Candace M. Newman

Progress in the use of remote sensing for coral reef biodiversity studies

Coral reefs are hotspots of marine biodiversity, and their global decline is a threat to our natural heritage. Conservation management of these precious ecosystems relies on accurate and up-to-date information about ecosystem health and the distribution of species and habitats, but such information can be costly to gather and interpret in the field. Remote sensing has proven capable of collecting information on geomorphologic zones and substrate types for coral reef environments, and is cost-effective when information is needed for large areas. Remote sensing-based mapping of coral habitat variables known to influence biodiversity has only recently been undertaken and new sensors and improved data processing show great potential in this area. This paper reviews coral reef biodiversity, the influence of habitat variables on its local spatial distribution, and the potential for remote sensing to produce maps of these habitat variables, thus indirectly mapping coral reef biodiversity and fulfilling information needs of coral reef managers.

Simple and effective monitoring of historic changes in nearshore environments using the free archive of Landsat imagery

The recently released archive of Landsat imagery can be used to detect historic changes in nearshore environments. We used a series of free Landsat images spanning the years from 1984 to 2009 to detect changes in the spatial extent of dominant substrate types, coral, algae, and seagrass, around Bawe and Chumbe islands in Zanzibar, and we compared the use of true-colour composites and supervised classifications. Results indicate temporal changes in the spatial extent of seagrass meadows are easily mapped with Landsat imagery, whereas temporal changes in algae cover and particularly coral cover pose greater challenges because of the similarities in spectral reflectance properties between the relevant substrate types. Supervised classification requires substantially more processing than the simple display of true-colour composites, but does not improve interpretation in our study. We suggest that historic Landsat imagery, obtained at no cost and processed minimally with free software, is the best available data source for studies of historic changes in the nearshore environments of East Africa.

Assessing the effect of management zonation on live coral cover using multi-date IKONOS satellite imagery

Change in live coral cover within the nine different management zones of the Marine Protected Area (MPA) around Bunaken Island, Indonesia, was estimated using IKONOS satellite image data for 2001 and 2004. For both years, field data were used to classify image pixels on a semi-quantitative scale based mainly on coral cover. The resulting substrate maps had overall classification accuracies of 78% and 81%, respectively. Change was estimated for each zone using a post-classification comparison. Results indicated an increase in live coral cover in one zone, a decrease in two other zones, and no change in the remaining six zones. The results show no obvious influence of the MPAs zonation plan on changes in live coral cover in the three year period investigated, and the study demonstrates the use of remote sensing to determine change in live coral cover, to assess zonation effectiveness, and to guide management efforts.

Assessment of Beer's law of logarithmic attenuation for remote sensing of shallow tropical waters

Remote sensing of coral reef environments may provide ecosystem managers with an effective monitoring tool, however, it is currently limited by some unexplained changes to the signal received by the sensor. Existing water column correction techniques attempt to account for signal modification by using Beers law of logarithmic decay to account for attenuation of downwelling irradiance. Our objective for this study is to use downwelling irradiance values, collected in situ, to statistically evaluate Beers law and suggest the appropriateness of applying a constant value of attenuation to satellite or airborne imagery. Findings suggest different bottom types and depths may modify attenuation coefficients differently. As a result, Beers law of logarithmic decay is not appropriate for water column correction techniques for shallow water environments over bright surfaces.

Assessing the uncertainty of radiometric properties in coral reef environments

The spectral information within a single pixel from a satellite image of a coral reef environment can theoretically reveal information on primary and/or secondary environmental factors. That which is of most interest within an image to reef managers and remote sensing scientists is coral species or genera identification, and coral spatial distribution and health. Attempts to map these dimensions, however, have slowed due to the inherent uncertainty of interpreting spectral signatures, which are a function of not only single environmental factors, but as well, the dynamism of their interactions within an aquatic system. What remains uncertain in identifying corals within an image is the precision with which they can be differentiated from submerged features with similar reflectance properties. In attempt to address this uncertainty we revisit spectral derivative analysis of signatures collected within the tidal range of Bunaken Island, Indonesia. Through comparisons of five derivative equations our goal is to identify the one that reveals the greatest potential for spectral discrimination of narrowly defined categories. Results indicate that the second and third spectral derivative equations of smoothed data best preserve reflectance change with wavelength and allow for discrimination between features within narrow wavelength ranges.

Statistical characterization of the diffuse attenuation coefficient over tropical corals

Remote-sensing of submerged features can help to monitor coral reef ecosystems by monitoring spectral signatures within the visible spectrum. A satellite or airborne image could be used to delineate those areas of coral stress requiring immediate attention. One limitation, however, to the accuracy of this technique is that the water column over a submerged coral reef modifies the signal received by the sensor. The authors report on a study in Fiji were measured profiles of the diffuse attenuation coefficient are used to better understand the properties of the water column over various substrates. Downwelling irradiance and upwelling radiance in SeaWiFS channels were collected over coral reefs, debris surface, sand channels, and blue water with depths far beyond the range of the 70 metre cable. Principal components analysis of vertical water column profiles reveals that irradiance values can be used to distinguish between different bottom types based on the magnitude of component loadings. The technique provides consistent output when discriminating between bottom types at various depths. These results may be used to better define algorithms and are encouraging with respect to water column correction for image interpretation.