What is ocean acidification?

Ocean acidification, or “OA” is the name given to the lowering of the ocean’s pH due to the increase in atmospheric carbon dioxide (CO2) since the industrial revolution. The pH scale measures the acidity of a liquid and spans from 0 to 14. Liquids with a pH of zero are extremely acidic, while those with a pH of 14 are extremely basic. Freshwater is neutral and has a pH of 7, while lemon juice has a pH of about 2, and lye’s pH is 13. The pH scale is logarithmic, so a change from a pH of 8 to a pH of 7 means the solution has gotten ten times more acidic.

The ocean is slightly basic, with a natural pH of about 8.2. As carbon dioxide dissolves in water it forms carbonic acid, lowering the pH of the ocean. While the ocean is unlikely to become truly “acidic” (pH less than 7) , shell-making organisms struggle to make their shells with the pH decreasing. As OA progresses, shellfish, plankton, and even finfish are starting to show signs of stress. The pH of the ocean has changed from approximately 8.2 to 8.1 since the start of the industrial revolution, representing a 26% increase in acidity and a rate of change more than 100x faster than anything the Earth has seen for at least tens of millions of years. If CO2 emissions remain unchecked, the Intergovernmental Panel on Climate Change (IPCC) predicts that the ocean could become 150% more acidic over the next century.

Why are we at risk?

Alaska as a whole is especially vulnerable to OA due to our shallow seas, cold water (which can hold more CO2), high rates of primary production and large amount of glacial melt. A 2015 NOAA study on Alaska’s regional vulnerability to climate change and ocean acidification cited Southwest and Southeast Alaska as regions at the highest overall risk for negative economic and social effects from ocean acidification, as illustrated by the map to the right. The study noted both regions’ high dependence on healthy fisheries and lack of viable alternative employment opportunities. The study also noted each region’s high dependence on subsistence resources, most of which are predicted to be negatively impacted by OA.

In a nutshell, Alaska depends on its fish and fisheries, and Southeast Alaska depends on those resources more than most. OA is predicted to both decimate the small, shelled plankton that form the base of our marine food web and to directly limit growth rates and navigational ability in finfish. All shellfish species are also highly dependent on ocean pH and are likely to suffer as our waters become more acidic.

What are we doing in response to OA?

To measure current and long-term OA trends, the Sitka Tribe of Alaska will collect continuous pCO2, temperature, and salinity data using a Burke-o-Lator (BoL). Ocean water is pumped from Sitka Harbor through PVC pipes up into the lab where the BoL analyzes these parameters 24/7. The BoL can also measure the concentration of the mineral aragonite—a form of calcium carbonate that is critical to shell formation—as well as how much carbon dioxide gas is dissolved in the seawater and the total amount of carbon from non-organic sources.

The BoL also has the capability of measuring discrete samples from other locations, which is being utilized by SEATOR partners to observe how OA is affecting their communities. The short term goal of our ocean observing program is to help Tribal leaders develop ocean acidification and climate change adaptation plans for their communities. Only with current, high-quality information can we accurately predict and react to changes in marine traditional resources.

Ocean Acidification Data from Hoonah Harbor (as of 1/17/2020)

-Analyzed at Sitka Tribe of Alaska’s Environmental Research Lab using a Burke-o-Lator OA system

34 samples analyzed to date

54 samples are in inventory waiting to be analyzed


Since there are several large gaps in analyzed data it is difficult to identify strong trends in the OA data. About 50% of the samples have low Aragonite values (below 1), which indicates a difficult environment for shellfish to create shells.
This data is not intended to be published; It is for informational purposes only. Additional information is needed before data is ready

This data is not intended to be published; It is for informational purposes only. Additional information is needed before data is ready to be shared.

Definition of Terms

Sample Temp: Sample temperature at the time of sample collection

Salinity: Salinity as measured in the STAERL lab.

Analysis Temp: Sample temperature during analysis.

TA (Total Alkalinity): Measurement of all alkaline substances dissolved in water. Primarily Bicarbonates, carbonates, and hydroxides.

pCO2 @ SST: Partial pressure of CO2 at sea surface temperature.

pH: The “power of hydrogen” is a logaritmic scale (from 0-14) that expresses the acidity or alkalinity of a solution. A pH of 7 is neutral.

Ω Aragonite: Aragonite saturation state is a measure of the tendency for the mineral calcium carbonate (in the form or aragonite) to form or dissolve. An Ω value greater than 1.0 indicates supersaturation (favoring crystallization), while values less than 1.0 indicate undersaturation (favoring dissolution).

Graph Analysis

pCO2 @ SST: We typically expect to see a higher pCO2 in the colder months, and lower pCO2 in the warmer months. This depends on several factors, the main ones being primary production (ie photosynthesis) and temperature (ie cold water dissolves more CO2 than warm water).

pH: The ocean is slightly basic, with a natural pH of about 8.2. Since the industrial revolution, the pH of the ocean has fallen to about 8.1 on average. We are seeing more samples fall below this natural state, especially in winter conditions.

Ω Aragonite: Shellfish have a threshold of suitable aragonite saturation for growth at around 1.5-1.7 Ω. Anything below 1 Ω means shells tend to dissolve.

Total Alkalinity: A measurement of the concentration of alkaline substances, which buffer pH in the water by neutralizing acids. Higher concentrations can be caused by the dissolution of calcium carbonate, but generally we’d like to see values that indicate a healthy buffer to acidification (>1500).