Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> This paper assesses the reflectance difference values between the respective spectral bands in the visible and near-infrared (VNIR) of Sentinel 2A/2B Multi-Spectral Instrument (MSI) and Landsat 8/9 Operational Land Imager (OLI) sensors for seagrass, algae, and mixed species discrimination and monitoring in a shallow marine environment southeast of Bahrain Island in the Arabian Gulf. To achieve these, a field survey was conducted to collect samples of seawater, underwater sediments, seagrass (<i>Halodule uninervis</i> and <i>Halophila stipulacea</i>), and algae (green and brown). In addition, an experimental mode was established in a goniometric laboratory to simulate the marine environment, and spectral measurements were performed using an Analytical Spectral Devices (ASD) spectroradiometer. Measured spectra and their transformation using the continuum-removed reflectance spectral (CRRS) approach were analyzed to assess spectral separability among separate or mixed species at varying coverage rates. Afterward, the spectra were resampled and convolved in the solar-reflective spectral bands of MSI and OLI sensors and converted into water vegetation indices (WVIs) to investigate the potential of red, green, and blue bands for seagrass and algae species discrimination. The results of spectral and CRRS analyses highlighted the importance of the blue, green, and near-infrared (NIR) wavelengths for seagrass and algae detection and likely discrimination based on hyperspectral measurements. However, when resampled and convolved in MSI and OLI bands, spectral information loses the specific and unique absorption features and becomes more generalized and less precise. Therefore, relying on the multispectral bandwidth of MSI and OLI sensors, it is difficult or even impossible to differentiate or to map seagrass and algae individually at the species level. Instead of the red band, the integration of the blue or the green band in WVI increases their power to discriminate submerged aquatic vegetation (SAV), particularly the water adjusted vegetation index (WAVI), water enhanced vegetation index (WEVI), and water transformed difference vegetation index (WTDVI). These results corroborate the spectral and the CRRS analyses. However, despite the power of blue wavelength to penetrate deeper into the water, it also leads to a relative overestimation of dense SAV coverage due to more scattering in this part of the spectrum. Furthermore, statistical fits (<span class="inline-formula"><i>p</i><i><</i>0.05</span>) between the reflectance in the respective VNIR bands of MSI and OLI revealed excellent linear relationships (<span class="inline-formula"><i>R</i><sup>2</sup></span> of 0.999) with insignificant root mean square difference (RMSD) (<span class="inline-formula">â¤</span>â0.0015). Important agreement (0.63â<span class="inline-formula">â¤</span>â<span class="inline-formula"><i>R</i><sup>2</sup></span>â<span class="inline-formula">â¤</span>â0.96) was also obtained between respective WVI regardless of the integrated spectral bands (i.e., red, green, and blue), yielding insignificant RMSD (<span class="inline-formula">â¤</span>â0.01). Accordingly, these results pointed out that MSI and OLI sensors are spectrally similar, and their data can be used jointly to monitor accurately the spatial distribution of SAV and its dynamic in time and space in shallow marine environments, provided that rigorous data pre-processing issues are addressed.
Highlights
Seagrass meadows are identified as an important key for the characterization of environmental resources in estuarine and shallow coastal areas, and a fundamental health index allowing the assessment of coastal ecosystems
The reflectance signatures of seagrass and algae samples are similar to those of healthy vegetation canopy. These reflectance signatures exhibit slight absorption features near 450 nm and others stronger between 650 and 700 nm with a minimum at 670 nm caused by the chlorophyll; as well as a significant reflection between 520 and 600 nm due to carotenoid pigments and high reflectance in the NIR attributed to internal tissue structure (700 to 900 nm)
The Multi-Spectral Instruments (MSI) sensors onboard Sentinel satellites 2A/2B and the Operational Land Imager (OLI) instruments installed on Landsat 8/9 satellites are designed to be similar in the perspective that their data be used together to support global Earth surface reflectances coverage for science and development applications at medium spatial resolution and near-daily temporal resolution
Summary
Seagrass meadows are identified as an important key for the characterization of environmental resources in estuarine and shallow coastal areas, and a fundamental health index allowing the assessment of coastal ecosystems. The composition and density of their species depend largely on water depth, temperature, salinity, coastal substrate material, and light penetration (Dierssen et al, 2015). Adapted to grow in shallow seawater down to a depth of 20 m, where approximately only 11% of surface light reaches the bottom (Duarte and Gattuso, 2008), they play an essential role in the sustainability of global ecosystem biodiversity in most shallow near-shore areas around the world Seagrass beds play an important role in carbon storage (Novak and Short, 2020), as well as effective removal of carbon dioxide from the “biosphere-atmosphere” system, which significantly mitigates the climate change impacts (Duarte et al, 2013; Lyimo, 2016).
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