Australian researchers have found Envisat's MERIS sensor candetect coral bleaching down to ten metres deep. This means Envisatcould potentially monitor impacted coral reefs worldwide on atwice-weekly basis.
Coral bleaching happens when symbioticalgae living in symbiosis with living coral polyps (and providing themtheir distinctive colours) are expelled. The whitening coral may diewith subsequent impacts on the reef ecosystem, and thus fisheries,regional tourism and coastal protection. Coral bleaching is linked tosea temperatures above normal summer maxima and to solar radiation.Bleaching may take place on localised and mass scales – there was anextensive bleaching event in 1998 and 2002 likely linked to El Niñoevents.
"An increase in frequency of coral bleaching may be oneof the first tangible environmental effects of global warming," statesDr. Arnold Dekker of Australia's Commonwealth Scientific and IndustrialResearch Organisation’s (CSIRO) Wealth from Oceans Flagshipprogram."The concern is that coral reefs might pass a criticalbleaching threshold beyond which they are unable to regenerate."
Aerialor boat-based observation is the current method of detecting bleaching,but many reefs are either inaccessible or simply too large (the GreatBarrier Reef has an area of 350 000 square kilometres) for an eventthat happens within a fortnight. Bleached corals may rapidly becolonised by blue-green to brown algae, more difficult to distinguishfrom live coral.
Repetitive, objective and broad-scale satellitecoverage is the alternative. At this week's MERIS/AATSR Workshop inFrascati, Italy, the CSIRO team presented initial results usingEnvisat's Medium Resolution Imaging Spectrometer (MERIS). MERISacquires images in 15 different spectral bands at 300 m resolution.
"Coralbleaching needs to be mapped at the global scale," Dekker adds."High-spatial resolution satellites can only do it on a few reefs dueto cost and coverage constraints. We need a system that has appropriatecoverage and revisit frequency, with a sufficient amount of spectralbands and sensitivity. There is no more suitable system than MERIS."
Theteam studied Heron Island reef at the southern end of the Great BarrierReef, site of a University of Queensland research station. ValidatingMERIS Full Resolution mode results, they found that observed changes inlive coral cover were correlated to an existing bleaching event.
Theoreticalstudies indicate that for each complete 300-metre pixel of coral underone metre of water it is possible to detect a 2% bleaching of livecoral. MERIS should remain sensitive to detecting from 7-8% bleachedcoral even under ten metres of water.
"MERIS Full Resolutioncovers the world every three days, a bottleneck for global monitoringcould be data processing," Dekker concludes. "However satellite sensorsmeasuring sea surface temperature such as Envisat's Advanced AlongTrack Scanning Radiometer (AATSR) can be applied to prioritise reefsthat are subject to sea temperature heating anomalies-thus focusing theMERIS based bleaching detection.
Australia's Great Barrier ReefMarine Park Authority has expressed interest in the project. Australianscientists plan to progress to perform MERIS monitoring of bleachingevents up to the scale of the whole Great Barrier Reef.
Cite This Page: