Satellites & Bleaching Bleaching Threshold

The "bleaching threshold" concept is crucial for understanding the Coral Reef Watch products, and we find that it is often misunderstood. So this whole section will focus on a careful definition of this concept, before we move on to more data products.

You will remember from the coral bleaching section that corals are very sensitive to warm temperatures. So to look for areas at risk for bleaching, we would start by looking for places that are warmer than normal – places with a positive SST anomaly. But it turns out that we can be even more specific than that. Scientists have shown that corals start to become stressed when the SST is 1°C warmer than the highest monthly mean temperature (Glynn and D’Croz, 1990). Let’s look again at one of the Virtual Station time-series graphs to see what this “maximum monthly mean” (MMM) concept means.

The light-blue crosses represents the mean temperature for each month. Now we need to look for the warmest month in the seasonal cycle; for the Seychelles, the mean temperature reaches a maximum of 29.5°C in April. So for this coral reef site, the maximum monthly mean is 29.5°C (shown as a light-blue horizontal dashed line). One degree above the maximum monthly mean is called the “bleaching threshold” temperature. The bleaching threshold is shown on our virtual station time-series graphs as a solid light blue horizontal line:

When the sea surface temperature is warmer than the bleaching threshold temperature, the corals will experience thermal stress. This thermal stress is the main cause of mass coral bleaching.

Try our hands-on Bleaching Threshold exercise to learn more.

(top)