| Under the rapid development of global economy, the lake eutrophication has become a global environmental problem. During recent years, European countries and the USA have been vigorously promoting the research and enaction of nutrient criteria and control standards for lake eutrophication. How to develop and enact lake eutrophication control standards for one specific lake is still a challenging task for lake scientists and management agencies, as well as a frontier problem around the world. China has adopted the Environmental quality standards for surface water (GB3838-2002) on lake water quality management for a long time. TN and TP are always used as nutrient indicators in eutrophication control. Though GB3838-2002 plays a good role in lake eutrophication control within China, it still has some obvious disadvantages which reflect the extreme weakness on nutrient criteria and eutrophication control standards research, and cannot meet the current requirements of lake eutrophication control in China. Firstly, GB3838-2002 only includes physical and chemical indicators of water quality, which cannot well express the overall lake ecosystem status and trends. Secondly, GB3838-2002 provides the same evaluation method of TN and TP standards for all kind of water bodies not considering geographic characteristics of lakes, which is known as "one size fits all". Therefore, it is not integral and comprehensive to adopt GB3838-2002 in lake eutrophication control and management. It would be much better to add biological indicators (e.g. algal Chl-a) and comprehensive indicators (e.g. transparency) to the current nitrogen and phosphorus concentration indicators, which can help ensure a sustainable development of lake water quality improvement and ecosystem health, and comply with the current treatment method as "One lake, One policy".The Lake Erhai Basin is located in the Yungui plateau in southwesten China, taking on a variety of functions, such as the provision of drinking water, the retention of flood water, cultivation, irrigation, fisheries, and tourism. Unfortunately, rapid economic development in the basin has resulted in serious deterioration of the lacustrine environment. Now this lake is in the early stage of eutrophication. This study first investigated the long-term pattern and causes of water quality in Lake Erhai for understanding the eutrophication evolution characteristics and regime shift of the lake. Based on the long-term water quality analysis, the study focused on how to develop, nutrient criteria and eutrophication control standards for Lake Erhai, which would provide a valuable reference to nutrient criteria establishment of other lakes in our country.The main results of this study are as follows:(1) Long-term (in recent 20 years) social-economic statistical data were analyzed to demonstrate the developing process of Lake Erhai Basin. More intensive agricultural practices, including livestock, crop planting, et al., had increased nutrient loading in the lake from 1990 to 2012. In particular, the socio-economic development has been speeded up remarkably after 2000. The TN loads and TP loads entering the lake from livestock and crop planting covered 66.1% and 74.9% respectively. Dramatic increases in nutrient loading have fueled the accelerated eutrophication of lakes, causing harmful algal blooms.(2) The long-term (1990a-2013a) field monitoring data from Lake Erhai were analyzed to illustrate the evolution characteristics of Lake Erhai. During the past 20 years, the water quality of Lake Erhai is deteriorating on the whole level. Although there are improvements during the last two years as the TN and TP level recovered to Grade II, the secchi-depth (SD) transparency is still remaining on low level. The increase of algae is indicating the execrating trend of lake eutrophication. Most water quality indicators exclusive of TP had abrupt changes duringthe period of 2001-2003. The TN concentration in recent ten years (2004-2013) was around two times of that determined in 1991-2000. The algal Chl-a content increased by dozens of times since 2001. The SD transparency in recent ten years (2004-2013) was about one half of that determined in 1991-2000. Consequently, Lake Erhai had a obvious regime shift during the period of 2001 to 2003 and was enduring a sensitive period shifting from mesotrophic to eutrophic lake.(3) For avoiding the shortcomings of traditional statistical methods such as ordinary-least-square regression, quantile regression was used to investigate the stressor-response relationship between nutrient levels and algal biomass in Lake Erhai. Though water temperature was still the primary limiting factor of algal biomass, nutrients (e.g. N, P) had replaced much limiting effects of temperature since 1995.The degree to which the lakes were P-limited increased with increasing mean total P concentration. P started to take the place of N and became the most important limiting factor of algal growth only standing beside the temperature. Furthermore, the limiting relationships among the nutrients, temperature and algal biomass had obvious seasonal pattern. In summer, N and P had strong interactive impacts on the algal growth. In autumn, all the three factors had interactive impacts on the algal biomass.(4) Based on the results of quantile regression analysis, traditional linear regression method, and Pearson’s correlation analysis, it is appropriate to choose TN, TP, Chl-a and secchi-depth (SD) transparency as lake eutrophication control indicators of Lake Erhai. Firstly, the reference lake method, lake population distribution approach, historical reference approach, trisection method were adopted to find the reference state of four environmental indicators. Their probability density distribution curves were ploted to evaluate the values of each reference state. The average value of all reference states exclusive maximum value and minimum value was determined as the nutrient criteria:TN 0.23 mg/L, TP 0.015 mg/L, CODmu 1.4 mg/L, Algal Chl-a 1.5 mg/m3, SD 430 cm. The criteria of of TN, TP, Chl-a and SD were directly determined as the recommended primary standards of the four indicators. The temporal comparison approach (the years of 1991-2000 as reference time with better state, and the years of 2005-2013 as worse state) was applied to calculated the recommended secondary standards:TN 0.44 mg/L, TP 0.022 mg/L, Chl-a 6.3 mg/m3 and SD 250 cm, respectively.(5) In order to verify the feasibility of recommended nutrient criteria and standards, an eutrophication model based on the Mike-Ecolab was developed to model the stressor-response relationships between external nutrient load reduction and water quality indicators (e.g. TN,TP, Chl-a, and SD transparency) in Lake Erhai. Four scenarios of external nutrient (e.g., TN, TP) load reduction with different reduction intensity were designed. TN concentration in Lake Erhai had abrupt decline under all four scenarios, and reached the secondary standard (0.44 mg/L) after 2015. With very intensive nutrient reduction (e.g. scenario 3 and scenario 4), TN can reached the primary standard (0.23 mg/L) after 2030. In contrast to TN, Tp concentration had smaller decline, didn’t reach the secondary standard (0.022 mg/L) until 2030 with most intensive nutrient reduction (e.g. scenario 3 and scenario 4) and was far below the secondary standard line under the condition of scenario 1 and scenario 2. Though algal biomass indicated by Chl-a was kept decreasing along with time gone by and with the more intensive nutrient reduction, annual average Chl-a concentration was above 9.0 mg/m3 which was far higher than the secondary standard (6.3 mg/m3) in 2030. Consequently, the secondary standard of Chl-a was suggested to be relaxed from 6.3 mg/m3 to 9.0 mg/m3. SD transparency had slow increase after 2015 and only attained the secondary standard until 2030 with most intensive nutrient reduction (e.g. scenario 3 and scenario 4)(6) Finally, a system dynamic model was developed by a software Vensim, in order to design reasonable external nutrient load reduction strategies in the Lake Erhai Basin and make it to meet the requirements of nutrient load reduction for attaining new standards Based on the main external pollution source of Lake Erhai and sensitivity analysis of social-economic parameters adjustment, a candidate external nutrient load reduction strategy was proposed, which coverd the livestock quantity reduction, increasing poultry in place of livestock, transformation from drought fields to paddy ones, promoting the intergrated processing and utilization of livestock manure, increasing the scale of greenhouse cultivation, et al. If the candidate external nutrient load reduction strategy was excuted, the TN load into lake and TP load into lake could be reduce to 1275 ton/yr. and 122 ton/yr in 2025 respectively, which were very close to the nutrient capacity under the secondary standards and achieved the nutrient reduction goals of scenario 3. Furthermore, economic feasibility analysis on new secondary standards had been conducted. The results was indicated that the gross investments each year were about 2.5% of GDP in the basin. Consequently, the local economic capacity can bear the implementation of candidate nutrient load reduction strategy. |
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