With oceans becoming more acidic worldwide, scientists are getting creative in designing experiments to study them. For example, one group at the University of Washington is using giant plastic bags to study ocean acidification.
Each bag holds about 3,000 liters of seawater and sits in a cylinder-like cage for stability. The group at UW, made up of professors and students, is controlling carbon dioxide levels in the bags over a nearly three-week period, during which they are looking at the effects of increased acidity on organisms living near the San Juan Islands.
“These mesocosms are a way to do a traditional experiment you might do in a lab or classroom,” said Jim Murray, professor of oceanography at the University of Washington. The structures, he said, make it possible to bring a part of the natural environment under controlled conditions.
“Ocean acidification is happening, and it’s clearly due to increasing CO2 in the atmosphere,” said Murray. “The big unknown is the impact to biology.”
The 6-meter polyethylene bags are sealed at their bottom ends. Five meters are submerged and 1 meter remains above the surface. Metal frames give structure to the bags and stabilize the vessels during rough water conditions.
There are nine bags being studied. Three are held at the carbon dioxide levels of water near UW’s Friday Harbor lab – 650 parts per million. For comparison, the current concentration of CO2 in the atmosphere is at 390.
Another three bags are kept at 1,250, while the last three will be allowed to drift throughout the study’s course. Drifting occurs because organisms within the bag alter CO2 levels as they take it in as food.
A LI-COR sensor measures light around the mesocosms to help control algae blooms within the bags, and a CTD sensor measures the temperature and salinity of water in each bag daily. The group controls carbon dioxide levels by bubbling the gas through seawater before adding it to the bags.
“If you start bubbling your experiment, organic matter begins to concentrate on the bubbles and makes changes in the experiment,” said Murray.
Murray said keeping the gas concentrations constant helps make sure that the experiment’s results reflect the effects of elevated carbon dioxide. Allowing the levels to drop over the course of the three-week period wouldn’t make sense when it came time for evaluating results.
Murray said the study, which is about halfway through, is yielding interesting results so far, but the full findings won’t be available until after its completion. It’s interesting research, he said, but notes that it’s also an immersive learning experience for students. Pupils fulfill their credit hour requirements by taking just one class and are at Friday Harbor almost every day. The students talk about their work via a class blog.
“Students get to learn about collecting, sampling and analyzing results,” said Murray. “And it’s great because they can see if they want to go on in research.”
Top image: The mesocosms are attached to a dock at UW’s Friday Harbor lab (Credit: University of Washington)