26. Isla del Coco, Costa Rica, Eastern Tropical Pacific

Mesophotic coral ecosystems (MCEs), at ocean depths between 40 and 150 m, have been found throughout the tropics but are different in the Eastern Tropical Pacific (ETP). This observation is based on submersible dives that have explored several ETP mesophotic areas. The ETP is characterized by a shallow thermocline (approximately 50 m depth), reduction in light availability at deeper depths due to the accumulation of particulate matter at the thermocline, cold temperatures, low oxygen concentrations, and low aragonite saturation state (Ω_arag) that may explain the absence of zoothanthellate corals below 40 m. Isla del Coco is located 500 km off the Pacific coast of Costa Rica. The MCEs of this island have been explored using the DeepSee submersible. There is a sharp change in benthic fauna, in both soft and hard bottoms, at approximately 50 m depth. The combination of the shallow thermocline bringing cold and low pH waters to near the surface plus the turbidity (low light) at the thermocline leads to a situation where zooxanthellate corals do not generally thrive below about 15–25 m depth (maximum around 40 m) and do not build reefs in deeper waters. The substrate between 50 and 150 m is densely covered with red algae (down to 90 m), octocorals, black corals, azooxanthellate corals, and calcareous hydroids. This chapter is the first synthesis of mesophotic environments of any ETP site and characterizes the MCEs of Isla del Coco National Park.