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Introduction

An important contribution to coral reef research is to improve spectral distinction between various health states of coral species in areas subject to harmful anthropogenic activity and climate change. New insights into radiative transfer properties of corals under healthy and stressed conditions can advance understanding of ecological processes on reefs and allow better assessments of the impacts of large-scale bleaching and disease events. The goal of the proposed research is to quantify the relationship between the coral reef ecosystem optical properties and coral reef ecosystem health using new remote sensing capabilities. Our objective is to examine the spectral and spatial properties of hyperspectral sensors that may be used to remotely sense changes in reef community health. In situ reef environment spectra (healthy coral, stressed coral, dead coral, algae, and sand) are input into our coral radiative transfer (RT) model (currently under development) to distinguish important spectral characteristics of corals. The RT model is being developed to identify the optical properties indicative of stress exhibited by coral that could lead towards better ecological assessment, extent, and forecasting of detrimental health conditions of the reef environment. Further, the RT modeling will be linked to remote sensing data to better predict coral reef health in remote areas where only remote sensing data is available.

Study Site

Figure 1. 2000 IKONOS image of Andros Island, Bahamas and Long Rock study site.

In situ spectra were collected in July and August 2002 at the Long Rock site in the Andros Island, Bahamas coastal zone coral reef (Figure 1). Fieldwork was performed in coordination with the Navy's Atlantic Undersea Test and Evaluation Center (AUTEC) on Andros Island at the Long Rock study site where University of Miami investigators have extensive ground-truth data and hyperspectral data collected with the Portable Hyperspectral Imager for Low Light Spectroscopy (PHILLS) sensor. In situ spectra collection (Figure 2) included spectra of healthy, unhealthy, and dead coral, substrate (sand), seagrass, and algae collected by a hand-held portable spectroradiometer (GER 1500) incased in underwater housing (Underwater Video Vault). Additionally, spectral measurements over a range of water depths (Figure 3), relief, and bottom types are compared to help quantify bottom-water column influences. Downwelling irradiance and upwelling radiance were measured simultaneously by a Tethered Spectral Radiometer Buoy (TSRB) instrument and water column optical properties were measured using the AC-9 instrument. Further, digital camera and video camera images were collected.

Our primary emphasis is on Acropora palmata (or elkhorn coral, Figure 4), a major reef building coral, which is prevalent in the study area, but is suffering from white band disease (Figure 5). A. palmata is currently being proposed as an endangered species because its populations have severely declined in many areas of the Caribbean. In addition to the A. palmata colonies, we have collected spectra of at least seven other coral colonies that exist within the study area, each with different coral community composition, density of corals, relief, and size of corals. We collected nearly 1,300 spectra including species spectral library and target mixtures at various depths. Coral spectral reflectance will be input into the RT model, to provide the absorption spectrum for distinguishing spectral characteristics of coral health.

In situ measurements address basic questions about the interactions of downwelling radiation with coral reef substrates as well as provide the necessary ground-truth data to calibrate airborne hyperspectral images. We focused on both small-scale (cm) species-specific reflectance measurements (spectral library) as well as intermediate scale (meter) reflectance measurements at distance from target for mixed spectra of entire micro-habitats. All field aspects included optical properties (AC-9 instrument) (Figure 6) and light field parameters (TSRB instrument) (Figure 7) so that in situ measurements of coral reef benthos can be used to test algorithms that correct for water column and atmospheric properties.

Three coral reef habitats were examined: shallow reef crest (<3m water depth) dominated by A. palmata, intermediate depth fore reef zone (7-12 m water depth) dominated by Montastraea annularis, and deep (20-30 m) marginal reef dominated by Montasraea franksi.