New research published in the journal Geophysical Research Letters contemplates the impact that ocean acidification is having on coral reefs globally. Although this topic is nothing new, it has been difficult to understand the specific impact of acidification as it stands apart from that of warming seas, because rising ocean temperatures also affect corals. In the new study, researchers from Woods Hole Oceanographic Institute (WHOI) aim to isolate those variables.
According to the study, which analyzed corals in the Great Barrier Reef as well as two reefs in the South China Sea, corals around the world are suffering from osteoporosis. In other words, they have reduced densities of their skeletons due to the rising acidity of the ocean.
"This is the first unambiguous detection and attribution of ocean acidification's impact on coral growth," says lead author and WHOI scientist Weifu Guo. "Our study presents strong evidence that 20th-century ocean acidification, exacerbated by reef biogeochemical processes, had measurable effects on the growth of a keystone reef-building coral species across the Great Barrier Reef and in the South China Sea. These effects will likely accelerate as ocean acidification progresses over the next several decades."
Using a numerical model, the researchers isolated the impacts of acidification to demonstrate that it has caused a 13% decline in the skeletal density of massive Porites corals on the Great Barrier Reef and a 7% decline in the South China Sea since 1950. The researchers say that this method could be extrapolated to other reef systems around the world in order to identify reefs that need the most mitigation.
"The corals aren't able to tell us what they're feeling, but we can see it in their skeletons," comments co-author and WHOI scientist Anne Cohen. "The problem is that corals really need the strength they get from their density, because that's what keeps reefs from breaking apart. The compounding effects of temperature, local stressors, and now ocean acidification will be devastating for many reefs."
In addition to carbon dioxide emissions, agricultural runoff and sewage exacerbate the fall in pH that so affects corals’ densities. “Our finding reinforces concerns that even corals that might survive multiple heatwaves are structurally weakened and increasingly vulnerable to the compounding effects of climate change,” write the authors. Focusing on reducing land pollution and runoff provides another path for protecting coral reefs, they say.