Bivalve Shells Buffer Ocean Acidification
Casco Bay Study Suggests Way to Counteract Clam Population Losses
May 20, 2009
Professor Mark Green of Saint Joseph’s College in Maine, led the research team which first made the connection between ocean acidification and juvenile clam (Mercenaria mercenaria) mortality.
They then showed that adding crushed clamshells to the shellfish beds reduced mortality levels in juvenile bivalves on their Southern Maine coastal study plots in the Casco Bay area.
The Connection Between Ocean Acidification and Shellfish Mortality
In a paper published in the July 2009 issue of Limnology and Oceanography, entitled Death by Dissolution: Sediment saturation state as a mortality factor for juvenile bivalves, Green et al showed that seed clams (0.2–0.6 mms) in sediment which is under-saturated with calcium carbonate and aragonite suffer very heavy mortality levels.
Most researchers had assumed that the loss of juvenile shellfish was a result of predation but this study was able to link losses of immature bivalves to shell dissolution in coastal areas where acidic sediments predominate. In addition, as the clams’ shells are weakened in this dissolution process, the bivalves are more easily preyed upon.
The clams suffering from dissolving shells continued to forage like the unaffected control animals in the study, so starvation is not likely to be the ultimate cause for the mortality. It is not clear yet whether bacterial infection, thermal issues, disruption of osmo-regulation or other problems are to blame. What is clear is that acid sediments, which are low in calcium carbonate and aragonite, do not support the smallest classes of shellfish. And ocean acidification, partially as a result of eutrophication and acid rain, is increasing the number of areas where sediment is under-saturated.
Crushed Clamshells Buffer Acid Sediments and Increase Juvenile Bivalve Survival
In another part of the study, adding crushed clamshell to a field study site in the Casco Bay area raised sediment alkalinity and increased the number of clams moving into the treated areas versus the control plots. The juvenile shellfish seem to be cued to move to this more optimum habitat. The clams may drawn to the shelter of the shells and receive the secondary benefit of lowered shell dissolution mortality or they may be drawn by the buffered sediment itself. Either way, this improved habitat increases survival rates.
There has been a focus on the effects of ocean acidification on high profile species such as corals, which are also threatened by direct human impacts from sunscreen use and diving. This study adds to that the significant environmental impacts that acidification has on shellfish. Beyond the commercial value of fresh shellfish for human consumption, many bivalves are important food sources for other marine species. Returning shells from bivalves harvested for human uses presents an elegant solution to one aspect of the ocean acidification problem.
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