Bottom Reflectance in Ocean Color Satellite Remote Sensing for Coral Reef Environments

Abstract

Most ocean color algorithms are designed for optically deep waters, where the seafloor has little or no effect on remote sensing reflectance. This can lead to inaccurate retrievals of inherent optical properties (IOPs) in optically shallow water environments. Here, we investigate in situ hyperspectral bottom reflectance signatures and their separability for coral reef waters, when observed at the spectral resolutions of MODIS and SeaWiFS sensors. We use radiative transfer modeling to calculate the effects of bottom reflectance on the remote sensing reflectance signal, and assess detectability and discrimination of common coral reef bottom classes by clustering modeled remote sensing reflectance signals. We assess 8280 scenarios, including four IOPs, 23 depths and 45 bottom classes at MODIS and SeaWiFS bands. Our results show: (i) no significant contamination (R rscorr < 0.0005) of bottom reflectance on the spectrally-averaged remote sensing reflectance signal at depths >17 m for MODIS and >19 m for SeaWiFS for the brightest spectral reflectance substrate (light sand) in clear reef waters; and (ii) bottom cover classes can be combined into two distinct groups, "light" and "dark", based on the modeled surface reflectance signals. This study establishes that it is possible to efficiently improve parameterization of bottom reflectance and water-column IOP retrievals in shallow water ocean color models for coral reef environments.

Spectral Reflectance of Palauan Reef-Building Coral with Different Symbionts in Response to Elevated Temperature

“…The similarity of the reflectance spectra from distinct host species and symbionts further supports past remote sensing studies employing inversion of a representative endmember (where an optical model is applied to field reflectance data) to identify coral and benthic constituents [1][2][3][4]71,72]. The relationship between integrated reflectance and symbiont concentration may prove useful to assess or monitor reef condition and the potential for bleaching.…”

Turbidity in Apalachicola Bay, Florida from Landsat 5 TM and Field Data: Seasonal Patterns and Response to Extreme Events

“…Both Cat Point and Dry Bar stations are located on the hard bottom substrate (with no vegetation) of productive oyster bars [72]. The large uncertainties in reflectance measurements in relatively clearer water scenarios indicated the possible contamination due to the variability in bottom reflectance at each pixel [73].…”

Parameterization of bottom spectral reflectance for shallow water ocean color inversion models

“…Very clear waters often characterize coral reef environments, thus light reflectance of the bottom becomes an issue. However this is often not the case in coastal systems that are relatively turbid (Reichstetter et al, 2015a).…”

“…Carbonate concentrations vary from approximately 20% near the coast to more than 80% on the middle and outer shelves (Mathews et al, 2007). Carbonate sand is generally considered the brightest substrate (Reichstetter et al, 2015a) and therefore, has the greatest influence on the accuracy of IOP retrievals in shallow water environments.…”

“…The current implementation of SWIM for the Great Barrier Reef (GBR), Australia, has been developed with the requirement of two specific regional input datasets, bathymetry and benthic albedo: Reliable bathymetry data at 100 m spatial resolution are available over the full extent of the GBR (Beaman, 2012). Prior to this study, an existing benthic biodiversity database (Pitcher, 2007) was used to derive the bottom reflectance signatures for a simple two-component "light" and "dark" reflectance map (Reichstetter et al, 2015b).…”