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Upwelling: a coral safety net?

Randall, C. J., Toth, L. T., Leichter, J. J., Maté, J. L., & Aronson, R. B. (2019). Upwelling buffers climate change impacts on coral reefs of the eastern tropical Pacific. Ecology. doi:10.1002/ecy.2918

Corals tend to bleach when the water temperature warms. With climate change, it is becoming increasingly difficult to escape the heat. So, corals essentially have to adapt or relocate to cooler water in order to survive. This is exactly what Randall’s team is witnessing; corals are preferring cooler water in the Gulf of Panama.

Sensitive to Stress

Coral reefs support marine biodiversity in tropical areas, protect coasts from powerful ocean storms, and attract ecotourism. But it isn’t easy being a coral. The coral animal forms a partnership with tiny algae, which provide them with nutrients. When the ocean water gets too hot, however, these algae desert the coral and the coral bleaches and dies. Most corals can only live in a narrow range of temperatures. Since they are sessile, or unable to move, rapidly warming temperatures are a great threat to coral reefs.

But if you look at corals over a long timeframe, they do “move” to cooler water. An individual coral cannot walk around and relocate, but the baby coral larvae can. And some coral larvae prefer to settle in areas that have access to cool water. These areas are called upwelling zones. Upwelling is when cool, nutrient rich, deep water is brought up to the surface to replace warmer water. This can happen when trade winds near a mountain range push warm surface water offshore. In the Gulf of Panama, this is precisely what occurs (Figure 1).

Figure 1. Upwelling diagram for the Gulf of Panama. Source: C. Sartor

Gulf of Panama: Coral Refuge?

Randall and team’s study area is the Pacific coast of Panama. In the east, the Gulf of Panama has cooler water due to upwelling. In the west, the Gulf of Chiriquí is warmer since there is weaker upwelling of cool water. They looked at coral growth, cover, and survival in the two areas after a coral bleaching event. Overall, they found that corals favored the Gulf of Panama, with more intense upwelling.

Figure 2. Panama map. Source: Wikimedia Commons.

But it wasn’t always this way. Before 2016, corals seemed to prefer the Gulf of Chiriquí. It was stable, warm, and nutrient poor. The corals grew 20% faster in the Gulf of Chiriquí than in the Gulf of Panama. So, what happened in 2016? Well, a disturbance called El Niño made the water in the Gulf of Chiriquí warmer than ever before. Consequently, corals preferred the cooler, Gulf of Panama. After the extreme El Niño disturbance, corals in the Gulf of Panama grew faster, survived longer, and covered more surface area than before. The upwelling the Gulf of Panama basically served as a coral refuge after the sea surface temperature increase.

Figure 3. Coral cover in the Gulf of Panama and the Gulf of Chiriquí is dominated by Pocillopora damicornis. Source: Wikimedia Commons.

Will all the world’s corals begin moving to areas with upwelling? Upwelling may be a temporary refuge for corals, but these sensitive organisms are still in danger. With rising temperatures, upwelling is not a sustainable solution. Temperatures will likely become too extreme for even the most resilient corals to withstand. The real solution is still to help slow climate change.





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