(Daily, experimental, version 0.7)
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 a coral's ability to tolerate light stress. 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 suite based on coral physiology. Called the Light Stress Damage (LSD) product suite, it was originally released in October 2012 for the Greater Caribbean region. It is now in its seventh experimental iteration (version 0.7) and has been expended to include the eastern Pacific Ocean. The suite is presently comprised of two products: 'LSD Index' (top left image in each regional 2 by 2 image set) and 'Total Daily Stress' (top right image in each set). These products combine satellite measurements of light (or Photosynthetically Active Radiation (PAR); lower left image in each set) and sea surface temperature (SST) (bottom right image in each set) to monitor aspects of coral photosystem health in terms of light and temperature stress.
Corals acclimate to light within a few weeks. (The LSD product suite 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: (1) the addition of stress due to an excess of light if for instance, the light levels jumped from cloudy conditions one day to sunny conditions the next, or (2) 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 stressors, such as temperature. In the summer, corals are therefore more susceptible to stress than they are in the winter.
The 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. These negative values are converted into positive values ranging from 0 to 0.4 and are a measure of the distance below the threshold.
These values represent the Total Daily Stress value and are plotted at the top right image for each of the Caribbean and Pacific. During a stress event, positive Total Daily Stress values are accumulated and plotted as an LSD Index (top left image for each of the Caribbean and Pacific) with values ranging from 0 to >4. Once the LSD Index climbs above 0, it is expected that corals will have begun to pale. As the value continues to increase, corals will begin to bleach. The exact value of the LSD Index for ecologically significant bleaching is yet to be determined; however, anecdotal evidence suggests that it will be somewhere around 1.
The v0.7 LSD product suite was released in September 2016. In this new version, we have maintained the color bar, introduced in September 2015, of the LSD Index image (top left image for each of the Caribbean and Pacific) to help coral reef managers better understand the onset of light/temperature stress, its buildup, and the eventual resultant coral bleaching (and associated mortality). We used a graduation of dark to light blue to signify the beginning and buildup of stress as well as the onset of bleaching. At LSD Index = 1, the color changes to light yellow, signifying that ecologically significant levels of bleaching have been reached. The color then transitions to red at about LSD Index = 3, at which time widespread coral bleaching with some mortality is expected. Finally, the color transitions to pink at about LSD Index = 4, where it is expected that most if not all corals on the reef are bleached and mortality is widespread. Please note that the LSD Index is yet to be calibrated against bleaching observations from the field; however initial indications from locations such as Looe Key National Marine Sanctuary (Florida Keys, 2015) are that the onset of bleaching from light and temperature stress, as described by the LSD Index, may be reasonably accurate.
The experimental Light Stress Damage product suite 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.