White Band Disease on Acropora palmata (Elkhorn Coral) Coral Hydrocoral:  Millipora species (Fire Coral) Diploria species (Brain Coral) with White Plague Disease. Gorgonian species (sea fan). Nassau Grouper (Epinephelus striatus)
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Clues to Coral Reef Health: Spectral Analysis and Radiative Transfer Modeling of Coral Reef Ecosystem Health


Liane Guild1, Barry Ganapol2, Philip Kramer3, Roy Armstrong4, Art Gleason3, Juan Torres4,
Lee Johnson5, Toby Garfield6, and Brian So7

1NASA Ames Research Center, 2University of Arizona, 3University of Miami, 4University of Puerto Rico, 5California State University Monterey Bay, 6San Francisco State University, and 7Lowell High School, San Francisco

 Introduction | Study Site |Radiative Transfer Modeling | Operating version of coral RT model |Next Phase of RT Modeling | Remote Sensing | Publications and Press Releases | Related Links | Contact



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.

  • Coral reefs are important for ocean productivity and C cycling and provide coastline protection and nursery grounds for marine fisheries.
  • World's reefs have shrunk to 1/2 - 1/10 of original size due to climate and human impacts.
  • Coral reefs respond immediately to environmental changes and are considered canaries of the ocean.
Goal

-Quantify the relationship between the coral reef ecosystem optical properties and coral reef health.

-Optimize sensor capabilities to remotely sense coral reef health.


Objective 1

Determine representative spectra of healthy, diseased, and dead coral, and dominant algal species at the level of airborne sensors.

 

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Study Site


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

Figure 2. Collection of in situ spectra using the portable spectroradiometer.

Figure 2. Collection of in situ spectra using the portable spectroradiometer.

 

Figure 3. Collection of spectra at depth. Spectralon panel at bottom is used as a reference standard.

Figure 3. Collection of spectra at depth. Spectralon panel at bottom is used as a reference standard.

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.

Figure 4. Acropora palmata with white band disease.

Figure 4. Acropora palmata with white band disease.

[Figure 5. White band disease on Acropora palmata.]

Figure 5. White band disease on Acropora palmata.

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.

 

Figure 6. AC-9 instrument.

Figure 6. AC-9 instrument.

Figure 7. TSRB instrument.

Figure 7. TSRB instrument.

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Radiative Transfer (RT) Modeling

 

 

Airborne reflectance spectra are composites of reflectance:

  • atmosphere
  • sea surface
  • water column
  • the bottom (sediments, seagrass, coral).

Airborne reflectance spectra are composites of reflectance:atmospheresea surfacewater columnthe bottom (sediments, seagrass, coral).

 

 

RT modeling comprises light scattering in the coral reef community including scattering in the water and atmosphere.

 

RT modeling comprises light scattering in the coral reef community including scattering in the water and atmosphere.

 

 

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Operating version of coral RT model:

Coral spectral reflectance, calculated from in situ coral spectra, is input into a RT model, CorMOD2 (CM2) (Figure 8), based on a leaf radiative transfer model.


In CM2, input coral reflectance measurements produce modeled reflectance through an inversion at each visible wavelength to provide the absorption spectrum (Figure 9).


Initially, we imposed a scattering baseline that is the same regardless of the coral condition and that coral is optically thick and no light is transmitted through coral.
 

Figure 8. CorMOD2 coral specific RT model.

Figure 8. CorMOD2 coral specific RT model.

 

 

Figure 9. Absorption profiles output from the coral radiative transfer model.

Figure 9. Absorption profiles output from the coral radiative transfer model. Healthy Acropora palmata and A. palmata with white band disease (WBD) profiles are distinct.

Phase 2 for coral RT model: Is this spectral distinction of coral health apparent above the water column and atmosphere? We will integrate spectra over hyperspectral sensor channels.

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Next phase of RT modeling

• Scattering profile is specified from transmission measurements from coral surfaces.
• Absorption profile is constructed from individual absorbing coral components and coral biochemical concentrations.
• RT between coral surfaces characterized by non-rotationally invariant photon transport.
• Need orientation of coral surfaces (CAD) like a leaf angle distribution.
• Need a coral surface area per unit bottom area (CAI) comparable to a leaf area index.
• Determine the appropriate substrate and diffuse photon source from the water column.

 

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Remote Sensing

Objective 2

Investigate the appropriate spatial scale for detecting coral features and health characteristics in hyperspectral data.

 

Figure 10. Portable Hyperspectral Imager for Low Light Spectroscopy (PHILLS) data.  Band Combination 54 (647.5 nm), 37 (565.9 nm), 12 (445.3 nm).
Figure 10. Band Combination 54 (647.5 nm), 37 (565.9 nm), 12 (445.3 nm).
Portable Hyperspectral Imager for Low Light Spectroscopy (PHILLS) data is under investigation for distinction of coral reef features and variation in spectral characteristics for healthy and unhealthy coral. The PHILLS sensor has 128 channels with 65 bands in the visible range


Spatial scale degradation of hyperspectral data will be applied for assessment of sensor resolution and distinction of reef features.

 

Barracuda Diploria clivosa (Knobby Brain Coral) Lobster Gorgonian with Flamingo Tongue Parot fish Fire Coral

 

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Meeting Abstracts
Guild, Liane, Toby Garfield, and Barry Ganapol, Lee Johnson, Roy Armstrong, and Philip Kramer, 2002, “Clues to Coral Reef Health: Spatial and Spectral Remote Sensing Detail”, International Conference on Remote Sensing of Marine and Coastal Environments, Miami, May 2002.

Guild, Liane, Barry Ganapol, Philip Kramer, Roy Armstrong, Art Gleason, Juan Torres, Lee Johnson, and Toby Garfield, Clues to coral reef health: integrating radiative transfer modeling and hyperspectral data, Eos. Trans. AGU 83(47), Fall Meet. Suppl., Abstract OS71A-0264, 2002.

Press Release
Guild, L., 2002. NASA Devising Method to Remotely Monitor Ocean Environment, NASA News Release: 02-127AR, Dec. 6.

Project Funding: 2002-03 NASA Ames Research Center Directors Discretionary Fund

Photos by: Juan Torres, Art Gleason, Liane Guild.

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Links:

NASA Ames Press Release: NASA devising method to remotely monitor ocean environment.

2003 Deep Coral field trip to USVI and Puerto Rico - San Juan Star Press Release: includes two picture!

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Responsible Official:
Liane Guild, Ph.D. lguild@mail.arc.nasa.gov
MS 242-4, NASA Ames Research Center
Moffett Field, CA 94035 USA

Web Design:
Brian So

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Page last updated: July 18, 2003.