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Ocean Acidification Effects on Two Predator-Prey Relationships
Reference
Appelhans, Y.S., Thomsen, J., Pansch, C., Melzner, F. and Wahl, M. 2012. Sour times: seawater acidification effects on growth, feeding behavior and acid-base status of Asterias rubens and Carcinus maenas. Marine Ecology Progress Series 459: 85-97.

Background
The authors correctly write that "the impact of seawater acidification on calcifying organisms varies at the species level," and they state that "if the impact differs between predator and prey in strength and/or sign, trophic interactions may be altered," which consequence, if true, could raise havoc with many marine ecosystems as currently constituted.

What was done
In a study designed to explore the potential for such interactions to occur within the brackish western Baltic Sea, Appelhans et al. investigated the impacts of three different seawater pCO2 levels (650, 1250 and 3500 ppm) on the growth of two predatory species (the common sea star Asterias rubens and the shore crab Carcinus maenas), in addition to which they determined whether the quantity or size of prey consumed (the blue mussel Mytilus edulis) was affected.

What was learned
As described by the five German scientists, "growth of Mytilus edulis was generally very low and not significantly affected by acidification." However, they say that "a trend toward a lower shell mass with increasing seawater pCO2 was observed," such that "the mean maximum breaking resistance of mussel shells was significantly lowered by ~20% at the highest level of 3500 ppm."

As for the predators, they report that "acidification did not provoke a measurable shift in prey size preferred by either predator." Also, they found that intermediate acidification levels (corresponding to 1250 ppm CO2) "had no significant effect on growth or consumption in either predator species," but they report that the highest acidification level (corresponding to 3500 ppm CO2) "reduced feeding and growth rates in sea stars by 56%, while in crabs a 41% decrease in consumption rates of mussels could be demonstrated."

What it means
"Interestingly," as Appelhans et al. conclude, "the enhanced vulnerability of mussels seems to be neutralized by the decreased consumption of the predators under high acidification." And they say that "these results illustrate that different stress effects on interacting species may not only enhance but also buffer community level effects," further emphasizing that "when stress effects are similar (and weak) on interacting species, biotic interactions may remain unaffected."

Reviewed 19 December 2012