The Gulf of Maine is and always has been an essential part of the coastal New England economy. Throughout history, the bounty of the Gulf of Maine has fluctuated due to fishing pressures, technology and species’ popularity. As these changes have affected some parts of the Gulf of Maine ecosystem more than others, New England fishermen and lobstermen have adapted by shifting their efforts to different parts of the ecosystem. But what would happen if the entire Gulf of Maine ecosystem changed at once?
We may soon find out. Excess carbon dioxide, or CO2, in our atmosphere is causing ocean acidification. The ocean absorbs atmospheric CO2, and when CO2 mixes with seawater, carbonic acid develops. Increasing CO2 in the atmosphere has subsequently increased the acidity of seawater, lowering its pH. And the oceans are expected to become even more acidic over the next 200 years, as CO2 levels in the atmosphere continue to rise. In fact, atmospheric CO2 levels are predicted to nearly double over that time.
Research by Justin Ries at the University of North Carolina at Chapel Hill and his colleagues Anne Cohen and Daniel McCorkle at the Woods Hole Oceanographic Institution has shown that shelled organisms exhibit different responses to increasingly acidic marine environments. Mollusks such as bay scallops, whelks, periwin-kles, oysters, conchs, quahogs and softshell clams build their shells more slowly as the amount of atmospheric CO2 increases. And in some cases, the shells of these organisms actually begin to dissolve away, rendering them virtually defenseless to the crushing claws and jaws of their predators.
Furthermore, Ries showed that some of the organisms lose more than just their shell strength as seawater becomes more acidic. Under high CO2 conditions, quahog shells had fewer ridges, conch shells had smaller knobs and the spines of pencil urchins became truncated. These organisms are thought to have evolved their ridges, knobs and spines for burrowing, stability and motility, respectively. Without them, these animals would be even more vulnerable to predators.
Meanwhile, Ries’ research also showed that the American lobster, the blue crab and the gulf shrimp actually increased the rate at which they build their shells as seawater becomes more acidic.
At first glance, these findings suggest that although ocean acidification is clearly bad for most prey species (such as quahogs, bay scallops and softshell clams), it may turn out to be good for predators such as the American lobster. Take another look. With thinner shells, mollusks may become more accessible to alternative predators such as fish and rays. Lobsters, therefore, may face increased competition for food even as their food gets easier to eat. This combination of increased competition and overpredation of the weakened prey species may ultimately leave lobsters hungry.
Although the experiments suggest that lobsters may produce heavier shells in more acidic seawater, this does not necessarily mean that they will thrive in such conditions. If lobsters are putting more energy into building their shells, they may be diverting energy from other essential functions such as muscle development and re-production. What good is a strong shell if the lobsters are losing muscle, struggling to find food and reproducing less successfully?
Furthermore, bivalves play a key role in filtering suspended particles from coastal waters. Should bivalve populations decline as a result of CO2-induced ocean acidification, the quality of coastal waters may follow. This would be bad for lobsters and lobster-eaters alike.
The bottom line is that the net effect of ocean acidification on marine ecosystems will likely be more severe than the sum of the responses of individual species. At the ecosystem level, it is unlikely that the weakening of some species (the mollusks) will be offset by the strengthening of others (the crustacea). Even as lobsters start building heavier shells, they are likely to be negatively affected by a decline in the abundance and vitality of their prey, by increased competition for their weakened food-stock, by declining water quality and by a loss of muscle mass and fecundity as they divert energy to shell-building.
Kelsey Abbott of Freeport is a writer and biologist. Justin Ries is a marine biogeochemist at the University of North Carolina-Chapel Hill.