A second summer of coral heat stress looms over Costa Mar reefs

The Costa Mar Reef Monitoring Network's station at Punta Negra, on the peninsula's southern coast, recorded a surface water temperature of 28.4 degrees Celsius on Wednesday morning—half a degree above the historical median for late June and approaching the threshold at which coral stress typically begins. The measurement, taken at 06:30 local time when the water is coolest, suggests that afternoon temperatures may already be climbing into the range where zooxanthellae—the symbiotic algae that give coral its color and much of its nutrition—begin to expel themselves from their host tissue.

Last summer, a heat event in August and September turned the upper reef zones pale and triggered a partial bleaching across twenty percent of the monitored area. The recovery has been slow; some coral colonies remain stressed, and the network's director, Dr. Carmen Restrepo, said in an interview this week that a second consecutive heat event could tip colonies that survived last year's stress into permanent mortality. The implications ripple outward: bleached reefs lose their capacity to support fish populations, which collapses the dive-tourism operations that form the backbone of coastal employment.

The network is testing new rapid-response protocols this summer—hourly temperature logging at four key sites, twice-weekly dive surveys to catch early signs of stress, and direct communication with the dive cooperatives so they can shift their routes away from vulnerable zones if bleaching begins. The question is whether these tools will matter, or whether the underlying heat is simply beyond the reef's capacity to endure.