Abstract
An autonomous surface vehicle instrumented with optical and acoustical sensors was deployed in Kane'ohe Bay, HI, U.S.A., to provide high-resolution, in situ observations of coral reef reflectance with minimal human presence. The data represented a wide range in bottom type, water depth, and illumination and supported more thorough investigations of remote sensing methods for identifying and mapping shallow reef features. The in situ data were used to compute spectral bottom reflectance and remote sensing reflectance R_(rs,λ) as a function of water depth and benthic features. The signals were used to distinguish between live coral and uncolonized sediment within the depth range of the measurements (2.5 m to 5 m). In situ R_(rs,λ) were found to compare well with remotely sensed measurements from an imaging spectrometer, the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS), deployed on an aircraft at high altitude. Cloud cover and in situ sensor orientation were found to have minimal impact on in situ R_(rs,λ), suggesting that valid reflectance data may be collected using autonomous surveys even when atmospheric conditions are not favorable for remote sensing operations. The use of reflectance in the red and near infrared portions of the spectrum, expressed as the red edge height 〖REH〗_λ, was investigated for detecting live aquatic vegetative biomass, including coral symbionts and turf algae. The 〖REH〗_λ signal from live coral was detected in Kane'ohe Bay to a depth of approximately 4 m with in situ measurements. A remote sensing algorithm based on the 〖REH〗_λ signal was defined and applied to AVIRIS imagery of the entire bay and was found to reveal areas of shallow, dense coral and algal cover. The peak wavelength of 〖REH〗_λ decreased with increasing water depth, indicating that a more complete examination of the red edge signal may potentially yield a remote sensing approach to simultaneously estimate vegetative biomass and bathymetry in shallow water.
Highlights
Coral reefs are among the most diverse and productive ecosystems worldwide (Odum and Odum, 1955) and provide a variety of goods and services to many tropical and sub-tropical coastal nations (Spurgeon, 1992; Moberg and Folke, 1999)
The approach consisted of instrumenting a small autonomous surface vessel (ASV) with synchronized optical and acoustical sensors, deploying the system to collect in situ observations in high spatial resolution on and around a small patch reef within Kane’ohe Bay, using the observations to derive shallow benthic properties of ecological importance, including water depth, benthic cover and reflectance, relating these properties to computations of water column reflectance, and comparing the in situ measurements with similar remotely sensed observations corrected for atmospheric effects
In situ measurements of remote sensing reflectance were shown to reasonably match surface reflectance derived from atmospherically corrected measurements from an imaging spectrometer deployed at high altitude
Summary
Coral reefs are among the most diverse and productive ecosystems worldwide (Odum and Odum, 1955) and provide a variety of goods and services to many tropical and sub-tropical coastal nations (Spurgeon, 1992; Moberg and Folke, 1999). For shallow water aquatic applications, data sets that include water optical properties, depth, and benthic cover are needed that span appropriate length scales (e.g., >1,000 m) and with sufficient spatial resolution (e.g.,
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