Use of in situ and airborne reflectance for scaling-up spectral discrimination of coral reef macroalgae from species to communities

Abstract

In principle, a priori knowledge of organism-scale spectral signatures for key ecological end-members is a basic requirement for identifying coral reef benthic communities using hyperspectral remotely-sensed imagery. Spectral signatures of end-members are now relatively well known for predominant reef taxa (coral, algae) and for the background of the living communities (e.g. sediments). What remains unclear is whether the criteria for spectral discrimination between endmembers at the millimeter or centimeter scale remain valid when attempting to process images at several meters resolution. In other words, is it possible to scale-up spectral criteria of identification from species/organisms to communities? We address this issue with in situ and 'compact airborne spectrometer imager' (CASI) hyperspectral measurements of the tropical marine flora of 2 South Pacific Ocean coral reefs. Targets were the dominant algal species and communities encountered in the shallow (0 to 3 m) barrier and fringing reefs of Moorea Island and the outer margin of the rim of Rangiroa Atoll (French Polynesia). Stepwise wavelength selection and linear discriminant analysis highlighted the key non-redundant wavelengths necessary to achieve good separation between the predefined ecological groups. Comparison of the wavelengths identified from in situ and airborne measurements allowed definition of a subset of common wavelengths that were robust to changes in spatial scale and still provided excellent discrimination and classification accuracy between the ecological groups. These results suggest that continuous spectral signatures acquired in situ at the centimeter scale can be used to select key discrete wavelengths for remote-sensing observations of communities at the meter scale despite the spatial heterogeneity in benthic cover and the resulting spectral mixing.

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.…”

“…The relationship between integrated reflectance and symbiont concentration may prove useful to assess or monitor reef condition and the potential for bleaching. Such algorithms may be implemented using hyperspectral sensors from airborne platforms like the Portable Remote Imaging Spectrometer (PRISM) [1,38], proposed future satellite sensors, and through the use of underwater hyperspectral imagers [32,33,47,48]. However, the implementation of the algorithms will be complicated by the effects of the intervening water column, sea-surface, and atmosphere, as well as the spectral and spatial resolution of the sensor itself [24,35].…”

“…Indeed, a variety of models and applications have been used to assess coral reefs from satellite and airborne multispectral imagery [1][2][3][4]. However, the concept of a "species" of coral is not quite accurate when considering spectral reflectance characteristics.…”

Mapping Coral Reef Benthos, Substrates, and Bathymetry, Using Compact Airborne Spectrographic Imager (CASI) Data

“…High spatial and spectral resolution remote sensing instruments can theoretically detect the subtle spectral differences between the different coral reef benthos and substrate types [5][6][7]. The Compact Airborne Spectrographic Imager (CASI) is a high spatial resolution hyperspectral airborne sensor, that has been used to map coral reef environments using both unsupervised [8] and supervised [9][10][11] classification approaches.…”

Ground-Level Classification of a Coral Reef Using a Hyperspectral Camera

“…Most of them have focused on hyperspectral data that benefit from the ability to detect unique spectral features [27][28][29][30][31]. Hyperspectral resolution, defined as imagery with tens to hundreds of bands, requires a different analytical approach to multispectral imagery or digital photography.…”