Spectral discrimination of coral reef bottom types

Degradation of coral reefs is a major environmental problem worldwide. There is a strong management need for cost-effective assessment of environmental health and reef conditions over large regions in remote areas. Remote sensing could be an ideal tool for monitoring coral reefs and related ecosystems in cases where different coral reef substrates are spectrally resolvable. The aim of the present paper is to study optical properties of different coral reef substrates and to estimate how much remote sensing can provide for monitoring of coral reef health. Satellite sensors such as Landsat MSS and TK as well as SPOT, have been used successfully in mapping reef extent and geomorphological zones. Hyperspectral airborne instruments have been successfully used for mapping of submerged coastal vegetation and corals. It is obvious that sensors with low spatial and/or spectral resolution are only capable of mapping coral reefs in general. Increased resolution enables the mapping of reef geomorphological zones and better resolution allows mapping of reef sub-zones or habitat. Assessing the biological health of a coral reef requires at least a capability to distinguish living coral (with symbiotic algae, zooxanthellae) from dead coral covered by overgrowth of benthic algae. The problem is that optical properties of most of coral reef substrates are determined by phytoplankton pigments (mainly chlorophyll a). Symbiotic algae and not the coral polyp tissue mainly cause the colour of living hard and soft corals. Dead corals, rubble and even sand particles are covered with a thin layer of algae. Mud contains different algae. Separating the above mentioned substrates from each other and from red, green and blue-green algae (cyanobacteria) and seagrasses is only possible if we are able to detect the influence of accessory pigments typical to each algae class. The problem is that the spectral information required to resolve different substrates has to be obtained with very good spectral resolution and through water column of variable depth and optical quality.