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Natural pH Fluctuations in a Coastal Environment

Paper Reviewed
Baumann, H., Wallace, R.B., Tagliaferri, T. and Gobler, C.J. 2015. Large natural pH, CO2 and O2 fluctuations in a temperate tidal salt marsh on diel, seasonal, and interannual time scales. Estuaries and Coasts 38: 220-231.

According to Baumann et al. (2015), projections of ocean acidification, in which the average pH of the open ocean is predicted to decline by 0.3 pH unit over the next century, have "heightened the need to better understand the sensitivity of marine organisms to low pH conditions." As a result, numerous ocean acidification experiments have been conducted on various marine organisms, producing a wide range of results. This "complexity of organism responses to elevated CO2," continue Baumann et al., "appears to stem, in part, from insufficient knowledge and thus appreciation of the scales of natural pH variability experienced by marine organisms in their habitats." Coastal environments, for example, generally experience greater fluctuations in pH than the open ocean. And since the majority of ecologically and economically important marine species spend a vast portion of their life cycles in coastal environments, the authors say there is a great need to "characterize [such] marine habitats in terms of their short- and long-term pH variability."

In an effort to fill this knowledge gap, the team of four American researchers embarked on a journey to "characterize the patterns and magnitudes of diel, seasonal, and interannual fluctuations in pH and dissolved oxygen (DO) in an undisturbed tidal salt marsh adjacent to Long Island Sound, using a multiyear, high-frequency data set." Flax Pond (40.96°N, 73.14°W), a 1 km2 tidal salt marsh located on the north shore of Long Island Sound, served as the specific study site, where data were collected between April 2008 and November 2012. And what did those data reveal?

As shown in Figure 1, large fluctuations in pH occurred at Flax Pond on both daily and seasonal time scales. The daily pH range varied from a low of 0.22 unit during the winter to a high of 0.74 unit in the summer. Seasonally, the highest pH values occurred in February (average of 8.19) and the lowest values (average of 7.59) occurred in August. Thus, average pH conditions in Flax Pond "decline from early spring until late summer by approximately 0.6 units" and "average diel pH fluctuations exceed 0.7 units and commonly approach one unit of magnitude in July and August." Yet even more extreme fluctuations in pH were found to occur within a single tidal cycle. The right panel of Figure 1 presents a three day record of detailed measurements that reveal a pCO2 fluctuation "between ~350 µatm and nearly 4,000 µatm within one tidal cycle." Thus, within a few short hours the marine life within Flax Pond was subjected to a pH fluctuation that reached values as low as 6.9, which is nearly 1 full pH unit below the predicted decline by the end of this century.

Figure 1. Left panel: monthly averages of the daily pH range from Flax Pond salt marsh, averaged across the period 2008-2012. Center panel: monthly mean pH values (black circles connected by the red line with ±1 SD shown in shading) from Flax Pond salt marsh averaged across the period 2008-2012. The blue line on the y-axis indicates the open ocean pH decline since the mid-1800s, while the red line indicates the projected decline between now and the end of the 21st century. Right panel: Variations in pH (lower panel, red line) and pCO2 (upper panel, green line) from Flax Pond salt marsh over five tidal cycles across a three day period in late July, 2012. Adapted from Baumann et al. (2015).

With respect to the cause of these large variations, Baumann et al. state they are chiefly biologic, the magnitude of which is "modulated by tides and the time of day, with the most acidic and hypoxic conditions occurring during low tide at the end of the night" due to community respiration. During the day, the pH rises as photosynthetic marine life assimilate CO2 and incoming water of a higher pH from the open ocean flows into the marsh from the incoming tide.

In discussing the implications of their findings, the American researchers note that the highly variable pH conditions observed in Flax Pond, though higher in magnitude, are characteristic of observations made in other coastal locations. However, such fluctuations have "yet to be adequately represented" in laboratory ocean acidification experiments, where the pH change is near unanimously held at a constant value and may falsely project a marine organism's response to ocean acidification out in the real world of nature.

Perhaps the most significant finding to be gleaned from this analysis, however, is that fact that despite current daily and seasonal pH fluctuations that often exceed -- and by more than two-fold -- the projected 0.3 unit decline that is expected by acidification alarmists to wreak havoc on marine ecosystems between now and the end of the century, this salt marsh is teeming with life, including the marsh's "main founding angiosperm Spartina alterniflora, ribbed mussels (Geukensia demissa), crabs (Seasarma reticulatum, Uca pugnax, Uca pugilator, and Dyspanopeus sayi), and forage fish like Atlantic Silversides, Menidia menidia, and Killifish, Fundulus spp. (Hovel and Morgan 1997), which comprise major prey items for transient predators like striped bass or bluefish (Tupper and Able 2000). Clearly," Baumann et al. continue, "such marsh organisms and their offspring cope with frequent periods of high CO2 and low oxygen conditions in their habitat, as well as with large diel to seasonal fluctuations in both parameters. Hence, many tidal salt marsh organisms and -- more generally -- coastal marine species may prove to be largely insensitive to elevated CO2 levels (Frommel et al. 2012; Hendriks et al. 2010; Hurst et al. 2013), particularly levels mimicking the predicted increase in average open ocean conditions over the next 300 years (i.e., up to 2,000 µatm; Riebesell et al. 2010)."

Frommel, A., Schubert, A., Piatkowski, U. and Clemmesen, C. 2012. Egg and early larval stages of Baltic cod, Gadus morhua, are robust to high levels of ocean acidification. Marine Biology 160: 1825-1834.

Hendriks, I.E., Duarte, C.M. and Álvarez, M. 2010. Vulnerability of marine biodiversity to ocean acidification: a meta-analysis. Estuarine, Coastal and Shelf Science 86: 157-164.

Hovel, K.A. and Morgan, S.G. 1997. Planktivory as a selective force for reproductive synchrony and larval migration. Marine Ecology Progress Series 157: 79-95.

Hurst, T.P., E.R. Fernandez, and J.T. Mathis. 2013. Effects of ocean acidification on hatch size and larval growth of walleye pollock (Theragra chalcogramma). ICES Journal of Marine Science 70: 812-822.

Riebesell, U., Fabry, V.J., Hansson, L. and Gattuso, J.P. 2010. Guide to best practices for ocean acidification research and data reporting. Publications Office of the European Union: 260.

Tupper, M. and Able, K.W. 2000. Movements and food habits of striped bass (Morone saxatilis) in Delaware Bay (USA) salt marshes: Comparison of a restored and a reference marsh. Marine Biology 137: 1049-1058.

Posted 16 December 2015