Dissolved oxygen of lake reflects the dynamic balance between the oxygen release process under the photosynthesis between atmospheric oxygen and plants and the oxygen consumption process in biological respiration. Low level dissolution of oxygen will not only affect lake’s oxidation-reduction potential, which will further increase the release of nitrogen and phosphorus nutrition and the toxicity of toxic heavy metals, but also influence the distribution and growth of fish and invertebrate animals. Consequently, a relatively high-level dissolution of oxygen is a key and essential element to maintain a sound lake ecosystem. As early as in 1957, Hutchinson put forward that “In comparison with any other chemical parameters, dissolved oxygen analysis enables the limnological scholars to make a better knowledge of characteristics of lakes, and combined with the data of lake transparency, color and other morphological features, scholars may deepen their understanding of lakes.”
Lake warming incurred by global warming and eutrophication caused by human activities will change the thermodynamic structure and dissolved oxygen stratification of lakes, and aggravate the formation of anaerobic environment in the bottom of the lake and then affect other chemical and biological process of the lake ecosystem, thus finally influencing the structure and function of the ecological system. With the financial support by the lake thermal process coupling program under One-Three-Five Strategic Planning of CAS, Zhang Yunlin’s research panel from Nanjing Institute of Geography Limnology has made great progress in the research on how the dissolved oxygen stratification in deep lake influences the world’s climate change and the eutrophication response of lakes. Relevant research finding has been carried on the newly published Water Research (Water Research, 2015, 75: 249-258).
Researchers have found that Qiandao Lake is confronted with dual threat namely global warming and lake eutrophication. In terms of climate warming, the average temperature in Qiandao Lake area in 1980~2013 rose by 1.67°C, an increase of 0.49°C every 10 years. Temperature rose evidently in spring and summer when the lakes are more prone to stratification, and the autumn came second and there was no apparent trend warming in winter. In terms of lake eutrophication, with the development of society and economy, application of watershed land has been changed, discharge of industrial and agricultural waste effluents and domestic sewage has been increasing, and eutrophication of lakes has been gradually expanding, which has made the water transparency declining. In the past 26 years, the transparency of Qiandao Lake declined by 0.3m every 10 years, especially in summer when lakes are more prone to stratification, the water transparency declines at a higher rate than average throughout the whole year.
Through analyzing the relation between the depth of oxycline, which is a dissolved oxygen stratification parameter of lake, and temperature, thermal structure, as well as transparency, it is revealed that during the stabilization and weakening stage (July ~ February)of thermocline, the depth of oxycline significantly negatively correlates with the surface temperature and the thickness of thermocline, while significantly positively correlates with the depth of thermocline (figure 1); the depth of oxycline significantly positively correlates with the degree of transparency (figure 2). Lake warming caused by climate change has reduced the depth of oxycline by 1.65m in the thermal stratification regression stage of Qiandao Lake in the past 34 years, while the lake eutrophication aroused by human activities has reduced the depth of oxycline by 2.78m in the past 26 years. Therefore, the dual threats imposed by climate change and human activities will reduce the depth of oxycline, aggravate the dissolved oxygen stratification of lakes, which is harmful to water exchanges between upper and lower level, leading to the formation of an environment where water at upper level is deoxygenated while water in the bottom is anaerobic, thus finally deteriorating the water quality of lakes.
