Part of my job at Island Institute is to elevate the questions and observations of people who make a living from or are otherwise intricately connected to the Gulf of Maine. Often these curiosities impact their livelihoods. One question I’ve recently gotten from multiple people is whether ocean acidification is linked to a drop in successful mussel sets along the coast.
We used to hear more about ocean acidification, but with the warming of the Gulf of Maine progressing so rapidly, much of the focus has been on ecosystem impacts of rising temperatures. Both have the same root cause: excess carbon dioxide emissions.
Oceans have absorbed a quarter to a third of the carbon dioxide released into the atmosphere since the Industrial Revolution. When carbon dioxide is absorbed by seawater, it forms carbonic acid, which lowers pH and increases acidity. Acidification is also exacerbated in coastal zones by freshwater runoff that carries nutrients and pollutants from land. With more intense rainstorms occurring with climate change, these runoff events and the associated degradation of water quality are impacting coastal fisheries and aquaculture businesses.
Warming ocean temperatures, to a point, result in fairly predictable responses from marine organisms (of course, with exceptions), including explosions of both native and invasive warmth-loving species, faster growth, earlier maturity, and shifts in distributions of species.
The chemistry of the ocean is harder to study—we can’t feel it, and it’s difficult to measure. Its impacts on marine organisms, particularly in combination with multiple other stressors, are difficult to pinpoint, to say the least.
So it was good news to learn, almost a decade ago, that the rapid warming was actually improving the buffering capacity of the Gulf of Maine and depressing ocean acidification.
But if you were thinking ocean acidification is complicated, try digging into the drivers of global ocean circulation changes, factoring in climate change and natural cycles, how they all impact Gulf of Maine temperatures, and why we are in a temporary respite from that rapid warming that progressed in earnest in 2010.
In short, there are three major water masses entering the Gulf of Maine, and their traits are very different. These include warmer, saltier, higher alkalinity water from the Gulf Stream, and cooler, fresher, less alkaline water from the north Scotian Shelf Water at the surface and Labrador Slope Water at depth. Gulf Stream water has been predominant since 2010, but in late 2023 there was an influx of colder, fresher deep water into the Gulf of Maine.
As is typical, this change was noticed first by people working on the water. In the winter of 2024, a Maine fisherman asked me what was up with the cold, fresh water at depth. He showed me and other scientists data from one of the offshore NERACOOS monitoring buoys (part of NOAA’s Integrated Ocean Observing System). This prompted investigation into the pause in warming—a pause that has persisted to date and according to some ocean models may continue over the next decade.
It’s not clear what this tempered warming will mean for acidification in the Gulf of Maine, but when mussel farmers start asking whether ocean chemistry could be contributing to their failed sets, we listen.
We know that larval shellfish, across the board, do not like higher acidity. Juveniles of many species show negative effects, such as reduced growth, and within adults, there is a high degree of variability, with sublethal effects such as changes in immune function.
With ocean climate patterns less stable over extended periods, changes in ocean chemistry are something to watch for. We are learning that these climate stressors vary geographically, emerge according to changes in oceanography, and uniquely impact marine species and thus peoples’ livelihoods.
Long-term monitoring platforms remain critical for improving our understanding, as do observations from fishermen and aquaculturists. These business owners can be ready to explore and implement local adaptation strategies including using hatchery-reared seed, buffering water in hatcheries, breeding for more tolerant strains, and pursuing multi-trophic aquaculture practices where one species may improve growing conditions for another.
Marine ecosystems respond in real time, and the faster we humans can catch up, the better.
Susie Arnold is the senior marine scientist at Island Institute, publisher of The Working Waterfront. She may be contacted at sarnold@islandinstitute.org.
