Planetary Boundary on Ocean Acidification Breached: What It Means and What, if Anything, Can Be Done.

Figure 5. The pH, temperature, tidal height and salinity of seawater measured in situ at ∼ 1 m below low water at Chowder Bay, Sydney over a multi-day period. Grey and white boxes indicate the time between sunset and sunrise. Red dots indicate values of reference samples collected adjacent to the device and measured using spectrophotometry

But like many phenomena, there is more to ocean acidification than meets the eye, and analogous to the relationship between the atmosphere and climate change, the relationship between the oceans and OA is a highly complex one. As you might have expected, the pH of the sea is not uniform, as there are several factors apart from atmospheric CO2 that contribute to pH. Biological processes such as photosynthesis and respiration have a direct and immediate impact on pH. When marine organisms that conduct photosynthesis extract CO2 from the water, they rapidly decrease its acidity, which raises its pH.

On the other hand, when Cells respire, they release CO2 into the water, which lowers its pH. This daily oscillation is clearly visible in the pH graph at the upper left (Runcie et. al, 2018, Technical note: Continuous fluorescence-based monitoring of seawater pH in situ, Biogeosciences, 15, 4291–4299), that shows pH increasing as the day progresses, but dropping sharply after sunset every night. Temperature and, by extension, geographic location also influence how much CO2 the ocean can absorb, and therefore, how much acidification will occur. Colder water can absorb more CO2 than warmer water, which is why polar and colder, higher-latitude waters tend to have a lower pH and higher acidity. Ocean currents and circulation patterns also affect the distribution of dissolved CO2 and can lead to local variations in pH.

As can also be seen in the lower right diagram above, sea water pH has a weak, but significant, direct correlation with salinity. In general, the more saline the water, the higher the pH - in other words, the saltier the water, the more alkaline (less acid) it will be. This factor plays a major role in oceans bordering desert regions where evaporation rates are high, or, at the opposite extreme, in seas that are fed by major rivers or melting ice, which add copious amounts of fresh water, reducing ocean salinity and consequently, lowering pH. However, to be very clear, although the CO2 in the atmosphere is increasing the acidity of our oceans, the oceans are still very much alkaline, meaning that the current acidification is gradually decreasing the alkalinity of the oceans (from an average pH of 8.2 during the pre-industrial era to a current average pH of around 8.1). So cutting GHG emissions should be the overarching priority.

Interestingly, some of the richest and most productive marine ecosystems of the world function optimally at a pH below (i.e., more acidic than) what would be considered the average pH of the open ocean. Coral reefs, which are among the most sensitive to ocean acidification, are healthiest at an average pH of between 8.0 and 8.4. Coral reefs experience significant diel (daily) fluctuations in pH and dissolved oxygen due to the balance of photosynthesis and respiration. During the day, photosynthesis by coral-bound algae (zooxanthellae) consumes carbon dioxide, leading to a higher pH and higher oxygen levels. At night, photosynthesis stops, but respiration continues, consuming oxygen and releasing carbon dioxide, which lowers pH and creates hypoxic (low oxygen) conditions. Corals can tolerate daily fluctuations in pH to around 7.8 but are negatively impacted by the long-term effects of ocean acidification. Higher sea-surface temperatures compound this effect by causing corals to expel the algae (zooxanthellae) living in their tissues causing the coral to turn completely white (coral bleaching). When this occurs, the carbon dioxide from coral respiration is not depleted by algal photosynthesis and contributes to the acidification.

Estuaries are another biodiversity-rich marine ecosystem that thrive under naturally lower pH ranges (on average, ranging between a pH of 7.0 to 8.6). Estuarine pH varies significantly, from fresh river water (around 7.0) to more saline seawater areas, influenced by freshwater inflow rates, sediment load, tides, and biological processes. Twice each day, in response to the gravitational pull of the moon, high tides bring alkaline seawater into the estuary, increasing the pH, and roughly six hours later as the tides ebb, freshwater reclaims the estuary, lowering the pH once again. Mudflats, which are generally exposed only during low tide, are another natural coastal ecosystem with slightly more acidic conditions (pH 7.7) than open ocean water. Serving as a rich habitat for molluscs, crustaceans and burrowing worms and even fish, mudflat pH is influenced by factors like sediment alkalinity, organic matter breakdown, and the presence of other minerals and ions.

In the same way, Mangrove ecosystems tend to be marginally more acidic than open seawater - averaging a pH of 8.04, but have been observed to have lower pH levels as well. Like estuaries, mangrove pH levels can be influenced by tidal cycles and the release of organic matter from the mangroves, leading to lower pH levels compared to offshore seawater. Seagrass beds, likewise, thrive under fluctuating pH levels (fluctuating between 7.9 to 8.9) than the open oceans. Because of the high rates of photosynthesis in the seagrass meadows during the daylight hours, these marine ecosystems have a buffering effect, helping to stabilize the water chemistry compared to non-vegetated areas.

While the overall trend of ocean acidification is causing a global decline in ocean pH, which is impacting all marine ecosystems, the complexities across ecosystems, and their natural ranges of pH, as affected by salinity, temperature and biodiversity, remain areas of interest and study. In particular, the ecosystem services provided by vegetation such as mangroves and seagrass, together with macro and microalgae, in regulating daily pH variability through the balance between photosynthesis (which increases pH) and respiration (which decreases pH) point to the importance of understanding, protecting and preserving healthy and functional marine ecosystems as an effective buffer to localized OA.