Reef stress signals worsen as water temperature climbs

The Costa Mar Reef Monitoring Network reported on Monday that water temperatures across the region's three primary monitoring stations have reached levels not seen in July since 2016. The Punta Blanca station, which sits above the largest shallow-water coral formation in the region, recorded a peak temperature of 30.8 degrees Celsius on July 14. The seasonal average for mid-July is 29.2 degrees, meaning this week's water is roughly 1.6 degrees warmer than normal.

Heat stress in coral manifests as bleaching, a process in which the coral expels the symbiotic algae that gives it colour and most of its energy. Mild bleaching events are reversible if temperatures fall within days. Sustained heat at 30.8 degrees or higher can trigger permanent damage. The network's director, Dr. Alejandra Fuentes, said the readings were concerning but not yet critical, provided temperatures decline in the next ten days.

The warmth coincides with the onset of Costa Mar's rainy season, which typically begins in May and runs through November. Paradoxically, the rain itself may have contributed to the warming: freshwater runoff from inland rivers has created a thin, warm surface layer that traps heat and slows the mixing of cooler deep water into the shallows where the reef lives.