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Climate Change

A regional perspective on ocean acidification

Article: Persistent spatial structuring of coastal ocean acidification in the California Current System. Chan, F.,Barth, J. A.,Blanchette, C. A.,Byrne, R. H.,Chavez, F.,Cheriton, O.,Feely, R. A.,Friederich, G.,Gaylord, B.,Gouhier, T.,Hacker, S.,Hill, T.,Hofmann, G.,McManus, M. A.,Menge, B. A.,Nielsen, K. J.,Russell, A.,Sanford, E.,Sevadjian, J.,Washburn, L.
Scientific Reports, 7, 2526 (2017), doi: 10.1038/s41598-017-02777-7

 

Think Locally

When you search topics online relating to climate change, ocean acidification is a common result. It has also been featured multiple times here on Oceanbites including two recent articles about the impacts of ocean acidification on sharks and predator behavior. The majority of research to date focuses on species impact and global changes. Scientists have a clear understanding of how global CO2 changes will impact ocean acidification. However, the nuanced changes on a local scale are a different story.

In this study, researchers measured pH in the intertidal waters off the western coast of the United States. They found a wide variety in pH across the northern California and Oregon coast (Figure 1). The goal of their study was to look at this spatial pattern to see if coastal management programs could be amended to better protect the ecosystem moving forward.

Methods

Chan et al. measured intertidal pH from April to October over two years (2011-2013). Figure 1 shows the sampling locations along the coast. They also measured dissolved oxygen (DO), temperature, and salinity.

Figure 1: Data from Chen et al. a) Map of study sites with magenta lines representing coastal shelf depths of 75 m, 100 m, 200 m, b) wind-driven cross-shelf surface transport from January (J) to December (D), c) Event scale changes in pH for the 2013 upwelling season with the white * representing the current global surface ocean pH, d) daily wind stress at 44.65oN matching the period in c, e-g) the lower 5th percentile which represents the most severe pH values for the three years, h) pH variability. The white lines in c and e-h represent the station locations in a. (Source: Chen et al., 2017)

During the 2011 sampling season, a NOAA West Coast Ocean Acidification cruise was nearby and their pH measurements were used as well to look at continental shelf water.  You can look at the cruise sample sites here.

How low can you go?

The current global mean surface ocean pH is 8.1. The lowest value Chen et al. found was 7.43 (a change of about 350%)! Overall 18% of the recorded values over the sample period were below 7.8. A pH of 7.8 is expected in the global surface ocean for an atmospheric CO2 concentration of 850 ppm (Do you know what the value is today? It was 406 ppm on June 29th). Remember that pH is measured on a logarithmic scale so a change of 0.1 equals about a 30% change. You can learn more about the pH scale and the impacts on the ocean here.

Not a straight line

Researchers found that there was a wide variety in pH in both time and space. There were daily pH changes up to 0.8 units and event (2-10 days) and seasonal (15 – 40 days) cycles. This could be a problem for marine life because rapid changes in pH are stressful. Further work is needed to investigate how marine life adapts to natural fluctuations in pH to understand how an overall lowering in pH will impact these species. Figure 2 compares these local changes (solid circles) to a larger survey (open circles) that represents tropical, temperate, and polar surface waters. The trend shows that there is a greater variability in pH at the lower pH values measured on this study. This variance is due to a combination of local surf-zone and offshore factors.

Figure 2: pH variability (represented by coefficient of variability, cv) versus the pH minimum. Open circles are from a survey spanning tropical, temperate, and polar ocean surface waters. Closed circles are from this study. (Source: Chen et al., 2017)

One major offshore process is upwelling and downwelling. Upwelling brings cold, low-pH, nutrient rich water from deep in the ocean. These deep ocean waters have more CO2 in them, thus the lower pH, and are nutrient rich because of sinking nutrients collected as the water travels around the world. As expected, when winds are favorable for upwelling, the intertidal pH decreases quickly. During a downwelling event, which brings warm higher pH water, there is an increase in pH.

What about the shells?

There are numerous negative biologically impacts of ocean acidification. One is the dissolution of calcium carbonate. This will impact any ocean creatures that form calcium carbonate shells, including corals and oysters. One way scientists keep track of how the ocean’s changing chemistry impacts these shells is with aragonite saturation state a) which is the degree to which the seawater is saturated with respect to calcium carbonate. As long as the aragonite saturation state is greater than 1 calcium carbonate formation is favorable. An aragonite saturation state value of 1.7 is the commercial threshold for oyster larvae success. When it falls below this value larvae death becomes more likely.

Chen et al. found ~63% of the estimated aragonite saturation state fell below the 1.7 threshold. There were sample periods at all sites where the aragonite saturation state fell below 1.  At one of the most acidified sites (SH, Figure 1) 16% of the time the aragonite saturation state was below 1. It should be added that researchers had to estimate the aragonite saturation state values. Researchers used temperature measured at each site and mean salinity and alkalinity (measured at one site) for an indirect measurement.  Chen’s group is confident in their aragonite saturation state values because there was a strong correlation (r2 = 0.99) compared to bottled samples.

Think globally

Figure 3: Littlenecks collected from the water (Source: Wikicommons)

This study offers a glimpse into a regional community that will be impacted directly by ocean acidification.  Based on the above results, it would be advantageous for scientists to set-up other long-term monitoring sites at coastal upwelling regions around the world. Chen et al. also suggest that biological studies are needed to understand how this fluctuating pH will influence marine life. Clearly, predicting how local ecosystems will respond to ocean acidification is a challenge.

Who cares what happens to oysters off the western coast of the United States? These oysters, and other shellfish, are part of a large industrial system relying on their survival (Figure 3). Ocean acidification will hurt marine life and human livelihoods. Fisherman, clam diggers, oyster farmers, and more rely on marine life to support themselves and their community. It will be critical to understand the links between local and global changes in ocean acidification moving forward.

Victoria Treadaway
I am a PhD candidate at the Graduate School of Oceanography at the University of Rhode Island. I am an atmospheric chemist studying organic acids in the troposphere to better understand their role in ozone processing. I flew on a Gulfstream V and a C-130 all in the name of science!

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