Free Ocean CO2 Enrichment

In situ, controlled perturbation experiments, often conducted over weeks to months, can provide inference concerning the response of natural communities to ocean acidification that is difficult or impossible to derive from laboratory experiments. Studies conducted in situ can include the effects of potentially important factors such as natural variation in planktonic food resources, larval abundance, changes in predators or competitors, as well as oceanographic conditions (e.g. changes in upwelling intensity). Drawing on the experience of Free Air CO₂ Enrichment (FACE) experiments used to investigate the response of terrestrial plant communities to rising atmospheric CO₂ levels, the scientific community has developed an analogous approach, Free Ocean CO₂ Enrichment (FOCE) experiments, for studying marine communities, and to complement a range of experimental methods and technologies for ocean acidification studies research. FOCE was first proposed and implemented by researchers at the Monterey Bay Aquarium Research Institute (MBARI).

As studies of the consequences of ocean acidification for marine organisms and ecosystems expanded rapidly over the past decade, the methods employed to evaluate the effects of expected future changes in ocean chemistry have become more sophisticated. Initial studies frequently involved measurements of the survival or physiological response of individuals of marine species to large changes in pCO2 or pH, while held in small containers under laboratory conditions. This approach increased the level of understanding of the effects of these environmental changes on individual species but provided little information concerning the response of natural assemblages of interacting species, in which the direct impacts of ocean acidification as well as their cascading indirect consequences (e.g. changes in the intensity of interaction strengths among predators or competitors) may be evident. Pelagic mesocosm experiments that examined the response of natural plankton communities to controlled pH perturbations helped move methods of ocean acidification research toward more comprehensive studies of whole communities and embedded processes under mostly natural conditions.[2] The FOCE approach represents an analogous advance for benthic assemblages, by allowing examination of the direct effects of acidification on particular species, but also potential changes in interactions among species. Moreover, FOCE methods provide precise control of pH, while allowing many other parameters to vary naturally. Like mesocosm studies, FOCE methods exploit the advantages of studying a natural community under mostly natural ranges of environmental variability.

The key elements of any FOCE experimental units are perspex, partially open, chambers, a CO₂ mixing system, sensors to continuously monitor ambient and chamber pH, and a control loop to regulate the addition of gases or liquids to each experimental chamber.

The carbonate chemistry of seawater can be manipulated using different approaches to mimic future conditions.[3] It is possible to directly inject gases (pure CO₂ or CO₂-enriched air) but this is more difficult than delivering water to achieve precise pH control. Current FOCE systems lower pH using metered addition of CO₂-enriched seawater into the experimental chambers. pH is controlled as a constant pH offset relative to ambient values, maintaining natural variability, or as a constant value.

Other approaches have been used to manipulate the seawater carbonate chemistry in the field. In pelagic mesocosm experiments,[2] the carbonate chemistry is generally altered at the beginning of the experiment and subsequently drifts as a function of biological processes and air-sea gas transfer. CO₂ bubbling in open water has also been used.[4] This approach does not enable precise control of the carbonate chemistry because it does not include a device to ensure full equilibration of added CO₂ in seawater and its precise control. There are no experimental chambers to regulate water flow, and thus allows for natural near-bottom flow conditions, but it generates highly variable pH under variable current speed or direction. This approach is therefore more similar to natural CO₂ vents than to FOCE systems. This approach can be useful when organisms can not be enclosed in chambers and when they inhabit environments such as estuaries where pCO2 levels are naturally hyper-variable. The approach has inherent limitations but may allow greater replication, at lower cost.

Current users of FOCE systems have organized to release guidelines and best practices information for future users. Furthermore, the Monterey Bay Aquarium Research Institute will release an open source package to transfer FOCE technology to interested researchers (xFOCE). This package will comprise all engineering information required to develop cost effective FOCE systems.

Future development of FOCE systems will include the study of the combined effects of ocean acidification and other environmental factors such as temperature or the concentration of dissolved oxygen.

7. Ainsworth E. A. & Long S. P., 2005. What have we learned from 15 years of free-air CO₂ enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO₂. New Phytologist 165:351-371.

8. Barry J. P., Buck K. R., Lovera C., Brewer P. G., Seibel B. A., Drazen J. C., Tamburri M. N., Whaling P. J., Kuhnz L. & Pane E., 2013. The response of abyssal organisms to low pH conditions during a series of CO₂-release experiments simulating deep-sea carbon sequestration. Deep-Sea Research Part II: Topical Studies in Oceanography 92:249-260.

• xFOCE
• eFOCE