Paper Reviewed
Engstrom-Ost, J., Holmborn, T., Brutemark, A., Hogfors, H., Vehmaa, A. and Gorokhova, E. 2014. The effects of short-term pH decrease on the reproductive output of the copepod Acartia bifilosa - a laboratory study. Marine and Freshwater Behavior and Physiology 47: 173-183.

Copepods consist of a group of small crustaceans that are found in the sea, some of which are planktonic (drifting in sea water), some of which are benthic (living on the ocean floor), and some of which migrate between the sea surface and ocean floor, during which activity they can encounter large pH differences, as has been noted by Feely et al. (2010). In addition, Wootton and Pfister (2012) have found that upwelling along coastal areas and estuaries can lead to pH values that vary by as much as 0.25 pH unit in a 24-hour period plus a much larger 1.5 units, seasonally, over a 10-year period. Also, copepods perform diel vertical migration (DVM), which in the summer exposes them to a pH change greater than one full pH unit between surface and deeper waters, as has been documented by Fabry et al. (2008) and Olafsson et al. (2009), which according to Engstrom-Ost et al. (2014), is "due to benthic respiration, limited photosynthesis and low CO2 exchange with the atmosphere."

Desiring to learn more about the abilities of copepods to cope with such rapid and large changes in oceanic pH, the latter team of three Finnish and three Swedish scientists collected a large number of them via vertical net hauls from depths of 25 meters in the western Gulf of Finland, which they brought back to their laboratory in order to study the effects of pH variability on a number of copepod bodily functions. And what did they thereby learn?

Working with the species Acartia bifilosa, Engstrom-Ost et al. found there was actually a stimulating effect of lowered pH on their recruitment, which they said was indicative of the fact that "A. bifilosa copepods are tolerant to a significant pH gradient, similar to that experienced during DVM, upwelling episodes and changed mixing conditions in the Baltic Sea." And they thus concluded that "A. bifilosa "may have considerable capacity to adapt to future pH decline, such as may occur with ocean acidification," because they say "it is already adapted to pH gradients and shifting pH environments."

References
Fabry, V.J., Seibel, B.A., Feely, R.A. and Orr, J.C. 2008. Impacts of ocean acidification on marine fauna and ecosystem processes. ICES Journal of Marine Science 65: 414-432.

Feely, R.A., Alin, S.R., Newton, J., Sabine, C.L., Warner, M., Devol, A., Krembs, C. and Malpoy, C. 2010. The combined effects of ocean acidification, mixing, and respiration on pH and carbonate saturation in an urbanized estuary. Estuarine and Coastal Shelf Science 88: 442-449.

Olafsson, J., Olafsdottir, S.R., Benoit-Cattin, A., Danielsen, M., Arnarson, T.S. and Takahashi, T. 2009. Rate of Iceland Sea acidification from time series measurements. Biogeosciences 6: 2661-2668.

Wooton, J.T. and Pfister, C.A. 2012. Carbon system measurements and potential climatic drivers at a site of rapidly declining ocean pH. PLOS ONE 7: e53396.