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The Buffering of Coastal Marine Ecosystems from Ocean Acidification

Paper Reviewed
Bracken, M.E.S., Silbiger, N.J., Bernatchez, G. and Sorte, C.J.B. 2018. Primary producers may ameliorate impacts of daytime CO2 addition in a coastal marine ecosystem. PeerJ 6: e4739, DOI 10.7717/peerj.4739.

As reported in a new paper by Bracken et al. (2018), "the range in pH values recorded in many coastal systems is greater than the pH decline predicted by the year 2100." This situation occurs as low-pH waters from the deep ocean are transported to the coast via upwelling, resulting in pH values of the surface ocean that are lower than that predicted in even the most pessimistic of future ocean acidification scenarios. In addition, fluxes of dissolved inorganic carbon tend to be substantially greater in coastal systems because of ecosystem photosynthesis and respiration. Consequently, Bracken et al. write that "predicting the impacts of ocean acidification in coastal habitats is therefore complicated by the biogeochemical processes mediated by the resident biota, which drive local-scale pH variability," and which can "reduce some of the impacts of ocean acidification in coastal ecosystems."

Hoping to learn more about this complicated interplay in coastal ecosystem seawater pH, the team of four scientists set out to examine the impacts of the addition of pCO2 on the carbonate parameters of a rocky intertidal shoreline and how those parameters are influenced by the resident biota. To accomplish this design, Bracken et al. artificially increased the pCO2 content of ten macrophyte-dominated tide pools located on the northern side of Horeshoe Cove in the Bodega Marine Reserve in Sonoma County, California, USA, for eight hours on two consecutive days during low tide in the spring of 2016.

Results of their study revealed that "daytime changes in pH, pCO2, net ecosystem calcification (NEC), and O2 concentrations were strongly related to rates of net community production (NCP)." More specifically, they report that adding CO2 to the pools during daytime low tides "should have reduced pH and enhanced pCO2" of the tidal pools; but it did not. Instead, Bracken et al. found that "photosynthesis rapidly reduced pCO2 and increased pH," so that adding CO2 into the tide pools did not reduce the seawater pH, because the extra CO2 was absorbed by the biota (particularly macrophytes) via increased photosynthesis. Consequently, the effects of adding CO2 (i.e., ocean acidification), were, in the authors' words, "modified by feedbacks between NCP, pH, pCO2, and NEC," which modifications, they say, "underscore the potential importance of coastal macrophytes in ameliorating impacts of ocean acidification."

It would thus appear that adding CO2 via so-called ocean acidification can actually ameliorate the carbon limitation that exists in many coastal ecosystems, which addition further enhances their rates of photosynthesis and raises surrounding seawater pH during the day. And that chain of occurrences suggests that ocean acidification is not the boogeyman that many make it out to be.

Posted 14 September 2018