by Melissa Foley, Science Early Career Fellow and Erin Loury, Science Communication Intern.

Ocean acidification, or the increase of low-pH, corrosive ocean waters, certainly qualifies as a process that impairs water quality.   Now the Environmental Protection Agency (EPA) has created guidance to address ocean acidification under the Clean Water Act. 

On November 15^th, the EPA released a Memorandum to guide regions and states in reporting the impacts of ocean acidification under Section 303(d) of the Clean Water Act, which requires states to submit a list of impaired water bodies that do not meet water quality standards, and develop total maximum daily loads (TMDLs) for these waters.  A TMDL is a calculation of the maximum amount of a pollutant that a body of water can receive and still safely meet water quality standards.  In the case of ocean acidification, the main pollutant in question is carbon dioxide (CO₂).

Ocean acidification is quickly being recognized as the “other” CO₂ problem in addition to climate change.  What creates an acid ocean?  As atmospheric CO₂ continues to increase, the ocean is also absorbing more CO₂ from the atmosphere.  Dissolved CO₂ alters the ocean’s carbon cycle, making it harder for animals like molluscs and corals to form their carbonate shells and skeletons.   More dissolved CO₂ also increases the concentration of hydrogen ions in the ocean, which in turn decreases the pH, thereby creating “ocean acidification.”

Since preindustrial times, the pH of the ocean has dropped from 8.21 to 8.10, a decrease of nearly 25%. This decrease is largely attributed to three main causes: increased atmospheric CO₂; changes to ocean circulation and biological processes (such as increases in oxygen minimum zones); and local pollution and run-off sources.  These drivers of ocean acidification are not uniform throughout the ocean – rather, ocean acidification “hotspots” that range from tens to hundreds of miles in size are forming in multiple areas of the ocean.

Such hotspots appear to be most rapidly developing in the coastal ocean, where other impacts such as nutrient pollution can increase the rate of pH decline and contribute to increased cumulative stress on marine organisms. Coincidentally, the coastal ocean is where ocean acidification is likely to have the greatest effect on natural and human populations by impacting species of high ecological and economic importance, such as corals and oysters.  This highlights the need for states to address ocean acidification at the local level, rather than waiting for a global policy that would reduce atmospheric CO₂ emissions.

Currently, approximately half of the coastal states and territories have established marine pH water quality criteria for marine aquatic life based on the EPA’s recommended national criterion for pH.   The EPA recommends that ocean pH not exceed a range of 6.5 to 8.5 by more than 0.2 units.  Despite having quantitative criteria in place, only a handful of coastal states and territories have the high-resolution instruments necessary to measure the baseline pH level, determine the natural level of pH variation, and actually track changes in pH.  As a consequence, no states or territories have developed TMDLs for impaired waters. In addition, basic research on the consequences of decreased pH on marine organisms is not comprehensive, and has tended to focus on coral reefs and shellfish.   Finally, we need to develop comprehensive bioassessment methods to track the impacts of ocean acidification.

Although this new EPA Memorandum does not impose new regulations for pH in the ocean, it is still an important step in recognizing ocean acidification as a serious problem for our ocean and marine resources.  It also gives the go-ahead to states and territories that have access to reliable pH data to include acidified waters in their 303(d) list and to prioritize the development of TMDLs if the data allow.

The EPA issued a number of recommendations outlining how states can begin to incorporate ocean acidification into their 303(d) integrated reporting framework: (1) Request and gather existing and readily available data related to ocean acidification, including temperature, salinity, dissolved oxygen, nitrate, total alkalinity, and pH; (2) Develop assessment methods for evaluating impacts of ocean acidification on marine waters based on existing pH and biological water quality criteria; (3) Track the progress of federal efforts to develop assessment and monitoring methods for ocean acidification including the National Ocean Council and the Interagency Working Group on Ocean Acidification; (4) Develop bioassessment methods and/or biocriteria for aquatic resources where data are available; and (5) prioritize TMDL development with regards to ocean acidification.

This Memorandum was issued as part of a settlement agreement between the EPA and the Center for Biological Diversity [CBD vs. EPA, No 2:09-cv-00670-JCC (W.D. Wash)]. In this lawsuit, CBD challenged EPA’s approval of Washington State’s 2008 303(d) list because it did not include coastal waters as impaired for low marine pH.   Recognizing the problem of ocean acidification and addressing its causes on the local and regional scale is becoming increasingly important as coastal populations grow and the reality of a global agreement on CO₂ emissions wanes.  This EPA Memorandum gives states the latitude to address ocean acidification issues at a scale that is relevant to their marine resources and the health of their coastal waters.