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Ocean Acidification May Affect Lobster Molt, Reproduction

The fact that the Gulf of Maine is warming more quickly than the majority of the world’s ocean regions is alarming enough. But with that warming comes an equally threatening change to the Gulf’s waters, increasing acidity. The effect of a more acidic Gulf on lobsters was the subject of a conference at Bowdoin College in June, organized by the Friends of Casco Bay and the University of Maine. The cause of increased acidity is increased carbon dioxide (CO2) in the atmosphere. The world’s oceans naturally absorb atmospheric carbon dioxide; in fact, oceans have absorbed between one third and one half of the carbon dioxide emitted by human activities since the start of the Industrial Age in the mid-1800s. In seawater, CO2 reacts with H20 to form carbonic acid. Carbonic acid then breaks down into hydrogen ions and bicarbonate ions. Those hydrogen ions reduce seawater’s pH, thus making the water more acidic. The chemical process also reduces the amount of calcium carbonate available in seawater, a factor that has affected the shell-building abilities of shellfish. But what effect does all this chemistry have on Maine lobster?

The Maine Legislature created a panel to review the impact of ocean acidification on Maine marine resources. The panel’s 2015 report stated that the number one goal was to evaluate the impact of ocean acidification on commercially important species such as lobster, clams, oysters, and sea urchins. Rick Wahle of the University of Maine reviewed his research in concert with University of Prince Edward Island scientist Spencer Greenwood and colleagues on changes in the the Gulf’s lobster populations. Juvenile settlement has declined throughout the Gulf of Maine for the past four years, according to Wahle, while lobster landings continued to rise, particularly in eastern Maine. Wahle and other researchers are puzzled that there are fewer young lobsters settling to the bottom at a time when the number of eggs entering the system from The fact of more eggs entering the system from a greater number of fertile female lobsters is at an all time high. which the numbers of young settling on the bottom declines puzzles Wahle and other researchers. Could it be because of a warming Gulf of Maine has driven the young lobsters food, such as copepods, away? Or could an acidifying ocean have have harmed fourth stage lobster larvae? Wahle and colleagues are examining lobsters taken from three areas of the Gulf — Rhode Island, mid-coast Maine and the Bay of Fundy — to look at the animals’ genes. They want to see if increased water temperatures and ocean acidity have affected how those genes work and if those changes follow through multiple generations. While crustaceans as a group are largely resistant to ocean acidification, the long-term effects are not known, Wahle said. David Field, a senior research scientist at Bigelow Laboratory, noted that the effects of ocean warming and acidification on marine species are both direct and indirect. Indirect effects have to do with the availability of food, ability to fend off disease, and successfully reproduce, among other things. He noted that water temperature has a marked effect on Calanus finmarchicus, a species of copepod favored by everything from right whales to juvenile lobsters. Typically, the body of an adult C. finmarchicus will be 60% fat when it is in colder water. Raise the temperature of the water, and the the copepod will have less body fat and will also be smaller in size. Fields conducted experiments to see what happened to lobsters when subjected to warmer water and higher levels of dissolved CO2 corresponding to a more acidic environment. He found that in warmer, more acidic water, lobsters breathed more and ate a good deal more. To do so, the animals must curtail expenditure of energy on other things, like reproduction or growth. “Lobster are solving these problems in evolutionary ways. It’s mysterious,” Fields said. He also noted that as C. finmarchicus has shifted in distribution another copepod species, C. glacialis, has moved in, however, that species seasonal behavior is different from finmarchicus. Is there any direct impact on a young lobster’s health from more acidic seawater? Amalia Harrington, a PhD student at the University of Maine, conducted an experiment to look at what effect warmer water and increased acidity have on a lobsters’ blood and tissue. Harrington held 48 subadult female lobsters in normal seawater and some in more acidified water. She then raised the water temperature from 12.5oC. to 29oC. in 2.5 hours and measured the heart rate of each lobster. Typically, a lobster’s heart rate will go up in a steady line as the temperature increases then drop as the stressed animal begins to conserve its energy in the face of a mortal threat. But what Harrington found is that the heart rate of lobsters held in acidified water began to decrease at a lower temperature than those kept in normal seawater. The presenters emphasized that the impact of ocean acidification on lobsterlinks to the question of how such changes are affecting the larger Gulf of Maine ecosystem. Lobsters are not the only commercial species feeling the heat, so to speak. “Adaptation doesn’t come for free,” Fields said. “There are costs to any sort of stress.”


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