Can kelp forests protect the coastal environment from ocean acidification?
Rising levels of atmospheric carbon dioxide are changing the chemical composition of the ocean at a rate that is faster than any known time in Earth’s geological history. One of the major effects is ocean acidification, which threatens the entire marine food chain.
“We talk about kelp forests protecting the coastal environment from ocean acidification, but under what circumstances is that true and to what extent?” said study lead author Heidi Hirsh. “These kinds of questions are important to investigate before trying to implement this as an ocean acidification mitigation strategy.”
The researchers found that the water near the ocean’s surface had pH levels that were slightly higher, or less acidic, depending on the kelp canopy. However, these beneficial effects did not reach the ocean floor, where acidification has the strongest impacts on wildlife.
“One of the main takeaways for me is the limitation of the potential benefits from kelp productivity,” said Hirsh.
Kelp grows very quickly – up to 5 inches per day – and removes a large amount of carbon dioxide from the water in the process. The effects of giant kelp in Monterey Bay are also influenced by seasonal upwelling, when deep, highly acidic seawater is pulled toward the surface of the Pacific waters.
“It’s this very complicated story of disentangling where the benefit is coming from – if there is a benefit – and assessing it on a site-by-site basis, because the conditions that we observe in southern Monterey Bay may not apply to other kelp forests,” explained Hirsh.
In a marine laboratory experiment, the researchers set up pH sensors to gain a better understanding of chemical and physical changes in seawater.
“We are moving beyond just collecting more chemistry data and actually getting at what’s behind the patterns in that data,” said study co-author Professor Kerry Nickols.
“If we didn’t look at the water properties in terms of how they’re changing and the differences between the top and the bottom of the kelp forests, we really wouldn’t understand what’s going on.”
The use of high-resolution measurements of pH, dissolved oxygen, salinity, and temperature allowed the researchers to distinguish patterns in the seawater chemistry around the kelp forest.
The team was surprised to find that the water was actually less acidic at night compared to daytime measurements. The researchers believe this was caused by the upwelling of acidic, low oxygen water during the day.
“It was wild to see the pH climb during the night when we were expecting increased acidity as a function of kelp respiration,” said Hirsh. “That was an early indicator of how important the physical environment was for driving the local biogeochemical signal.”
While the benefits of the kelp forests did not appear to reach some of the most sensitive organisms on the seafloor, the researchers did establish that there was an overall less acidic environment within the kelp forest compared to its surroundings.
The study authors said fhe organisms that live in the canopy or could move into it are most likely to benefit from kelp’s local acidification relief. They also noted that the research serves as a model for future investigation about the ocean as a three-dimensional, fluid habitat.
“The current knowledge set is pretty large, but it tends to be disciplinary – it’s pretty rare bringing all these elements together to study a complex coastal system,” said Professor Rob Dunbar. “In a way, our project was kind of a model for how a synthetic study pulling together many different fields could be done.”
The study is published in the journal JGR Oceans.