Tuesday, February 4, 2014

Soft Shell Clams in the News, Pt 1

Soft-shell clams are important in Maine -- both ecologically and economically.  So it's  not surprising to see them in the news once in a while.  But recently I feel like I've seen a lot about them, here, and here, and here, to name a few.  So I thought I'd take some time to wade through these issues and explore their causes and what they might mean.



Bags of clams on the Freeport Town Docks.

Ocean Acidification:  Most of us are familiar with the threat of climate change, resulting from increased carbon dioxide in the atmosphere.  One consequence of high atmospheric CO2 levels that many people are unaware of is ocean acidification, but it's a relatively straightforward concept.  If you ever combined baking soda and vinegar, and were able to explain the resulting explosion of gas, you can understand ocean acidification.

1)  Atmospheric carbon dioxide concentration is increasing.  There is excellent evidence that atmospheric carbon dioxide levels are increasing consistently.  In 1960, there were 315 ppm of CO2 in the atmosphere; where as today the level is 396.81 ppm.  (The level considered safe and sustainable in terms of sustaining human culture and health is 350.  Yeah, that's a downer.  I would add that the vast majority of scientists, as in, virtually all, support the idea that our modern society is causing this increase in carbon dioxide.)    


The CO2 concentration last month was:


Atmospheric CO2


2)  Our oceans absorb about a third of the carbon in the atmosphere.  CO2 can exist as a gas in the atmosphere, but it also can dissolve into water, and is absorbed by our rivers, lakes and oceans.  The oceans have been an important buffer for the CO2 we've been pumping into the atmosphere, acting as a "sink".  

3)  Absorption of CO2 causes the oceans to become more acidic.  A few reactions go into this process, but the end result is a bunch of CO3 2- (carbonate) and H+ (positively charged hydrogen ions).  It's the conversion of the hydrogen in water to hydrogen ions that changes pH -- pH is a measure of how many of these H+ ions are around (a lower pH means a substance is more acidic and has more H+ ions).  Since before the industrial revolution, the oceans have dropped from a pH of about 8.2 to their present level of 8.1.  This might seem like a small difference, but the pH scale is logrhythmic, so this actually is a large change, in fact representing an increase in H+ ions of at least 25%.  

4) The end result of higher CO2 levels is a reduction in CO3 available for clams to make their shells.  Shellfish (and corals, urchins, and plankton, to name a few) have shells composed largely of CaCO3 -- calcium carbonate.  But they can only make these shells if the water is saturated with CO3.  With all the extra H+ ions around, CO3 (which is negatively charged) is hard to come by for clams.  Plus, their shells can dissolve in acidic water -- just like when you combine vinegar and baking soda (which is full of carbonate).  This is leading to thinner shells for many species (but some species actually do better with greater CO2 levels -- like lobster). 

On their way to the dinner table.
It's been a very busy week for me here at school -- a new semester is well underway, and this year my work load has doubled (no, my salary has not, unfortunately).  I think I'll save the green crab portion of this problem for next week!  Those papers aren't going to grade themselves, now, are they?

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