Increased spectral resolution enhances coral detection under varying water conditions

Abstract

Earth observation offers effective spatial and temporal coverage to monitor coral reefs in addition to in situ monitoring. Effective monitoring requires that significant substratum features are detectable by a sensor. This detectability is a function of the sensor spectral resolution, the depth and composition of the water column and the spectral characteristics of the substratum. Most broadband multispectral satellite sensors are ineffective in resolving reef substrata at depth due to a lack of spectral specificity. The aim of this simulation study was to quantify the level to which substrata can be classified by sensors with variable spectral resolutions over a range of depths and water qualities and also to improve and quantify the definition of substratum detectability (the measure to which a substratum can be resolved from the water column) and substratum separability (the measure to which a substratum pair can be resolved from each other and from the water column). Three sensors were selected, representing hyperspectral data (CASI with 30 spectral bands) and multispectral data (WorldView-2 with 8 bands, and QuickBird with 4 bands). Spectral separability of substratum reflectance spectra (convolved to the spectral resolution of the three sensors) were compared within two contrasting water columns (reef-oceanic and coastal) over a range of water column depths. Metrics for substratum detectability and substratum separability were determined. As spectral resolution increases from QB to WV2 to CASI, end-members can be resolved to greater depths (e.g. from two to six meters in a coastal water column). The additional three spectral bands in the visible part of the spectrum of the WV2 sensor, as compared to QB, increase the applicability of multispectral sensors to systematic coral reef remote sensing. Increase in water column attenuation, due to higher concentrations of water column constituents, causes loss of substratum detectability and substratum separability. This effect can be partly compensated for by increased spectral resolution. For example, although the CASI and WV2 sensor performed comparably in a very shallow coastal water column (e.g. at 2m depth), the higher spectral resolution of the CASI sensor enhanced spectral separability, resulting in higher substratum separability in deeper water than WV2. With more spectral bands, more substratum end-member reflectances are distinct from the water column signal to greater depth. This implies that higher spectral resolution will enhance bathymetry retrieval, especially within coastal waters. The quantitative framework of this study extends findings of previous contextual coral reef substratum mapping studies. It confirms that higher spectral resolution (i.e. WV2 and CASI) earth observation data significantly enhances coral reef classification capability to increased depths or to the same depth in a more turbid water column. The conclusions of this study can also be extended to coastal ecosystems.