(Daily experimental product, version 2.0)
Mass coral bleaching is a phenomenon driven by environmental stress. Currently, all satellite products used to monitor stress related to mass coral bleaching events are based solely on sea surface temperature (SST). Whereas temperature is clearly the main environmental driver of this phenomenon, it is actually the stress caused by excessive light that leads to bleaching. The link to temperature is via increased ambient temperature which reduces the tolerance level of corals to stress caused by light. So if the stress from light is already high, the addition of thermal stress can trigger bleaching.
NOAA Coral Reef Watch has developed a new experimental product that is based on coral physiology. This new product suite (originally released in October 2012; now in version 2.0) combines satellite measurements of light (or Photosynthetically Active Radiation (PAR); see bottom left image above) and SST (see bottom right image above) to monitor aspects of the health of the coral photosystem in terms of light and temperature stress.
Corals acclimate to light within a few weeks (the satellite product described here uses an acclimation rate of 6 days for half-acclimation). Since the acclimation is not instantaneous, quick changes in light levels from one day to the next will result in stress due to an excess of light if the light levels jumped from cloudy conditions one day to sunny conditions the next, or the removal of light stress (i.e. the ability for repair) if the reverse occurred. Since the effects of stress are cumulative, over the course of a year, the coral photosystem ranges from being far from stressed in the winter months, to being right on the threshold of too much stress in the summer. However, not all yearly cycles are the same and light stress levels differ from one summer to another.
The closer corals are to their stress threshold due to light, the more sensitive they are to other stresses such as thermal stress caused by elevated temperatures. In the summer, corals are therefore more susceptible to thermal stress than they are in the winter. This is also true of many other environmental stresses.
The Light Stress Damage (LSD) product suite tracks the effect of light stress from day to day. When thermal stress occurs, this is then added to the current levels of stress from light and the sum is compared to the stress threshold. If the total is above the threshold then the product returns a positive value; if it is below the threshold, it returns a negative value.
This is called the Daily Light Stress Damage value and is plotted as an image with values ranging from -150 to 50 mol quanta (see second top image above: Daily Light Stress Damage). During a stress event, positive Daily Light Stress Damage values are accumulated and plotted as an LSD Index image with values ranging from 0 to 400 mol quanta (see top image above: LSD Index). Once the LSD Index climbs above 0, it is expected that corals will begin to pale. As the value continues to increase, corals will eventually bleach. The exact value of the LSD Index for bleaching is yet to be determined; however, anecdotal evidence suggests that it will be somewhere around 200.
This experimental Light Stress Damage product was developed by NOAA Coral Reef Watch in close collaboration with the Universidad Nacional Autónoma de México, the University of Queensland, and the University of Exeter, with support from the Bleaching Working Group and Remote Sensing Working Group of the Coral Reef Targeted Research (CRTR) Program, the Australian Research Council, and the NOAA Coral Reef Conservation Program.