Seasonal Variation of Water Color at 30m Spatial Resolution in 42 Urban Lakes
Eutrophication has been a serious problem in China’s urban lakes. In this study, to monitor 42 lakes trophic state in Wuhan city, a Forel-Ule index (FUI) method based on Landsat 8 OLI data was proposed among different seasons. The result shown that the FUI calculated from Landsat 8 OLI reflectance spectral (Rrs) can substantially represent the lakes TSI, with accuracy of 79.37% for in situ experiment TSI dataset. And the TSI in urban lakes can be divided into 3 categories by a decision tree (oligotrophic, FUI 1-6; mesotrophic, FUI 7-9; trophic, FUI 10-21). Seasonal dynamics show that the trophic state of 42 lakes in summer is significant higher than other seasons, while 99.65% lakes are under the state of trophic. And the order of average FUI is summer (19) > spring (18) > fall (11) > winter (10). Our results can serve as an example for other lakes lie in Yangtze River basin, and support the formulation of effective strategies to reduce seasonal eutrophication. Introduction Urban lakes have played a key role in the development of the national economy, with functions of regulating regional climate, maintaining regional ecosystem and balance the diversity of traditional derivatives. In recent decades, unban lakes suffering serious problem of water eutrophication due to the anthropogenic pollution (Le et al. 2010). This process often accompanied with the rapid increase in the number of harmful algae, especially for the chlorella, cyanobacteria (Wang et al. 2019b), diatom, which would threaten other aquatic life and change the color or odor of the water body, and finally causing drinking water crisis. Therefore, the trophic state evaluation and monitoring of inland water have been a research hotspot in water ecology community for decades (Chao Rodriguez et al. 2014; Smith and Schindler 2009), Carlson (Carlson 1977) creatively invented a widely adopted numerical trophic state index (TSI) based on 5 water quality parameter, chemical oxygen demand (COD), Secchi depth (SD), total phosphorus (TN), total phosphorus (TP) and chlorophyll-a (Chl-a). And then the trophic state of inland water is divided into three categories, Oligotrophy (TSI: 0-30), Mesotrophy (TSI: 30-50), and Eutrophy (TSI>50). Satellite sensors for worldwide observations of natural waters, such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) (Woerd and Wernand 2015), the Medium Resolution Imaging Spectrometer (MERIS) (Woerd and Wernand 2015), the Moderate Resolution Imaging Spectroradiometer (MODIS) (Chen and Quan 2013), and Landsat 8 Operational Land Imager (OLI) (Pahlevan et al. 2014), gradually developed into a new way of water environment monitoring. These spectrometers are specifically designed for water quality monitoring which only contains narrow (≈10 nm) spectral bands, mostly in the visual domain (380–780 nm), to establish empirical formulas for water quality parameters and bands reflectances and detecting the physical and chemical composition in water, like total suspend matters (TSM), chlorophyll-a and other colored component. (Li et al. 2018; Zhang et al. 2014) The Forel-Ule index (FUI) is a traditional investigations of water color that commonly used in coastal waters, oceans, and regional seas. Based on the principle of FUI, water 21 FUI-classes numbered from 1 to 21 which represent the water color vary from dark blue to yellowish brown (van