"Energy and global warming news for December 24: Growing hypoxic zones reduce fish habitats; Ocean acidification may disrupt the marine nitrogen cycle"
One traditional Christmas Eve dinner is fish….
Billfish and tuna, important commercial and recreational fish species, may be more vulnerable to fishing pressure because of shrinking habitat according to a new study published by scientists from NOAA, The Billfish Foundation, and University of Miami Rosenstiel School of Marine and Atmospheric Science.
An expanding zone of low oxygen, known as a hypoxic zone, in the Atlantic Ocean is encroaching upon these species’ preferred oxygen-abundant habitat, forcing them into shallower waters where they are more likely to be caught.
During the study, published recently in the journal Fisheries Oceanography, scientists tagged 79 sailfish and blue marlin with satellite tracking devices in the western North Atlantic, off south Florida and the Caribbean; and eastern tropical Atlantic, off the coast of West Africa. The pop off archival satellite tags monitored horizontal and vertical movement patterns. Researchers confirmed that billfish prefer oxygen rich waters closer to the surface and will actively avoid waters low in oxygen.
While these hypoxic zones occur naturally in many areas of the world’s tropical and equatorial oceans, scientists are concerned because these zones are expanding and occurring closer to the sea surface, and are expected to continue to grow as sea temperatures rise.
“The hypoxic zone off West Africa, which covers virtually all the equatorial waters in the Atlantic Ocean, is roughly the size of the continental United States, and it’s growing,” said Dr. Eric D. Prince, NOAA’s Fisheries Service research fishery biologist. “With the current cycle of climate change and accelerated global warming, we expect the size of this zone to increase, further reducing the available habitat for these fish.”
Less available habitat can lead to more fish being caught since the fish are concentrated near the surface. Higher catch rates from these areas may give the false appearance of more abundant fish stocks. The shrinking availability of habitat and resulting increases to catch rates are important factors for scientists to consider when doing population assessments.
Researchers forecast that climate change and its associated rise in ocean temperatures will further increase the expansion of hypoxic zones in the world’s oceans. As water temperature increases, the amount of oxygen dissolved in water decreases, further squeezing billfish into dwindling available habitat and exposing them to even higher levels of exploitation.
Ocean acidification, the result of roughly a third of global CO2 emissions dissolving into the seawater and lowering its pH, has complicated and poorly understood consequences for ocean ecosystems. Scientists already know that a drop in ocean pH affects the carbon cycle, reducing the carbonate ions that organisms like corals, mollusks and crustaceans use to build shells and external skeletons. Now, a new study shows that a CO2-induced increase in acidity also appears to disrupt the marine nitrogen cycle. The finding, to be published December 21 in the Proceedings of the National Academy of Sciences, could have ramifications for the entire ocean food web.
The authors of the study examined a specific step in the marine nitrogen cycle, called nitrification, in which microorganisms convert one form of nitrogen, ammonium, into nitrate, a form plants and other marine microorganisms require to survive. Previous research studies on experimentally acidified freshwater and in the laboratory have suggested that reduced pH slows nitrification, and one study in coastal ocean waters showed that large pH decreases did the same. But no one had tried to experimentally simulate the more subtle pH changes predicted to occur in oceans due to the increase in atmospheric CO2 expected over the next 20-30 years, says lead author J. Michael Beman, a professor of oceanography and biogeochemistry at the University of California Merced.
Beman and his colleagues collected samples (six in total) from four separate ocean research locations in the Atlantic and Pacific Oceans, and induced pH decreases ranging from 0.05 to 0.14 in the experimental samples””either by bubbling CO2 through the bottles or adding dilute acid. The experimental nitrification rates were then compared to those in the controls. In the bottles to which CO2 was added, explains Beman, “basically, we exposed them to the future atmosphere in terms of CO2 composition.” The group treated some samples with acid “to make sure the effect we were observing wasn’t driven by experimental approaches.”
Nitrification decreased, compared to controls, in all experimental cases, with the effect ranging from an 8 percent reduction to a 38 percent reduction. “What we saw is almost uniform across the ocean, or at least in all the experiments we conducted, which seems to suggest this is fairly consistent effect,” says Beman. Importantly, in some cases the change was quite large. “So it could have a pretty substantial effect on how nitrogen is cycled in the ocean,” he says.
One potentially positive effect would be a reduction of nitrous oxide””marine nitrification is a relatively big source of this greenhouse gas. “But the larger, much more difficult things to predict are the connections to other organisms and processes,” says Beman. Less nitrification would make fewer nitrates available to the plants and other organisms that use them to make vital proteins, making it more difficult for them to thrive. This in turn means less food would be available to the animals that eat those nitrate-using organisms, and so on up the food web. But the food web is complex, and the precise implications of the study’s results are still unclear, he says.