Investigation Of The Regional Differences And Nitrogenous Cycling In China’s Lakes

Being a worldwide environmental problem, eutrophication has been paid more and more attention in past three decades. A lot of studies show that the occurrences of eutrophication not only correlate to the water quality but also associate with the geographic character, climate, hydrologic condition and aquatic ecosystem. The response of lakes to nutrients input varied greatly with the regions that lakes are located in, while the ecological regionalization and classification system of lakes’ eutrophication have not been constructed in China by now. The aquatic ecological areas are presently being regionalized by the district, water quality and eco-function. Thus, it is of great urgency to regionalize the lakes scientifically according to the regional character and lake type. Since the investigation of the difference between the lakes in China is the base of the eutrophication eco-region, the following aspects are studied in this paper:1. The eutrophic statuses of25lakes in Jiangsu, Yunnan, Xinjiang and Inner Mongolia were analyzed according to the field investigation. Combined with large amount literatures, the eutrophic evolution tendencies of these lakes were further discussed. About84%of the investigated lakes are eutrophic or being toward to eutrophication. The eutrophic levels of these lakes in different district were in the rank of Xinjiang<Jiangsu<Inner Mongolia, while the eutrophic levels of the lakes in Yunnan show a serious polarization phenomenon. The historic documents and field investigation showed that the eutrophic level of most of these lakes has been raised during the past three decades. It was found that the eutrophication of the lakes in arid region such as Xinjiang and Inner Mongolia may result from the trends of climatic warming and drying, while the eutrophication of the lakes in south of China may be caused by anthropogenic factors.2. The nitrogen and phosphate in25rivers surrounding Lake Taihu as well as in some typical lake zones such as Meiliang Bay and East Lake Taihu were analyzed during the hydrological year of2009-2010. Furthermore, the nitrogenous self-purification capacity of Lake Taihu was studied in combined with the investigation of water flow, cyanobacteria salvages and aquatic product outputs. In the whole hydrological year, the inflow and outflow fluxes of total nitrogen (TN) were7.00×104t and4.01×104t, respectively. About3.02×104t and0.20×104t nitrogen in water body were removed by denitrification and sediment adsorption during this hydrological year, respectively, suggesting a strong nitrogen self-purification capacity of Lake Taihu. The potential denitrification is stronger in western Lake Taihu (e.g., Meiliang Bay) than that in eastern Lake Taihu (e.g., East Lake Taihu), and is stronger in summer than that in other seasons. Thus, the nitrogenous self-purification capacity plays an important role in nitrogenous transference and transformation in Lake Taihu.3. To understand the effect of cyanobacterial blooms on nitrogen transformations in eutrophic Lake Taihu, nitrate transformations after the addition of cyanobacteria collected from Lake Taihu were traced in laboratory microcosms with the15N isotope addition method. About81.2%and98.4%of nitrate was lost when2×109and4×109cells L"1of cyanobacteria were added, respectively. Conventional denitrification was found to play a major role in the nitrate removal process while other pathways (e.g., dissimilatory nitrate reduction to ammonium [DNRA] and assimilation of nitrate into microbial biomass) were negligible. It was likely that the cyanobacterial respiration as well as their decomposition resulted in anoxic conditions and that cyanobacteria also served as a carbon source for denitrification. Based on the above results, it was estimated that109cells of cyanobacteria were enough for denitrifiers to remove0.53mmol nitrate in the eutrophic lake ecosystem. Therefore, cyanobacterial blooms have the potential for nitrogen removal by denitrification and this process could cause nitrogen limitation of primary production in summer in Lake Taihu.4. The origins of the dissolved organic matter (DOM) in27lakes were identified using UV-Vis absorbance, fluorescence spectroscopy, C/N ratios and isotopic technology. Fluorescence indexes (FI) and Biological Index (BIX) of DOM in the lakes of west Jiangsu were less than1.3or0.67, respectively, suggesting DOM a strong terrestrial origin. Significant correlation was found between the fluorescence intensities per unit organic carbon at different peaks, which indicated that the humic-like and protein-like materials in these lakes have a similar source. Considered the characterization of these lakes and the economic development, it can be deduced that most of these DOM are produced by agriculture and downstream industries and imported with inflow rivers. C/N, the stable carbon isotope (δ13C) values typical of C3plant debris in particulate organic matter (POM) and the fluorescence indices of DOM indicate that most of the OM in Lake Hulun is of terrigenous origin. It was deduced that about10.2%and7.3%of DOM were contributed by algae in September and January, respectively, according to the linear correlation between the concentrations of algae-derived DOM and the fluorescence intensities of tyrosine-like matter. According to stockbreeding development and climate change in Hunlun Buir Steppe, we deduced that the destruction of the grassland ecosystem by over grazing in specific locations and trends in climatic warming and drying were the main factors causing the increase of OM and nutrient concentrations in Lake Hulun. Both the allochthonous inputs and autochthonous production take a certain contribution to the DOM pools of Lake Taihu in December, which suggests that the autochthonous production is the main origin of DOM in Lake Taihu in the whole year. The DOM in the eutrophic lakes in Yunnan and Xinjiang such as Lake Dianchi, Lake Xinyunhu and Lake Qiluhu are predominantly allochthonous derived, while it is on the contrary in oligotrophic and mesotrophic lakes. 5. The algal growth models of Lake Taihu, Lake Dianchi and Lake Hulunhu were constructed. According to the models, it was found that the responses of the three lakes to nutrients inputs are ranked in the order of Lake Hulunhu, Lake Taihu and Lake Dianchi. Moreover, the factors influence on the trophic response of lakes were discussed. Base on the analysise of the factors, the investigated lakes were primarily regionalized and classified.Our study can provide the basis for the lakes regionalization and classification in China, and for the developing of nutrient criteria and standards.