Sep 19 2016
A black tip shark swims above a shallow reef of primarily dead coral skeletons at Palmyra Atoll. Research by Stanford scholars offers new insight into how climate change will affect reefs. (Image credit: Brian Zgliczynski)
Around the world ocean coral reefs are threatened by climate change, however not all reefs are affected equally. Justin Rogers, a postdoctoral researcher at Stanford’s Environmental Fluid Mechanics Laboratory, led a study that suggested when oceans begin to get warm, physical forces such as water flow and wave strength impact which reefs flourish and which die.
The research findings have been published in a report in the Limnology and Oceanography journal. The report provides a new understanding into the way climate change will influence reefs on a local level, and also suggests measures that conservationists can take to decrease the impact of warming on these delicate ecosystems.
We have known for a while that high water temperatures are harmful to coral reefs. 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.
Justin Rodgers, Postdoctoral Researcher, Stanford University
Coral reefs are one of the Earth’s biodiversity hotspots. They host numerous marine species to support sustainable fisheries, and their tall structures help to protect exposed coastal areas from storm waves.
Rogers was keen to learn how forces behind ocean water circulation would impact reef temperatures and ultimately, the health of the coral reef. Rogers and team set up a series of instruments - including velocity sensors to track wave speed, temperature sensors, and pressure sensors to measure the strength of incoming waves and tides - in the waters near the South Pacific atoll of Palmyra region.
“The idea was just to get a huge coverage over the reef,” Rogers said. Once all of the instruments were in place they were monitored for around three years to collect data.
Upon analyzing their atoll data, it became clear to Rogers and his team that the health of particular reef zones was directly linked to the wave, temperature, and pressure dynamics in those zones.
For a long time scientists have been aware that the increasing ocean temperatures influence coral health in a number of ways.
Zooxanthellae - a colorful, single-celled algae that live inside coral - typically provide the coral with a rich food source, and the coral provides shelter for the algae. However, when the ocean water warms the stressed corals expel the brightly colored algae, revealing the bone-white coral skeleton.
As a result, “bleached” coral tend to become more susceptible to disease and environmental impacts.
Oceans absorb more carbon dioxide when the temperatures increase, and when this occurs corals become less able to extract calcium carbonate from the water. Calcium carbonate is an essential mineral the coral needs in order to form and strengthen their skeletons. Consequently, their growth becomes stunted and they may even begin to dissolve.
The reefs that did thrive well over time – those displaying the maximum level of live coral cover – were the coral that received a sufficient flow of cooler water from the ocean away from the shore.
The Stanford team discovered that both tides and waves in the surrounding waters influence the flow rate around these high-performing reefs, with waves being the most important factor.
High temperature is very stressful to corals. If there’s not enough exchange of water from the open ocean, those areas do not do well.
Justin Rodgers, Postdoctoral Researcher, Stanford University
Not unexpectedly, in reef zones surrounding the atoll that had less moving water, coral cover was a lot sparser, as there was limited cool inflow to equalize rising water temperatures.
The researchers also noticed that the health of the coral was affected when there was high wave stress close to the outer edges of reefs – heavy pounding that causes physical damage. High wave stress may turn out to become more widespread as climate change progresses and sea levels rise. On the contrary, protected reefs that do not have to endure heavy pounding had better levels of healthy coral cover.
Rogers’ research indicates how coral die-offs in other, similar reefs may advance if ocean temperatures worldwide continue to increase. A 2016 U.S. government-funded study reports that oceans are already warming globally at an extraordinary rate. If this continues, more coral reefs will be affected or will perish, threatening fisheries in coastal areas and causing the equilibrium of undersea ecosystems to be disturbed.
Reefs exposed to heavy wave pounding and those found in stagnating water zones will likely be the first to go. “Areas of the reef that are not stressed will suddenly become stressed,” Rogers said.
Corals can endure short water temperature fluctuations, he added, but their resistance decreases when average temperatures stay high for weeks or months at a time, which is the case in areas with minimal ocean inflow.
“What really seems to affect corals are these long-term temperature changes. They create an environment where it’s difficult for the corals to thrive,” he said.
Although the projection seems grim, Rogers’ findings also propose realistic strategies to save as many reefs as possible. “At a local reef scale, we could look at promoting flow in some areas,” he said, though he warned that such intrusions would have to be vetted cautiously to avoid unintentional and unfavorable outcomes.
But with the Earth constantly warming, certain reefs could be so severely stressed that it would be doubtful that they will survive even with intervention. He recommends that limited conservation funding should be directed to reefs with high ocean inflow, which have a better chance at survival.
The reefs [with] a better chance of survival are the ones that have a better connection to offshore ocean waters. It would be helpful to say, ‘These areas of the reef are going to do better under climate change, so let’s focus our conservation efforts [there].'
Justin Rodgers, Postdoctoral Researcher, Stanford University
Stanford co-authors with Rogers on the article, “Thermodynamics and hydrodynamics in an atoll reef system and their influence on coral cover,” are Stephen Monismith, David Koweek, Walter Torres and Robert Dunbar.