The Albermarle Sound, the Pamlico Sound, the South River; these estuarine waters are an important feeding and nesting habitat for fish, birds and other wildlife. Most of the seafood that ends up on your plate spends part of its life in our estuaries so it makes sense to protect these habitats. But as beneficial as estuaries are for recreation and the economy, they are also very fragile. Today on the Down East Journal, we focus on just one of the many factors that can alter an estuarine ecosystem. Time to put your thinking caps on as we delve into the topic of ocean acidification.
Recent studies have shown that acidic environments affect the calcification of shellfish and can impair an organism’s ability to process environmental and biological cues. Associate Professor of Marine Environmental Science at the Institute of Marine Sciences in Morehead City Dr. Michael Piehler says acidification can be driven by many different aspects.
“The factor that we think a lot of the time is responsible for the big long term changes are changes in atmospheric concentration of carbon dioxide or CO2. And that equilibrates with the water and causes a decrease in pH. When there’s more CO2 in the water, you have lower pH’s.”
Dr. Piehler says a long term trend in carbon dioxide levels has led to an increase in ocean acidification on a global scale. But estuarine waters in eastern North Carolina are also highly susceptible to natural fluctuations in acidity.
“They lie between the oceans and the rivers and rivers tend to have lower pH’s. So if you have higher river flow, that's going to drive your pH’s lower. So that can cause acidity in and of itself.”
Algae that grow in estuaries take CO2 out of the water, which raises the pH.
“Estuaries are really dynamic systems when it comes to pH. So when you’re studying the affects of potential acidification, whether it be now or in the future, if you’re doing it in an estuary, you’re doing it in an already dynamic baseline.”
When the pH level drops, it also affects the water chemistry. For example, when the acidity increases, the concentration of calcium carbonate goes down, weakening shellfish, coral and crustaceans.
“It affects organisms that calcify. It’s been shown in past studies that not only does decreasing pH change organisms that calcify ability to do so, it doesn’t necessarily do it in a consistent fashion.”
Meaning acidity affects different species in different ways. A study by Northeastern University professor Justin Ries found that eastern oysters, which are common along our coast, responded negatively to acidic environments. Their shells became frail and researchers believed that would make them more prone to predation. Crabs, on the other hand, responded positively to acidic conditions and were able to form a harder shell.
“So this set up a situation in an oyster reef system where you have crabs that eat oysters where you could imagine it would be logical to assume if the crabs were going to get bigger and stronger and the oysters were going to get weaker, that this really important predator and prey relationship may have been skewed towards the predators.”
To test this hypothesis, Dr. Piehler and several other marine scientists set up an experiment in which crabs and oysters were grown in a range of acidities. Dr. Piehler says the normal pH in sea water is around eight, so anything less than that would be considered acidic.
“We had a series of different treatments in terms of pHs. One that was a control that was today’s ocean pH. We had a moderate pH treatment, which was around 8.04 pH units and then a high which was 7.05.”
Both the crabs and oysters were grown for 71 days in the varied levels of acidification. Then, the predation trials were set to begin. Based on past research, the group knew that oyster shells would be weaker and the crabs would be stronger. They predicted that the crabs would consume more oysters because they were easier prey. But, that’s not what happened.
“When we looked at the low pH treatments, the most acidic treatments, the crabs barely grazed at all. And it turns out the affects that we predicted based on the calcification work that Justin Ries had done in the past weren’t born out because there was a physiological effect on the crabs ability to forage. So it’s just another lesson in things are never quite as simple as they seem.”
So, crabs in high acidic environments are basically disoriented. Any calcification advantages crabs experience are overwhelmed by the inability to sense their prey.
“And we found that this affect was temporary. When we moved them back into water that was not as low pH, they regained their sensory abilities.”
This experiment gives Dr. Piehler and his team of researchers a better understanding of the dynamic estuarine ecosystem and the affect acidification has on aquatic wildlife.
“Estuaries have variable enough pH that it is entirely possible that there are areas within North Carolina estuaries that have had pH’s at the level that we saw these effects on crabs. So it’s possible that pH’s approaching seven, which was our low pH treatment could occur within North Carolina estuaries.”
If carbon dioxide emissions into the atmosphere continue to rise, Dr. Pieler says the acidity of our oceans and estuaries could increase too, exacerbating the current fluctuations in pH levels along our coast. But, it’s also possible that crabs and oysters can adapt to future ocean acidification, though, that’s unclear at this time.
For more information on the study, which was published by The Royal Society in May, go to http://rspb.royalsocietypublishing.org/content/282/1810/20150333
