最少，一日之內無甚變動，都在1 ppm左右，而且均維持在10~15m之水層內，變溫層中溶氧量最少的水層與浮游生物上下的移動無關，而導電度之分佈正好跟溶氧量呈相反之關係，溶氧量少而導電度大之情形與水中之有機物質懸浮在底水層上緣，受到微生物之分解有關。2月時變溫層消失，因湖體本身擾動和對流現象的影響，各層水域溫變化小，且溶氧量也相當充沛，均在6 ppm以上。4月變溫層初形成，其溶氧量保持在4~6 ppm之間，尚未有顯著的下降情形。溶氧量在一定水層中急劇減少，然後增加再減少之情形乃水域之自然恢復現象。