Coral reefs that don’t decline with lower pH means that lower pH is bad for coral reefs?

The Woods Hole Institute studied coral growth in a reef system with naturally lower pH’s and found healthy communities and seem to try conclude the opposite for future predicted ocean acidification.  The Palau Rock Islands lagoons are naturally acidic from subsurface venting.  The lagoons are known for their natural diversity species and corals.  The Wood’s Hole study is the latest of a number of studies of this lagoon. The abstract from the Science article is below.

Ocean acidification threatens the survival of coral reef ecosystems worldwide. The negative effects of ocean acidification observed in many laboratory experiments have been seen in studies of naturally low-pH reefs, with little evidence to date for adaptation. Recently, we reported initial data suggesting that low-pH coral communities of the Palau Rock Islands appear healthy despite the extreme conditions in which they live. Here, we build on that observation with a comprehensive statistical analysis of benthic communities across Palau’s natural acidification gradient. Our analysis revealed a shift in coral community composition but no impact of acidification on coral richness, coralline algae abundance, macroalgae cover, coral calcification, or skeletal density. However, coral bioerosion increased 11-fold as pH decreased from the barrier reefs to the Rock Island bays. Indeed, a comparison of the naturally low-pH coral reef systems studied so far revealed increased bioerosion to be the only consistent feature among them, as responses varied across other indices of ecosystem health. Our results imply that whereas community responses may vary, escalation of coral reef bioerosion and acceleration of a shift from net accreting to net eroding reef structures will likely be a global signature of ocean acidification.

They found a pH range from ~8.0 to ~7.6 standard units and Ω-aragonite ranges from ~3.7 to ~1.9.  They found some species variations that couldn’t really be assigned to pH differences and were similar to other reefs.  In general, they found a healthy, thriving coral system.  The negative seems to be increased “macrobioerosion” with decreasing pH.

Bioerosion is caused by organisms and can be supplemented by physical and chemical processes of erosion. The article doesn’t discuss the biological causes but states that it correlated with pH. It seems that bioerosion hasn’t stopped the reef from being healthy and striving and the authors don’t spend much time discussing bioerosion’s other potential causes or how other reefs compare.  Ω is an index of saturation calculated from calcium and carbonate ions and the solubility product (Ksp).  An Ω = 1.0 indicates the solution is saturated.  Ω>1 is supersaturated and the greater the number the more likely calcium carbonate will precipitate and the better for shell formation.  Ω=4 is supposedly ideal.

The reef is supposedly a natural lab to look at the future of ocean acidification since the pH range and Ω and a predictor of future ocean acidification from climate change.  The results seem to be opposed to the standard scare, but the article would have us believe otherwise.  The Wood’s Hole news release says the corals are thriving despite acidification.  In the news release they use the standard 0.1 pH unit decrease is a 30% increase in acidification. This is a (rounded) calculation straight from the definition of pH (pH=-log[H+] which, on rearrangement, gives the hydrogen ion concentration.  Any 0.1 decrease in pH is a 26% increase in hydrogen ion concentration (acidity).  On the pH scale, 7 standard units is neutral, below 7 is acidic and above 7 is basic.  Since the ocean pH is generally >7, it is basic.  Climate science needs to claim “acidification” so they have defined less basic as acidic.  Adding acid to a basic solution to decrease the pH was once called neutralization.  If I added acid to reduce the pH below 7, I was acidifying the solution.  It looks like definitions change to suit the need.  Wood’s Hole’s sea water chemistry is here.  And a pretty good write up on carbonate water chemistry is here.

It looks like these corals grow despite the accepted theory and have been growing for a long time since the Rock Islands are limestone.  The conclusions could be ocean acidification isn’t the problem here instead of growing “despite ocean acidification.”


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s