In a previous post we demonstrated how dendrograms can be used as effective tools for investigating the similarities and differences of reflectance spectra. Here we expand on our earlier discussion, and explore how dendrograms can be used to illustrate the decreasing separability of coral reef spectra with increasing water depth.
Coral reefs are known for their exceptional biodiversity, containing a complex array of mobile and sessile organisms. From a remote sensing perspective, however, coral reefs can be particularly challenging study areas, mostly due to the confounding effects of the overlying water column. Varying water depth and varying water properties can both contribute significant complexity to the interpretation and identification of features on the sea floor.
Given this complexity, it is therefore necessary in remote sensing to simplify coral reef ecosystems into a collection of generalized components, each representing a unique compilation of species and/or substrate types. When grouping species for analysis, and when interpreting image classification output, it is important to understand the spectral similarity – or dissimilarity – of different image features.
As an example, let’s first consider an example where reef habitat composition is represented by four fundamental components: coral, sponge, sand and submerged aquatic vegetation (SAV). As shown in our previous post, the average in situ spectra of these four components exhibit unique reflectance characteristics and can be readily differentiated at 0.1 spectral angle. However, with increasing water depth (approximated here using a semi-analytic model for clear tropical water) the separability of these components decreases. At 3 m water depth it becomes more difficult to differentiate sand from SAV, and coral from sponge; and at 10 m water depth analysis is essentially reduced to a two-component systems: sand versus coral, sponge and SAV.
Let’s now compare and contrast the above results with the same analysis applied to all of the individual spectra used to create these component averages. This includes measurements from 24 coral species, 10 sponge species, 3 SAV species and areas of sand. As shown below, many individual species (and in some cases small groups of species) can be readily differentiated when the overlying water column is not considered. However, when the effects of the water column are included, the ability to distinguish individual species diminishes significantly with increasing water depth.
While the relationship between water depth and spectral similarity is to be expected, what is particularly informative from these dendrograms is the ability to discern which species group together at different depths. For example, note that coral species do not always group with other coral species, but are observed to also group with both sponges and SAV. Additionally, because spectra do not necessarily group according to type, it becomes apparent that three spectral groups can be reasonably differentiated at 10 m rather than just two groups as suggested from the analysis using just averages for each component.
Such information can be immensely valuable for guiding image analysis, as well as aiding the interpretation of results. So if you’re working on remote sensing of coral reefs, it’s worth exploring the spectral characteristics of the dominant species in your study area, and investigating how spectral similarity changes with water depth.
Acknowledgement: Spectral data used in the above examples were collected using a GER-1500 spectrometer by M. Lucas at the University of Puerto Rico at Mayaguez for a NASA EPSCoR sponsored research project on the biodiversity of coastal and terrestrial ecosystems.