Ocean Acidification Linked to Declining Pacific Oyster Declines

For the past five seasons, millions of Pacific oysters have failed to reproduce successfully off the coast of Washington, causing a crisis for the industry. Oyster declines are a phenomenon not isolated to the Pacific Northwest; populations worldwide have been diminished by a combination of obvious factors such as coastal development, over-harvesting and water pollution from upstream sources. In the case of the Washington crisis, however, scientists believe that more may be at work than the usual suspects.

Wild oysters spawn in the shallow waters of bays and estuaries, producing larvae that must swim around and attach to a hard substrate. Often larvae will settle on the shells of other oysters, forming an oyster reef. Shellfish growers can then move the mollusks by hand to sheltered areas where they fatten for years until ready for market. For the past 5 years, however, few Washington oysters have survived that first, vulnerable phase between being spawned and settling on a surface.

Scientists testing water quality near the die-off sites found high levels of Vibrio tubiashii, an ocean-borne bacteria that targets larvae, and took this as the explanation. However, larvae failed to survive even after water-treatment systems were installed to control the Vibrio tubiashii, leading researchers to believe that there might be another factor contributing to the declines. Researchers noticed another abnormality: low pH levels in the water. Enter ocean acidification.

A new study from Smithsonian scientists, published 27 May in the journal PLoS ONE, discusses the specific threats to shellfish posed by changing water chemistry. Ocean acidification is to marine habitats as climate change is to terrestrial ecosystems: the environmental response to elevated levels of carbon dioxide. The ocean acts as an enormous carbon sink, absorbing about one third of the carbon that is released into the atmosphere. When CO2 interacts with water, it dissolves into carbonic acid that is then transformed into either carbonate or bicarbonate ions. According to the Smithsonian study, “increased acidity tips the balance toward bicarbonate formation and away from carbonate.” Oysters, as well as some other invertebrates, many plankton, and corals all use carbonate as the building block to produce shells and other calcified parts. Less carbonate in the water can translate into weak and deformed shells or coral structures prone to erosion.

In the Smithsonian study, lead scientist Whitman Miller placed larval oysters in "estuarine water that was held at four separate CO2 concentrations, reflecting atmospheric conditions from the pre-industrial era, the present, and those predicted in the coming 50 and 100 years" and then measured the growth over time. He found that one species of oyster had a significant decrease in calcium carbonate generation between pre- and post-industrial concentrations.

All this has led to speculation that ocean acidification may be contributing to the Washington oyster collapse. After all, larval oysters have shells made out of aragonite, which is weaker and easier to dissolve than the calcium carbonate that older oysters generate. Importantly, the Washington coast is the site of an upwelling of cold water that replaces surface water displaced by strong northerlies. Scientists have long predicted that deep, cold ocean water will be more acidic because it circulates less frequently and has more time to accumulate carbon dioxide. So the Washington coast may be experiencing pH levels that could become standard for the entire ocean as anthropogenic sources contribute more and more carbon to the ocean and acidification occurs at a faster and faster rate.

Production has improved in 2009 for oyster growers and hatcheries in the northwest. According to the Seattle Times, this may be related to the recent decrease in upwelling events.

source: Fishlink Sublegals