A new study showed that as oceans warm, physical forces like wave strength and water flow influence which reefs thrive and which die.
The study, led by Justin Rogers, a postdoctoral researcher at Stanford University's Environmental Fluid Mechanics Laboratory, and published this week in a report in the journal Limnology and Oceanography, offers new insight into how climate change will affect reefs on a local level.
"We have known for a while that high water temperatures are harmful to coral reefs," Rogers said. "What this paper illuminates for the first time is how waves can lower the water temperature and create better conditions for coral reefs to thrive."
The results, according to the researchers, hint at steps conservationists can take to reduce the impact of warming on these fragile ecosystems.
As coral reefs host thousands of marine species to support sustainable fisheries and are among the world's biodiversity hotspots, their towering structures help to protect vulnerable coastal areas from storm waves.
To understand how forces driving ocean water circulation would affect reef temperatures and, by extension, reef health, Rogers and his colleagues deployed a series of instruments in the waters surrounding the South Pacific atoll of Palmyra, including temperature sensors, velocity sensors to track wave speed and pressure sensors to measure the strength of incoming waves and tides.
"The idea was just to get a huge coverage over the reef," Rogers was quoted as saying in a Stanford news release.
After placing the instruments, the researchers monitored them for about three years to collect data. And while analyzing their atoll data, they realized that the health of particular reef zones was closely tied to the temperature, wave and pressure dynamics in those areas.
For instance, the reefs that did best over time were the ones that received an ample flow of cooler water from the ocean further offshore.
The researchers found that both waves and tides in nearby waters drive the flow rate around these high-performing reefs, with waves being the most significant factor. "High temperature is very stressful to corals," Rogers said. "If there's not enough exchange of water from the open ocean, those areas do not do well."
Conversely, in reef zones around the atoll that had less moving water, coral cover was much sparser, since there was little cool inflow to offset rising water temperatures.
In addition, the researchers found that coral health suffered when there was high wave stress around the outer edges of reefs, where heavy pounding inflicts physical damage. High wave stress may become more common as climate change proceeds and sea levels rise.
Reefs in stagnating water zones and those exposed to heavy wave pounding may be among the first to go. "Areas of the reef that are not stressed will suddenly become stressed," Rogers said.
Corals can withstand brief water temperature fluctuations, he added, but their resistance wanes when average temperatures remain high for weeks or months at a time, as they do in areas with low ocean inflow.
While the prognosis may be grim in a warming world, Rogers suggested that limited conservation funding is best directed to reefs with high ocean inflow, which still have a fighting chance.
