El Niño and Coral Bleaching

by Noah Pomeroy and Bernardo Vargas-Angel
photographs by Noah Pomeroy and Kara Osada-D'Avella

“I was sweating in my wetsuit!” “It was like diving in bathwater”… Such proclamations were common as everyone rinsed down gear after our first day of diving at Howland. Earlier that day, my fellow divers of the oceanography team, Oliver, Russell and Danny, popped up from their first dive and told me I’d be roasting in my 5mm wetsuit if I wore it on the next dive. Heeding their advice, I rolled backwards off our boat, “Steeltoe,” into the warmest water I’ve ever dived in. Learning to dive in frigid California waters while wrestling with half-inch-thick neoprene covering my body really made me appreciate being able to dive for an hour in swim trunks without so much as a chill. My SCUBA console gauge reported the water temperature at an exceptionally warm 86F (30C).

A subsurface temperature recorder
attached to the reef

Sure, we all knew it was hot, but we leave it up to our precisely calibrated instruments to tell us the detailed story of Howland’s water temperature since we last visited two years ago. During our dives that day, we recovered four subsurface temperature recorders (STRs) that we had attached to the Howland’s reefs two years ago. The deepest was installed at a depth of 126 ft, the shallowest was positioned at 23 ft. We installed four new STRs in place of those that we recovered at Howland so we may continue to monitor the in situ water temperature at this small isolated Pacific island. Recording temperature every 30 minutes, the high resolution data from the STRs clearly show its waters have been warmer that usual for the last 4 months. These elevated temperatures are likely due to the effects of the El Niño-Southern Oscillation (ENSO), a climate pattern that occurs on average every 2-7 years throughout the tropical Pacific. During ENSO, commonly referred to as “El Niño,” warm water from the western Pacific spreads eastward in the equatorial current. The name El Niño comes from Spanish "the boy" and refers to Christ as the warming period off the coast of South America typically begins in December, around Christmas time.

Such warming episodes have occurred for at least the past 300 years but strong events can have serious implications for the health of coral reefs. Although we all enjoyed the comfort of diving in Howland’s exceptionally toasty water, Howland’s coral may have a different take on the elevated water temperature.

Coral bleaching at Howland Island

During our first dives at Howland, we observed high levels of coral bleaching. Coral bleaching refers to the reduction in the intensity or complete absence of coloration within living coral. This reduction in color is due to loss of pigmentation, and/or the expulsion of the endosymbiotic single celled algae (zooxanthellae) that normally live within the coral tissue. This loss results in the white skeleton showing through the remaining translucent tissue. Bleached corals can appear pale, pinkish, bluish, or white as new fallen snow. Patterns of bleaching can vary, with only the upper surface or lower surface of the colony being affected. Bleaching can also vary along gradients as well as among different species, with some being more susceptible than others. Extensive bleaching has been attributed to exposure to increased water temperatures. However, bleaching is a generalized stress response and therefore high levels of ultraviolet radiation, salinity, turbidity, and sedimentation may also induce bleaching. Prolonged anomalously high water temperatures not only can result in widespread coral bleaching but can eventually cause the death of the coral.

During our surveys around Howland, we have observed bleaching affecting many coral species, with massive species appearing to be more resistant than branching and table corals. We have collected environmental and biological data pertaining to this event which is being compiled and analyzed to generate a peer reviewed publication.