Assessments of trophic state in lakes and reservoirs of Wuhan using Sentinel-2 satellite data

Abstract

With the rapid development of economy, many lakes in Wuhan have been polluted to different degrees and suffer from eutrophication. The main objective of this study was to conduct the monthly trophic state assessments of waters in Wuhan from March 2019 to June 2020 using 111 Sentinel-2 images. The Forel-Ule index (FUI) and empirical Gaussian process regression (GPR) were, respectively, applied to obtain monthly area percentage (AP) of waters with each trophic state. Both FUI-derived and empirical GPR-retrieved results showed that majority of water bodies (>90%) in Wuhan were in mesotrophic and eutrophic states. The GPR-retrieved results based on FUI-derived water types were more reliable than the retrieval without classification, which reduced RMSE of trophic-level index (TLI) from 9.2 to 5.8, and MAPE from 14% to 9% (N = 213). Severe eutrophication occurred in the summer and early autumn (June-October). Stepwise multiple linear regression analysis indicated that temperature and wind speed were the two most important meteorological factors influencing eutrophication variability: the temperature accounted for 63% and 55% dynamic eutrophication from FUIderived and GPR-retrieved results, respectively; the wind speed explained 44% and 52% variability of FUI-derived and GPR-retrieved results.

Algal Blooms in Lakes in China Over the Past Two Decades: Patterns, Trends, and Drivers

“…We can use the two‐dimensional CIE x – y chromaticity diagram to digitally specify the color as three RGB values (i.e., reflectance) using the following equations (Fairman et al., 1997): $$\begin{align*}X=2.7689\mathrm{R}+1.7517\mathrm{G}+1.1302\mathrm{B}\\ Y=1.0000\mathrm{R}+4.5907\mathrm{G}+0.0601\mathrm{B}\\ Z=0.0000\mathrm{R}+0.0565\mathrm{G}+5.5934\mathrm{B}\end{align*}$$ $$$x=X/(X+Y+Z)$$$ $$$y=Y/(X+Y+Z)$$$ where R, G, and B represent the reflectance at the red, green, and blue wavelengths, respectively. Water color reflects the first‐order description of water quality (Zhou et al., 2021), and the majority of reported algal blooms in inland freshwater lakes are greenish in color in China (Hou et al., 2022). We acknowledge that blooms with other colors, either cyanobacteria or other bloom species, were excluded from our CIE‐based method; such blooms were also identified in many regions, especially in brackish lakes (Dierssen et al., 2006; Lopez et al., 2008).…”

“…where R, G, and B represent the reflectance at the red, green, and blue wavelengths, respectively. Water color reflects the first-order description of water quality (Zhou et al, 2021), and the majority of reported algal blooms in inland freshwater lakes are greenish in color in China (Hou et al, 2022). We acknowledge that blooms with other colors, either cyanobacteria or other bloom species, were excluded from our CIE-based method; such blooms were also identified in many regions, especially in brackish lakes (Dierssen et al, 2006;Lopez et al, 2008).…”

Water Colour Changes in High-Elevation Alpine Lakes during 2017–2022: A Case Study of the Upper Orco Valley Catchment

“…This observation suggests a recurrent temporal variability of water colour, which may depend on seasonal cycles. In addition to climatic variables, certain boundary conditions, such as land cover and hydrology of the lake basin or the proximity of anthropogenic activities, are additional factors that influence water colour [19,20].…”

Expanding the Application of Sentinel-2 Chlorophyll Monitoring across United States Lakes

“…Since their launch in 2015 and 2017, respectively, these sensors have provided 10-to-60 m resolution, depending on the spectral band, with a combined 5-day revisit frequency. Previous studies indicate S2 may be well suited for deriving chl a in lakes and reservoirs, for example in Europe [35][36][37], South America [38][39][40], Asia [41][42][43][44], the U.S. [45,46], and across a global set of lakes, with a rigorous validation across North America [47,48]. However, these studies tend to focus on individual or a limited number of lakes.…”