Abstract
Algal bloom is a serious global issue for inland waters, posing poses a serious threat to aquatic ecosystems. The timely and accurate detection of algal blooms is critical for their control, management and forecasting. Optical satellite imagery with short revisit times has been widely used to monitor algal blooms in marine and large inland waters. However, such images typically are of coarse resolution, limiting their utility to map algal blooms in small inland waters. We developed a new method to map the spatial extent of algal blooms using sentinel-2 multispectral instrument (MSI) and Landsat operational land imager (OLI) images with higher spatial resolution but lower temporal resolution based on the concept of local indicator of spatial association. The mapping results was applied to measure the duration and frequency of algal blooms in Lake Taihu from 2017 to 2020. Our results show that the developed methodology is able to extract the spatial distribution of moderate algal blooms using near-infrared and red-edge bands (bands 6, 7, 8, and 8a of sentinel-2 MSI images or band 5 of Landsat OLI images) by comparison with MODIS FAI data ( R 2 = 0.888 for sentinel-2 MSI and R 2 = 0.85 for Landsat OLI, P < 0.05). However, the temporal resolution of combined Landsat OLI and sentinel-2 MSI images (i.e., up to 2–3 days) is insufficient to monitor algal blooms during the summer time in Lake Taihu due to cloud effects and rapid algal change. Our research has benefits for the management of small inland waters with complex water conditions.
Highlights
INLAND waters, including rivers and lakes, provide various ecosystem services, regulate microclimate, maintain biodiversity, and supply habitat for flora and fauna [1]
Among all NIR, red-edge and SWIR1 bands from Sentinel-2 MSI images, the extracted area of High-High clusters in band 5 was the largest, covering both algal bloom regions and turbid water areas (Figure 4; more example extractions in Supplementary Material B), which is significantly different from the area extracted from other bands (ANOVA P < 0.05)
More algal blooms extracted from bands 8, 7 and 6 are near the boundary of the spatial extents of the algal blooms from band 8a (Figure 4)
Summary
INLAND waters, including rivers and lakes, provide various ecosystem services, regulate microclimate, maintain biodiversity, and supply habitat for flora and fauna [1]. Around the world, algal blooms of high intensity and duration in inland waters are being frequently reported [4,5,6]. The causes for this phenomenon include nutrient enrichment from agricultural, industrial and urban runoff [7, 8], hydrological alteration from dam construction [4], and climate change [1, 6]. Satellite imagery with diverse spatial and temporal resolutions has great potential for timely and accurate algal bloom monitoring at large spatial extents, frequency, drifting rates and occurrence duration [6, 19, 20]
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