A bed of California mussels.

Salish Sea pH is dropping as carbon dioxide levels rise

1,340-year-old mussels tell the story, says SeaDoc-funded study

California mussels from the Salish Sea have undergone a serious transformation over the past millenium, indicates a recent scientific study funded by The SeaDoc Society. Their shells have become 30 percent thinner and are increasingly made of fossil fuel-sourced carbon.

“Corrosive water has recently been documented in the northeast Pacific, along with a rapid decline in seawater pH over the past decade,” wrote University of Chicago researcher Cathy Pfister and colleagues in the study abstract.

Fossil evidence indicates that ocean pH has fluctuated over the past 20 million years, but only within the range of 8.1 to 8.3. Pfister and fellow researcher Tim Wootton have measured pH values as low as 7.7 and 7.8 in the Strait of Juan de Fuca.

“What long-term effects in response to these drops in pH mean for ocean life is the million dollar question, but we are certain that some shelled organisms are going to be affected,” said SeaDoc scientist Ignacio Vilchis.

Concerned about rising carbon dioxide levels and plunging pH levels in the Salish Sea, The SeaDoc Society recently funded Pfister’s analysis of 1,340 to 1,000-year-old mussel shells from Makah middens on Tatoosh Island.

Because the earliest seawater pH recordings were taken in the 1990s, Pfister turned to the shells as a much longer-standing recording device for clues about the historic state of the ocean.

Probing carbon cycle changes

Many scientists believe that roughly a quarter of atmospheric carbon dioxide is absorbed into the world’s oceans. Higher sea CO2 levels result in a higher concentration of free hydrogen ions, which lowers seawater pH and takes up normally available carbonate, making it difficult for organisms to form their calcium carbonate shells.

Pfister’s study aimed to determine whether these observed carbon cycle changes have a historical precedent, or are caused by the carbon dioxide that enters the atmosphere when humans burn fossil fuels.

“Combusted fossil fuel carbon is composed of more of the lighter, naturally occurring isotope of carbon (12c), so we are able to see the signature of fossil fuel burning in current CO2 measurements,” she said.

When Pfister and colleagues compared modern, decade-old and midden shells, they found the carbon makeup has been rapidly changing from the heavier 13c form of carbon to the lighter combusted fossil fuel carbon, 12c.

Pfister said the changes have no historical precedent.

“[The research] indicates a change in the carbon cycle consistent with fossil fuel carbon making its way into the ocean,” Pfister said. “The rate of change in the ratio of carbon isotopes is also consistent with ocean pH decline. Carbon isotopes give you a hint of what is going on with carbon in the ocean; what concerns me is that we have a pH decline that is pretty severe in the area. Why is it going down at a rate faster than we would expect? Does it indicate more CO2 is going into the nearshore ocean than we expected?”

Pfister and colleagues also observed a more than 30 percent decrease in mussel shell thickness. Those data are part of a separate study on long term decline in shell calcification, which they expect to publish soon.

The complete manuscript of Pfister’s study is available for download online.

Ocean acidification and shellfish

Many scientists now draw a connection between the release of carbon dioxide through fossil fuel burning and an observed drop in seawater pH, a process termed “ocean acidification” that is interfering with reproduction of both cultured and wild shellfish in the Salish Sea.

“The oceans aren’t acidic,” says NOAA oceanographer Adrienne Sutton – at least not yet. “Acidification is a term that describes a process.” A pH below 7 is considered acidic.

Washington state shellfish hatcheries have grappled with this shift over the past few years, hiring scientists to figure out how to prevent massive die-offs of their shellfish larvae crop.

The alarm was initially sounded around 2008 by Oregon’s Whiskey Creek Shellfish Hatchery, which was experiencing larvae kills from low pH seawater. Since then, industry and federal efforts have funded ongoing scientific monitoring of these sea changes. Wild oysters in Puget Sound and off Vancouver Island are also struggling to reproduce.

In recent years Orcas Island shellfish growers have occasionally found it difficult to purchase spawn in order to seed their farms.

Sutton said surface sea waters tend to have the highest pH and oxygen saturation, and to be highly saturated with carbonate ions.

But as it gets deeper, ocean chemistry changes, and waters range from saturated to undersaturated. In some places, CO2 naturally bubbles from the sea floor and the seawater pH is as low as 7.5.

As the ocean absorbs more and more CO2, carbonate ions become increasingly less available. The saturation depth of aragonite, a key calcium carbonate mineral, has been measured getting shallower at a rate of 1-2 meters per year, so that organisms must find the aragonite they need in increasingly shallower ranges. Deeper water is also low in oxygen.

The situation worsens for shellfish when an ocean process called “upwelling” brings very deep, low pH water to the surface, and the low aragonite saturation causes shellfish mortality.

“This deep water used to stay deeper,” said Benoit Eudeline, chief hatchery scientist for Taylor Shellfish Farms, the country’s largest producer of farmed shellfish, with hatcheries located in inlets and bays throughout Puget Sound.

Eudeline said adding sodium carbonate to hatchery tanks can raise the seawater’s pH and carbonate saturation, but this only works on a limited scale. He avoids pumping fresh seawater into the hatchery during upwellings, often triggered by north winds. Eudeline said scientists have also proposed to breed shellfish for survival in a less basic environment.

Pacific Shellfish Institute president Betsy Peabody said the situation for shellfish worsens when nitrogenous nutrients in sewage and fertilizer enter the ocean, feeding algae that in turn deplete oxygen and raise seawater CO2 levels.

Further reading on ocean acidification in the Pacific Northwest

The Yale School of Forestry and Environmental Studies recently published a story by Elizabeth Grossman called “Northwest Oyster Die-Offs Show Ocean Acidification Has Arrived.”