Ocean acidification: a global issue affecting a Maine oyster farm

It’s easy to get lost in the zeros. Anthropogenic carbon dioxide (CO2) emissions from 1750 through the end of 2014 totaled 2,147.6 gigatons, which is equal to 2,147,600,000,000,000 kg. (1) If you convert all of this CO2 to coal and put it in railroad cars, the resulting train would wrap around Earth 4,642 times.

While the growth rate of CO2 emissions has decreased in the last few years—the first time this has happened during a period of economic growth (2) —2014 total emissions were 65 percent greater than in 1990 (3) and the rate of CO2 release was more than four times the average rate for the entire period from 1750-2014. This means that the train holding 2014’s CO2 would wrap around the Earth more than 75 times. At today’s rate we would wrap a new coal train around the globe about once every five days.

All of this CO2 doesn’t just stay in the atmosphere. About 25 percent of it dissolves in the world’s oceans where it forms carbonic acid. Since the start of the industrial revolution, this has resulted in a 30 percent increase in the average acidity of ocean surface waters. The rate of change in ocean pH has accelerated as carbon dioxide emissions have increased. Hopefully last year’s emissions are a genuine trend. If not, by the year 2100, ocean acidity is projected to double. This process of ocean acidification (OA) is occurring at a rate likely unprecedented in our Earth’s history.

OA is a new topic for scientific inquiry. Since the first publications in the early 2000s concerns about OA and funding for research have grown. After 14 years, we have more questions than answers about local acidification processes, how marine ecosystems will be affected by acidification, and what these impacts will mean for individuals and communities whose livelihoods depend on marine resources. The implications are enormous for food security and other socio-economic concerns not just here where we live in New England, but in just about every part of the world.

How does OA affect shellfish?

Shellfish are of special concern because they make their shells out of calcium carbonate (CaCO3) and are generally considered to be at risk from OA because formation of CaCO3 becomes increasingly difficult as the pH drops. Hydrogen ions (H+) increase when CO2 dissolves in water, and this causes a reduction in the availability of carbonate ions (CO3-2). Larvae are considered to be especially vulnerable.

Numerous recent studies show that bivalves are sensitive to acidification. As shown in Figure 1, survival of the free-swimming, larval phases of bay scallops and hard clams in an experimental system declines as CO2 in the water increases from pre-industrial atmospheric levels to atmospheric levels seen today (390 parts per million (ppm)) and those expected at mid-century and by 2100.

Complicating factors

In nearshore waters, other processes can add to the acidification caused by atmospheric CO2. For example, acidified water brought to the surface by upwelling nearly caused the collapse of the Pacific Northwest oyster industry until the problem was identified and hatcheries took corrective actions. Monitoring coastal seawater, with a broader offshore picture provided by the U.S. Integrated Ocean Observing System (IOOS) and the National Oceanic and Atmospheric Administration (NOAA) Ocean Acidification Program, has played a key role identifying and managing the problem by industry.

In Maine, increasing freshwater may pose a similar threat to nearshore shellfisheries because river water mixed with seawater is more acidic than ocean water alone (4). The annual rainfall in Portland, Maine, has increased by 1 inch about every 10 years since 1930 (Figure 2). (5) Perhaps the most dramatic change is in how this increased precipitation is falling. Over the last half century, there has been a 71 percent increase in the heaviest 1 percent of precipitation events.

At about 8.1, the average pH of seawater is slightly basic. Rainwater and runoff on the Maine coast can have a pH of 5 or even less, which is 1,000 times more acidic than seawater. (6) Large influxes of freshwater bring H+ to coastal waters and can cause substantial downward shifts in seawater pH. Freshwater runoff, along with the inexorable increase of CO2 emissions, represents a “one, two” punch for bivalve larvae.

At the Mook Sea Farm hatchery in Maine, beginning about 2009, we realized that low pH water was having an impact on the oyster larvae we raise. Fertilized eggs would periodically show poor survival, and many of the survivors were severely deformed. More often, larval populations would stall. After normal fertilization and early growth, they would stop feeding and growing and the larval period, normally 14 to 16 days, would last for an additional week or longer. Typically, these larvae would take longer to metamorphose from larvae to juveniles and exhibit lower survival rates than normal populations. The problems were especially evident after large runoff events.

Although we had no systematic documentation, it became clear that the water pumped into the hatchery is presently fresher. The relative saltiness of seawater is referred to as salinity, which ranges from 0 in most river water to 38 in very salty ocean waters. Thirty years ago, when the business was started, the salinity routinely ranged from 30 to 31. Now the salinities typically range between 27 and 29. In the Gulf of Maine, the concentration of dissolved CO2 is increasing at a rate of ~1.2 ppm per year (7), which translates to an increase of 36 ppm over 30 years.

Faced with larval production problems and recognizing these trends, Mook Sea Farm developed a suite of management/mitigation strategies similar to those employed by hatchery operators on the West Coast to control the carbonate conditions in the larval cultures. These strategies have successfully restored normal larval production. With help from the University of New Hampshire, a carbonate chemistry monitoring system is now operating that continuously measures the temperature, salinity, oxygen concentration, and pCO2 (concentration of dissolved CO2) of the water pumped into the hatchery.

The saturation level of aragonite (symbolized by the Greek letter Ω and generally considered to be a key measure of the suitability of water chemistry for larval shell formation) is calculated from these measurements. Figure 5 shows data collected in April and May 2014, estimated saturation levels when Mook Sea Farm started operations 30 years ago, and estimated contributions to the lower aragonite saturation today from both increasing freshwater and atmospheric CO2.

While we can manipulate conditions in our hatchery, we must question the fate of wild populations subjected to the steady movement of atmospheric CO2 into seawater, exacerbated by extreme variability caused by the increasing number of intense storms dumping more and more freshwater into the Gulf of Maine. As acidification progresses, we hypothesize that the success of bivalve larvae in coastal waters will become increasingly sporadic, reaching a point where some natural bivalve populations may disappear.

Taking action

After the near collapse of the Pacific Northwest oyster industry, the state of Washington took action, creating a Blue Ribbon Panel on Ocean Acidification to evaluate the current status of ocean acidification science, identify research and monitoring needed to understand OA effects, and recommend approaches to minimizing and adapting to OA’s harmful effects on the state’s marine resources.

Following Washington’s lead, Maine passed legislation in spring 2014 to form an Ocean Acidification Commission charged with a similar mission. Based on a review of the science (see report Appendix C) and an assessment of gaps, the Commission unanimously adopted goals and recommendations for Maine. These include: investing in monitoring and research; reducing carbon dioxide emissions; understanding and reducing land-based pollution contributing to acidification; increasing Maine’s capacity to mitigate, remediate, and adapt to impacts; and informing decision-makers, the public and other stakeholders about ocean acidification.

Recognizing the challenges of coping effectively with OA, the report recommends that the state “maintain a sustained and coordinated focus on ocean acidification.” The legislature is currently considering a bill to establish an ocean acidification council that will facilitate and coordinate implementation of the commission’s recommendations. Based on identified gaps, another bill has been submitted that asks Maine voters to pass a $3 million bond strengthening Maine’s capacity to research and monitor OA.

The enormity of the greenhouse gas problem may seem overwhelming, but lack of action means the carbon dioxide train will continue accelerating. We must start by slowing down the train. Global solutions are needed, and local and regional initiatives underway in Maine and other coastal states have helped increase badly needed federal funding for monitoring and research and focus attention on the seriousness of the underlying issue of greenhouse gases. A “Clean Energy Revolution” will safeguard the health of our waters and the fragile ecosystems on which we depend and, as with other transformative action throughout human history, it will afford economic opportunities yet to be imagined.

B. Mook & J. Salisbury, Earthzine, 26 May 2015. Article.

  • Reset


OA-ICC Highlights

%d bloggers like this: