The Effect Of Climate Warming On Nutrients Dynamic In Different Trophic Lakes

Over the past century, a significant climate change which warming was the main feature has swept the world. The global surface temperature has risen about0.74℃in the past century and will continue to rise3-5℃in the next100years. Studies have shown that global warming has many effects on freshwater ecosystem including (i) physical and chemical characteristics of water column;(ii) chemical recycling process of nutrients in the sediment and (iii) species density and composition of biota. At present, our studies mainly focused on paleoclimate evolution and the effects of climate change on terrestrial ecosystems, however, research about the impacts of climate warming on lake ecosystems is lacking. Many shallow lakes in the middle and lower reaches of Yangtze River were much effected by eutrophication. Meanwhile, studies have shown that the annual mean temperature in this region will increase much higher than global average level in the future considering the effects of greenhouse gas increase only. Wuhan is located in the middle reaches of Yangtze River in the eastern Jianghan plain, it’s known as the "city of a hundred lakes" because the abundant water resources. Therefore, it provides a suitable basis to study the response mechanisms of lake ecosystems to climate warming. This study first analysed the observation data of the temperature, rainfall, wind speed and the sunshine duration in Wuhan during the last62years to explore the impacts of global warming on Wuhan. Secondly, an indoor microcosm system consisted18℃and22.5℃temperature regimes was set up to explore the effects of warming on the nutrients dynamic in sediment of a hypereutrophic shallow lake, Lake Nanhu. At last, four treatments with two different temperatures (ambient, ambient+4.5℃) and two different trophic levels (high nutrient level, low nutrient level) were set up using the sediment from the high nutrient level lake-Lake Nanhu and the low nutrient level lake-Lake Liangzihu. An outdoor mesocosm system was built and functional variables (the dissolved oxygen of sediment-water interface, total alkalinity and inorganic nutrients flux, etc.) were monitored every month to study the impacts of global warming on the sediment system of different trophic lakes. The results are as follows: (1) Both the wind speed and sunshine duration in Wuhan from1951to2012showed a significant declining trend. Reduced wind speed was caused by the rising temperature and fast urbanization process. The reduced sunshine duration was due to the increased aerosol concentration from anthropogenic emissions in the air. No trend was shown in any precipitation factors during1951-2012in Wuhan and the annual precipitation was stable. However, the maximum daily precipitation fluctuated strongly and there may be some cyclical characteristics. The temperature factors from1951to2012in Wuhan including annual average air temperature, average highest air temperature, average minimum air temperature, extreme minimum air temperature and average air temperature anomaly showed a significant increasing trend, except for the extreme maximum air temperature. The main reason of the rising trend was the joint effect of global warming and urban heat island.(2) An indoor microcosm system was set up to explore the effects of climate warming on the nutrients dynamic in sediment of a hypereutrophic shallow lake, Lake Nanhu. The results indicated that in eutrophic shallow lakes, warming can accelerate the release of phosphorus from the sediment into water and the increase of the phosphorus content in the water would further increase eutrophication. Rising temperature can accelerate the convention from nitrate to ammonia under aerobic conditions, but it didn’t show any significant effect on nitrogen dynamic at sediment-water interface owing to the denitrification and benthic biological absorption. However, further studies were need to explore the climate warming effect on nitrogen dynamic in lakes.(3) The response of nutrient dynamic at sediment-water interface to warming in different trophic lakes was explored using an outdoor temperature controlled enclosure simulation system. Results showed that during the whole experiment, the pH decreased and electrical conductivity increased significantly which suggested that warming can increase the level of ionization ion at sediment-water interface. Meanwhile, warming can enhance the microbial activity and accelerate the decomposition of organic nitrogen, therefore, will stimulate the release of total nitrogen at interface. The electrical conductivity at sediment-interface in high nutrient level shallow lakes was higher than low nutrient level shallow lakes owing to the higher concentrations of inorganic nitrogen and phosphorus. In addition, the change of nitrogen and phosphorus fluxes can be more sensitive to the stronger anaerobic environment in high nutrient level lakes. More sedimentation of decaying debris of macrophytes in low nutrient level shallow lakes may cause the levels of organic carbon and nitrogen significantly higher than those in high nutrient level shallow lakes.