Aquatic ecosystems are under stress from various stressors, such as pollution, human activity, climate change, and changes in land use. More than half of the world's population engages in activities that exacerbate aquatic stresses, including anthropogenic eutrophication and algal blooms and lives close to water bodies.
Aquatic ecosystems, including seawater, estuaries, reservoirs and lakes, face several problems due to urban population development, expanding urbanization, agricultural and industrial activities, and global climate change.
Importance of Water Quality
Numerous studies have shown that one of the most significant potential hazards to civilization is the erosion of water quality. To define water quality and locate the origin of any pollution or contamination that might result in a decline in water quality, biological, chemical and physical criteria are often utilized.
Water Quality Assessment
Regular water quality assessments may be made utilizing satellite-based remote sensing, which has shown to be a successful and valuable method.
Over the last 40 years, there have been numerous notable advancements in using remote sensing technologies to evaluate water quality. However, studies that used the correlation between water quality metrics such as chlorophyll content, total suspended particles, or optical depth often employed empirical, semi-analytical models and machine learning methods.
What is Chlorophyll-a?
The primary pigment in phytoplankton is chlorophyll-a, which can be identified due to its absorption peaks at 438 nm and 676 nm. Because it reflects the health of aquatic ecosystems, chlorophyll-a concentration is often employed to evaluate water bodies' primary productivity or eutrophication level.
Phytoplankton or algal levels are often assessed using the metric chlorophyll-a in aquatic habitats. Although algae are a naturally occurring component of aquatic ecosystems, uncontrolled algal development may result in unsightly issues, including surface scum, unpleasant scents, and lowered dissolved oxygen levels.
Some algae also create toxins that, in large quantities, may be dangerous to the public's health. As a result, retrieval of chlorophyll-a concentrations at a synoptic scale is essential for managing and assessing water quality.
Multispectral Instrument (MSI) to Evaluate Aquatic Environment
An advantageous instrument for researching and evaluating the aquatic environment is the Multispectral Instrument (MSI) mounted on Sentinel-2 satellites. It has been used to establish links between the spectral reflectance of water bodies and water quality measures. Several methods have been created to determine the amount of chlorophyll-a in water using various multispectral satellites utilizing either individual or ratios of spectral channels.
Most empirical methods for retrieving water quality parameters need regional calibration for the proper optical properties of various locations. As a result, their effectiveness in seas with different optical characteristics and larger scales may be constrained.
Using Sentinel-2 MSI Photos GEE Platform
Using Sentinel-2 MSI photos on the Google Earth Engine (GEE) platform, this study examines the anthropogenic impact on chlorophyll-a concentration in Ha Long Bay in Ha Long City from 1 January 2019 to 30 June 2021.
The method based on the OC-2 band created for Bengal Bay was used in the research and calibrated for the chl-a concentration estimate in Cua Luc Bay. Cua Luc Bay may be subjected to the OC-2 algorithm since both bays have similar topography and climate.
How the Study was Conducted
81 Sentinel-2 Level-2A photos were processed in the GEE data repository for this investigation. To guarantee that the research regions were not impacted by cloud cover and its shadows, the photos were filtered based on the percentage of cloud cover less than 50% and carefully reviewed.
For calibration, chlorophyll-a information for Cua Luc Bay was gathered from the National Oceanic and Atmospheric Administration (NOAA). Applying a threshold enabled the Normalized Difference Index (NDWI) to be generated for each picture and used to distinguish between water and land regions.
Chl-a layers were initially calculated for chl-a determination utilizing the model created for a prior investigation. The accuracy was assessed by comparing the values of random NOAA points with equivalent points in the chl-a layer.
Significant Findings of the Study
To immediately examine the effects of development, tourism, coal mining and aquaculture activities on the chl-a development in the coastal waters of Ha Long Bay, this study successfully utilized the OC-2 from the previous research.
This study successfully demonstrated how to calibrate the OC-2 model using NOAA chl-a data. However, further research is advised to enhance model prediction and replication of the finding for other comparable locations that need a longer evaluation time and include in situ gauged data.
Optical remote sensing manual atmospheric correction is essential for investigations of the coastal environment.
Reference
Nguyen Hong Quang , Minh Nguyen Nguyen , Matt Paget, Janet Anstee, Nguyen Duc Viet, Michael Nones and Vu Anh Tuan (2022) Assessment of Human-Induced Effects on Sea/Brackish Water Chlorophyll-a Concentration in Ha Long Bay of Vietnam with Google Earth Engine. Remote Sensing. https://www.mdpi.com/2072-4292/14/19/4822/htm
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