Two-Decade Reconstruction of Algal Blooms in China’s Lake Taihu HONGTAO DUAN,* ,† RONGHUA MA, † XIAOFENG XU,* ,‡ FANXIANG KONG, † SHOUXUAN ZHANG, † WEIJUAN KONG, § JINGYAN HAO, † AND LINLIN SHANG † State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, China, 210008, Ecosystem Dynamics and Global Ecology (EDGE) Laboratory, School of Forestry and Wildlife Sciences, Auburn University, Auburn, Al, 36849, and Department of Geography Information Sciences, Nanjing University, Nanjing, China Received November 17, 2008. Revised manuscript received March 15, 2009. Accepted March 18, 2009. The algal blooming in the inland lakes has become a critically important issue for its impacts not only on local natural and social environments, but also on global human community. However, the occurrences of blooming on larger spatial scale and longer time scale have rarely been studied. As the third largest freshwater lake in China, Lake Taihu has drawn increasing attention from both public and scientific communities concerning its degradation. Using available satellite images, we reconstructed the spatial and temporal patterns of algal blooms in Lake Taihu through the past two decades. The blooming characteristics over the past two decades were examined with the dynamic of initial blooming date being highlighted. The initial blooming dates were gradually becoming later and later from 1987 to 1997. Since 1998, however, the initial blooming date came earlier and earlier year by year, with approximately 11.42 days advancement per year. From 1987 to 2007, the annual duration of algal blooms lengthened year by year, in line with the substantial increases in the occurrences of algal blooms in spring and summer months. The algal blooms usually occur in northern bays and spread to center and south parts of Lake Taihu. The increases in previous winter’s mean daily minimum temperature partially contributed to the earlier blooming onset. However, human activities, expressed as total gross domestic product (GDP) and population, outweighed the climatic contribution on the initial blooming date and blooming duration. This study may provide insights for the policy makers who try to curb the algal blooming and improve the water quality of inland freshwater lakes. Introduction Global environmental change is one critical issue facing human beings and has been recognized as a result of anthropogenic activities (1, 2). The eutrophication in inland freshwater lakes is one of the most severe environmental problems (3), not only due to the significant ecological functions of inland lakes (4), but also due to the ecological services, including freshwater supply, fishery, and flood mitigation, on which nearby human society depends (5). Since the alteration of water quality primarily results from economic development and sewage treatment, the eutroph- ication in inland lakes has become one of the most widespread environmental and social problems for all countries around the world (2, 6). For example, lakes such as Victoria in Africa, Okeechobee in the United States, Taihu in China, and the Baltic Sea in Europe could be headed in the direction of becoming perennial algal soups (5). Lake Taihu, the third largest freshwater lake in China, is one of the main sources for nearby residents’ drinking water, and one of the most severely polluted freshwater reservoirs in China where the algal blooms have been heavily studied (5, 7, 8) (see Figure 1). The recent bloom in summer 2007 affected the Wuxi City nearby significantly; more than 1 million people were short of drinking water (5). This event made Lake Taihu one of the hot topics not only in China (9, 10), but also in global community (5, 11). Although the Chinese government has begun to recognize the importance and urgency of studying and managing eutrophication in the Lake Taihu and has provided some funding to support this kind of research, a long-term and large-scale campaign against the eutrophication in Lake Taihu is still in its infancy (2, 5). The algal bloom has long been recognized as the result of importing nutrients, mainly as nitrogen and phosphorus (5, 12-14). Furthermore, the major nutrient sources have been recognized as industrial activities, for example, sewer- age, livestock drainage, soil nutrients and losses of fertilizers in drained agricultural lands (15). Taihu Basin is identified as one of the regions in China featured as high population density and the high gross domestic product (GDP) per capita (16). For example, in 2000, the GDP in this area accounted for 10.3% of national GDP; the local per capita GDP was 3 times of that nationwide with a 7 times average population density than nationwide average population density (17). Along with the advancing of our understanding and the development of new techniques, more and more studies have recognized the complex integration of many environmental factors on algal blooming, which have not been fully understood and need to be thoroughly investigated (11). Currently, eutrophication has been partially attributed to climatic influences such as the increasing temperature (14). Although the identification of these factors has been achieved, quantitative understanding and mechanical exploration of eutrophication are still lacking. A number of studies on algal blooms have been conducted since the 1960s (18-27). However, most of them were traditionally performed by taking ship-borne water samples and analyzing the samples in a laboratory or by doing on- site measurements (20). Due to the coarse sampling fre- quencies and limited sampling points, it is difficult to comprehend the temporal and spatial pattern of blooms for one entire lake, which were usually featured by outbursts uneven in time and space scales (28). The blooms can increase the aquatic content of chlorophyll-a and yield critical impacts on the light reflection of the water surface, and aquatic vegetational albedo, which can be detected by satellite. Therefore, it is possible to retrieve the algal blooming on the basis of shifts in ecological information of aquatic system at large spatial scale and long time series (28). Actually, satellite- derived data has become a powerful tool and has been preliminarily utilized in understanding algal blooming (8, 28). * Address correspondence to either author: Phone: 86-25-86882161 (H. D.); 1-334-844-8068 (X. X.). Fax: 86-25-57714759 (H. D.); 1-844- 1084 (X. X.). E-mail: htduan@niglas.ac.cn (H. D.); xuxiaof@auburn.edu (X. X.). † Nanjing Institute of Geography and Limnology, CAS. ‡ Auburn University. § Nanjing University. Environ. Sci. Technol. 2009, 43, 3522–3528 3522 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 10, 2009 10.1021/es8031852 CCC: $40.75  2009 American Chemical Society Published on Web 04/08/2009