Acidification of oceans may contribute to global declines of shellfish

Professor Christopher J. Gobler, Ph.D., and Ph.D. candidate Stephanie C. Talmage of the School of Marine and Atmospheric Sciences at Stony Brook conducted experiments to evaluate the impacts of past, present and future ocean acidification on the larvae of two commercially valuable shellfish: the Northern quahog, or hard clam, and the Atlantic bay scallop. The ability of both to produce shells partly depends on ocean water pH. Previous studies have shown that increases in atmospheric CO₂ levels can lower the ocean's pH level, causing it to become more acidic.

"In general, the study of ocean acidification on marine animals is a relatively new field. Ocean acidification has been going on since the dawn of the Industrial Revolution but it has been investigated as a process for less than a decade," Dr. Gobler said. "People have known about rising levels of CO₂ and have been talking about that for decades but had originally assumed the oceans would be able to maintain their pH while they were absorbing this CO₂." The largest contributor to CO₂ in the atmosphere and oceans is the burning of fossil fuels, Dr. Gobler said.

While previous studies have demonstrated that shellfish are sensitive to the increases in CO₂ projected for the future, "the extent to which the rise in CO₂ that has occurred since the dawn of the Industrial Revolution has impacted these populations is poorly understood," the researchers wrote.

While studying the impact of rising global temperatures on shellfish the researchers shifted their focus to another worldwide threat. "Temperatures have risen about 8 percent since the dawn of the Industrial Revolution but carbon dioxide is up 40 percent, increasing over 100 parts per million," Dr. Gobler said.

The researchers reported that larvae grown at approximately pre-industrial CO₂ concentrations of 250 ppm had higher survival rates, grew faster and had thicker and more robust shells than those grown at the modern concentration of about 390 ppm. In addition, larvae that were grown at CO₂ concentrations projected to occur later this century developed malformed and eroded shells. The findings may provide insight into future evolutionary pressures of ocean acidification on marine species that form calcium carbonate shells, the authors wrote.

"CO₂ entering the ocean decreases the availability of carbonate ions (CO₃−2) and reduces ocean pH, a process known as ocean acidification," the authors wrote. "These changes in ocean chemistry may have dire consequences for ocean animals that produce hard parts made from calcium carbonate (CaCO₃)."

Other calcifying organisms impacted by ocean acidification include coccolithophores, coral reefs, crustose coralline algae, echinoderms, foraminifera, and pteropods, the authors wrote.

In their experiments with the Northern quahog, Mercenaria mercenaria, and the Atlantic bay scallop, Argopecten irradians, the scientists introduced different levels of CO₂ gas to filtered seawater taken from Shinnecock Bay, NY, USA. Shellfish larvae grown under near preindustrial levels of CO₂ (250 ppm) displayed the highest rates of metamorphosis, growth, and survival, they found. Those grown under higher levels developed thinner shells. The high CO₂ "severely altered the development of the hinge structure of early stage bivalves. As CO₂ levels increased from approximately 250 to 1,500 ppm, there were dramatic declines in the size, integrity, and connectedness of the hinge." That can impact the ability of the shellfish to feed, they wrote. This research was conducted on the Southampton campus of Stony Brook.

"Our findings regarding the effects of future CO₂ levels on larval shellfish are consistent with recent investigations of ocean acidification demonstrating that calcifying organisms will experience declines in survival and growth, as well as malformed CaCO₃ shells and hard parts," they wrote. "However, our examination of the development of larval shellfish at levels of CO₂ present before the industrialization of the planet provides important insight regarding the potential effects ocean acidification has had on calcifying organisms during the past two hundred years."

Together with rising global temperatures, pollution and algae blooms, ocean acidification can have a devastating impact on shellfish populations, Dr. Gobler said. "There a lot of efforts right now to bring back our shellfish in New York and around the world, but this study demonstrates this could be more difficult than anticipated," he said. While some threats such as overharvesting can be dealt with through limits on licenses and off-limits areas, "when you're dealing with a global phenomenon it's harder to counteract."