An acid test for fisheries

The oceans cover 70 percent of Earth. At the surface, the ocean and the overlying air continually exchange energy and chemicals. Anything humans put into the atmosphere eventually ends up in the ocean, including carbon dioxide.

Ocean uptake or “storage” accounts for approximately one-third of the carbon dioxide added to the atmosphere in modern times. “We used to think this was a good thing,” said Dr. Richard Feely of the National Oceanic and Atmospheric Administration’s (NOAA) Pacific Marine Environment Laboratory at a presentation at the University of Maine in 2008, “but what we now know is that there is a serious biological impact of ocean CO₂ uptake.” As carbon dioxide has increased in the atmosphere at an accelerated rate, so has the amount absorbed by the ocean (525 billion tons, half in just the last 30 years, according to NOAA).

In the ocean, carbon dioxide (CO₂) bonds with water (H₂O) to form carbonic acid, a weak acid that readily breaks apart into its components, bicarbonate and hydrogen. The result is more and more hydrogen ions floating freely throughout the world’s oceans (30% more than what existed at the start of the Industrial Era and on schedule to increase 100 percent by 2100), which are responsible for an increase in the ocean’s acidity, a process known as acidification. More acidic conditions have been experienced before in deep history, but the current rate of acidification, as measured at the Mauna Loa observatory in Hawaii and inferred from samples of fossil shells, is ten times faster than at any time since the demise of the dinosaurs 65 million years ago.

More hydrogen and more carbon tied up as bicarbonate means there is less carbonate available. Carbonate is what some organisms-clams, oysters, lobsters, urchins, coral, even certain plankton-use along with calcite and aragonite to build their shells and skeletons. Normally, there is plenty of carbonate dissolved in seawater, and marine species take it directly from their watery environment. Under more acidic conditions, carbonate becomes harder to come by and shell-building can require more energy. Under very low carbonate conditions, the water becomes a corrosive environment and shells can begin to dissolve.

Acidification isn’t uniform but varies depending on local and regional conditions. Polar regions are most at risk, as is the Pacific Ocean along the west coast, and other regions where natural upwelling brings to the surface deep ocean water that is naturally more acidic and empty of carbonate. But increasing acidity is a threat to oceans everywhere. In the Gulf of Maine, where so much of the natural resource economy depends on harvesting shelled animals, acidification has the potential for devastating impacts: weak shells, pitted shells, eroded shells, animals using too much energy trying to build shells, deformed algae, decreased fertilization rates in oysters, misshapen oysters, sluggish squids. Jellyfish, however, thrive in a more acidic sea. “Moving towards a high CO₂ world is running evolution in reverse,” said Feely, taking us back to a previous, more primitive world where food webs are simplified and microbes rule.

Research by Dr. Mark Green, a marine science professor at Saint Joseph’s College of Maine, suggests that “death by dissolution” is the leading cause of mortality for juvenile (seed) clams in southern Maine estuaries. In the coastal zone, marine sediments are influenced by runoff from the land that can make them more acidic than the open ocean, and Green has documented such conditions in Casco Bay. “We’ve seen clams dissolve in the mud in certain areas. We then mimicked what we saw in the field in the lab, and we saw exactly the same thing,” said Green. This finding challenges the common belief that clam flat losses are the result of predation. “Of course clams get eaten, but that can’t be what’s happening to all of them,” he said. Green has had success in the lab improving clam set by buffering the sediments to decrease the acidity, and he is preparing for large-scale buffering experiments in Broad Cove this spring.

While the reasons for excess acidity in estuaries are different, they show what kind of things can be expected with acidification of the open ocean. Other studies show potential risks to the copepod Calanus finmarchicus, an important part of the Gulf of Maine food web, and the calcium carbonate-producing phytoplankton Emiliania huxleyi.

But there’s another shell-producing organism that might be affected by ocean acidification: the American lobster, the pillar of Maine’s fisheries for the last two decades. Will ocean acidification affect lobster’s ability to reproduce, grow and build new shells? Will extra energy spent building shells impact other basic functions or make lobsters more vulnerable to shell disease? Surprisingly little research has been done to date on the impact of increased acidity on lobsters.

In 2008 Justin Ries and colleagues in Anne Cohen’s laboratory at the Woods Hole Oceanographic Institution compared the effect of increased CO₂ levels on American lobster and 17 other shell-building marine species. After sixty days living in tanks with varying amounts of CO₂, lobsters were among only a handful of species that appeared to enhance shell-building (calcification) in the high CO₂conditions that negatively affected hard clams and soft-shell clams whose shells began to dissolve.

It’s not clear why lobsters were able to maintain shell building. These experiments were conducted in much warmer water than occurs along the Maine coast, and further research is needed.

In the United Kingdom, researchers at the national lobster hatchery conducted a series of laboratory experiments to study the critical period between when European lobster larvae are released from eggs (Stage I) to when they are ready to settle to the bottom as juveniles (Stage IV). The larvae were held in seawater in the laboratory in containers with varying levels of added CO₂, fed once a day, and allowed to develop for 28 days.

According to lead investigator Dominic Boothroyd, initial findings indicate that the larvae raised in the higher levels of CO₂ developed lighter weight shells (carapace), compared to those raised in the control environment. Other indicators, such as shell length and survival, did not appear to be impacted by CO₂ levels, at least during the 28-day study period. The next priority for Boothroyd’s team is to investigate any impacts of acidification on juvenile benthic stage and adult lobsters. Boothroyd notes that the European lobster is a different species than the American lobster, which may respond differently to acidification.

Given the potential for devastating impacts on Maine’s most valuable marine species, fishermen and communities along the coast will likely be looking to learn more about how acidification will look in our region.

This article is made possible in part by funds from Maine Sea Grant and the Oak Foundation.

Heather Deese holds a doctorate in oceanography and is the Island Institute’s director of marine programs. Catherine Schmitt is communications coordinator for Maine Sea Grant.