Cold water bleaching - Florida 2010

When we think of coral reef bleaching it is understandable for most to only think of warm water bleaching events as these are the ones we typically learn about in the media. A good example of this is the widely reported warm water bleaching that occurred as a result of the 1997/1998 ENSO event. However, this post aims to look at some cold water events that can be equally as disruptive to coral reef ecosystems, looking at studies that consider exposure under the opposite threshold. That threshold is 16 degrees, temperatures below this can cause fast widespread coral death unprecedented in extent and degree of severity. 

in January 2010 the biggest cold-water event occurred in the Florida Reef Tract (FRT), an extreme anomaly that caused rapid mortality of Acropora spp. An interesting paper on this by Lirman et al. aimed to investigate these impacts through conducting reef surveys. The coral mortality patterns they observed were directly related to satellite derived cold water temperatures, and their results showed severity of impacts similar in extent and intensity to global warm water bleaching events. The reason for the event was surface wind anomalies and southward advection of cold arctic air caused by unprecedented negative values of the Northern Atlantic Oscillation (NAO) index. It is important to mention that this isn't an exclusive cold water event for this area, Shinn reported mortality in 1962 where temperatures fell to 13.3 degrees and further reported events occur in 1981, 1969-1970 and 1977-1978, though all of these were more constrained to local levels than the 2010 event.

Figure 1. Oceanic El Niño Index (ONI) from 1986 to 2011 (source: Paz-Garcia et al, 2011)

The study reports that tissue mortality was high (30-40% in studied areas) and 7.8% of colonies sampled in 2010 were severely lacking of live tissue. They further noted that the Upper and Middle Keys were less severe, with the richest most diverse inshore assemblages being the most impacted (despite being well known as resilient). Another interesting finding was that the response of corals to warm water anomalies was a poor predictor of the response of corals to this cold water event, with those most resistant to warm water temperatures the most vulnerable to cold water temperatures. The paper in general provides a decent analysis of the spatial response of coral reef species to this event however it is limited to analysing stony corals and not soft corals and benthic organisms such as sponges.

The take-home message from this study is that cold water bleaching events are equally as destructive as warm water events but affect different coral species that may be less resilient to the cooler conditions.

Figure 2. Bleaching event of Pocillopora colonies in Punta Galeras (source: Paz-Garcia et al. 2011)

Interestingly as I read further articles on cold water bleaching, it appeared a much more frequent occurrence than first expected (see Figure 1). There also occurred a cold water bleaching event that in the Gulf of California in 2011, despite its time and extent being less severe to that of Florida 2010, a study by Paz-Garcia et al. notes that the same significant visual signs could be seen in 85% of the surveyed corals. Linked in to the Lirman study these academics wanted to explore the different responses of Pocillopora morphospecies (figure 2) to cold water events and analyse the differences between cold and warm water events. They concluded that P. damicornis responded most negatively and that  cold water bleaching events events needs more study in general.

I will conclude this post with a question I asked myself as I read around this topic, why, if cold water events are both heavy impacting and not as temporally spread out as once imagined, is there very little coverage both in the media and in academic study? are these ecosystems being ignored for protection despite being seemingly as vulnerable as those in warmer waters?