What is ocean acidification?

Ocean acidification refers to the process of seawater pH decreasing (“increased acidity”) as the ocean absorbs CO2. As atmospheric CO2 levels have increased since pre-Industrial times (the mid-1700s), the oceans have absorbed approximately 30% of anthropogenically-produced CO2. As a consequence, ocean pH has dropped from 8.2 to 8.1. While this may not seem like a major change, the logarithmic scale of pH means that this change actually represents a 30% increase in acidity.

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A graphic representation of the chemical changes produced by adding CO2 to water to cause ocean acidification (decreasing ocean pH). The top row represents the effects of ocean acidification on a pteropod, a typical shelled organism that will have trouble building its shell under lower pH (Image produced by the NOAA Pacific Marine Environmental Laboratory).

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(Top row) Past, current and future modeling of ocean surface pH along the California Current System for 1750, 2005 and 2050. Warmer colors represent lower pH values. (Bottom row) Aragonite saturation values at a depth transect off Central California, indicated by the white line in A-C for 1750, 2005 and 2050. Warmer colors represent lower aragonite saturation values, which have been found to correlate directly to decreased capacity to build calcium carbonate shells (Graphics reproduced from: Gruber, Nicolas, et al. (2012). Rapid progression of ocean acidification in the California Current System. Science 337.6091: 220-223).

Additional drivers

Ocean acidification due to increasing CO2 uptake by the oceans is compounded by additional localized drivers. These include:

  1. Increased regional-scale upwelling of high-nutrient, high-CO2 waters with increasing winds. Wind strength has increased in recent years, driving more persistent upwelling. The deep water that surfaces also contains CO2 absorbed by the oceans 30-50 years ago, meaning that our current CO2 production will have an effect for years to come.

  2. Nutrient inputs from land-based run-off. These inputs drive biological respiration, which in turn decreases O2 in the water.

  3. Nitrogen oxide and sulfur oxide production from motor vehicles, ships and electrical utilities. These gases produce similar acidifying effects to CO2.

(Information from the Washington State Blue Ribbon Panel on Ocean Acidification document Ocean Acidification: From Knowledge to Action).

Impacts of Ocean Acidification along the West Coast

In the mid-2000s, shellfish farmers in Oregon and Washington began to notice the effects of ocean acidification as they watched their shellfish larvae struggle to grow and form sufficient calcium carbonate shells. After several summers of die-offs, the farmers, along with scientists at the NOAA Pacific Marine Environmental Laboratory, identified increasingly low-pH waters as a major deterrent to shellfish growth (Barton et al., 2012).

Shellfish hatcheries can now buffer their incoming seawater to produce pH levels optimal for larval growth, allowing them to combat acidic upwelling events in order to stay in business. Adult shellfish that are transplanted to open bays may be at increasing risk, however, as pH levels in upwelled waters continue to decrease to levels at which even tougher, fully-grown adult shellfish cannot handle.

In the meantime, West Coast agencies, including the WCGA, are continuing to help link shellfish farmers to available science and resources, to monitor these changes and to make decisions about how to mitigate the effects of ocean acidification.

Links to resources