Research On Temporal And Spatial Variation Of Algae Blooms And Its Driving Factors In Lake Dianchi Based On GIS/RS

In recent decades, with the rapid economic development, industrialization and urbanization, human activities are strengthening the impact on natural environment and ecosystem, this result in inland lakes or reservoirs facing a serious ecological problem in China, which is eutrophication. Eutrophication can not only leads to a serious threat to public health, but also restricts the development of economy and society. Lake Dianchi, which is a eutrophication lake located in Yunnan Province, has been listed in the "Three Rivers and Three Lakes" (which represents Huaihe River, Liaohe River, Haihe River, Lake Taihu, Lake Chaohu and Lake Dianchi) National Key Technology of Water Pollution Control and Governance Project. From the late 1980s, large area of algae bloom begins to emerge in Lake Dianchi, which brings a series of problems to local drinking water resources, irrigation and fishery. How to monitor algae bloom in a long time series accurately, identify the initial algae bloom date, reveal the temporal and spatial distribution characteristics and changes, analyse the causes of these changes, are becoming more and more crucial for Dianchi pollution controling.Remote sensing can observe large area of earth surface from space, which can’t be reached by ground surveys. Due to its advantages, like wide range of monitoring, low cost and short revisit time, remote sensing is gradually developing into one of the most important means in today’s water environmental investigation and research. MODIS can obtain surface images twice a day, and of a maximum resolution of 250m, the high temporal and spatial resolution make it a wide use in earth surface observation and dynamic monitoring.In this study, MODIS images from 2000 to 2014 are utilized to estimate algae bloom areas in Lake Dianchi through an algorithm called FAI. Then initial algae bloom date and spatial distribution characteristics are determined for each year, annual changes of algae cover and outbreak dates also be analysed. In the last, factors causing temporal and spatial variation of algae bloom are analysed from both meteorology and water quality. The main findings are as follows:(1) Average area of algae bloom is very small in January, February is the smallest algae bloom area month on average, except for 2003,2005 and 2006, there is no algae bloom occurred in February. From March, algae bloom begins to emerge, but the frequency is low and distribution area is not large. From April to August, the average area of algae bloom increases rapidly, and in August reaches its peak. From September, algae area begins to decrease, and reduces to a low level in December. Algae stops blooming at the end of December usually, but sometimes may continue to the next year.(2) Interannual algae initial bloom date variations shows a strong fluctuation. From 2000 to 2014, the bloom initiation date is delayed in general, with a speed of one day per year. It can also be divided into two phases from 2000 to 2014, the first stage is from 2000 to 2005, Dianchi algae outbreak shift to an earlier date, and reached the earliest in 2005, which is relative 40 days earlier than 2000; the second stage is from 2006 to 2014, algae outbreak shows a delayed trend, the bloom initiation date in 2014 is 50 days delayed relative to 2006.(3) Algae outbreak locations are relative in the north of Dianchi from February to May, mainly in Longmen Village, Haigeng, lake inlet of Panlong river and Huiwan. From the beginning of June, algae bloom expands southward, and in August reaches the southernmost. From September to December, blooms gradually moves northward and ends at the initial outbreak locations. But some years in January, algae widely distributed in the south of Lake Dianchi or enven the entire lake.(4) Spatial distribution of algae bloom is little changed between 2000 and 2009, but from the beginning of 2010, algae bloom shows a tendency moving to the south and east region.(5) This study also reveals that the varation of Dianchi algae bloom area during the year is drivened by temperature, precipitation and wind speed; Interannual bloom initiation dates mainly influenced by temperature.(6) Changes of wind speed and direction, especially in north and south direction, may cause the spatial distribution changes of algae bloom in Lake Dianchi. The increasing frequency of westerly may be the reason of algae cover moving to east. The higher nitrogen and phosphorus concentrations in north Dianchi may cause higher bloom frequency than south Dianchi. The planting of water hyacinth from 2011 may reduce the frequency of algae bloom in north Dianchi.