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Dissolved Oxygen (Volume Assessment)

Data gathered from 2010 to 2012 indicate that 35 percent of the combined volume of open-water, deep-water and deep-channel water of the Bay and its tidal tributaries met dissolved oxygen standards during summer months.

Dissolved Oxygen, Percent Goal Achieved for 2012, 3 YR Analysis

Map: Dissolved Oxygen, Percent Goal Achieved for 2012, 3 YR Analysis

Date created: Jul 08 2013 / Download

The 2012 Dissolved Oxygen Indicator showing Percent of Goal Achieved using the three year analysis method.

July 22, 2011

What is dissolved oxygen? Laura Fabian from the Maryland Department of Natural Resources (DNR) explains why oxygen is so important to underwater life and how it is used to measure water quality in the Chesapeake Bay.

Produced by Matt Rath
Music: “A Moment of Jazz” by Ancelin


Just as is the case for animals on land, oxygen is essential for all aquatic plants and animals to survive. In water, oxygen is present in a dissolved form.  Adequate concentrations of dissolved oxygen in water are necessary for healthy ecosystem function; without the required amounts of oxygen in the water to support healthy ecosystem function, the Bay’s ability to support aquatic life is compromised.

The necessary amount of dissolved oxygen varies by aquatic species, season and location within the Bay. Generally, aquatic animals need higher oxygen levels in shallow waters during spring spawning season.  Slightly lower oxygen levels are acceptable during other times of the year, particularly in deeper waters.

Concentrations of dissolved oxygen are also an important indication of levels of nutrient pollution in the Bay.  Low dissolved oxygen levels are primarily the result of excess nutrient pollution, which fuels the growth of algae blooms. These algae eventually die and sink to the Bay’s bottom, where they undergo the natural process of bacterial decomposition. During this process, bacteria use up the oxygen present in the water, leaving little for fish, shellfish and other forms of aquatic life.  In general, greater quantities of excess nutrients being delivered to the Bay result in larger algae blooms within the receiving waters, leading to an increased amount of areas with low-oxygen concentrations (i.e., dead zones).


The goal is for 100 percent of the Chesapeake Bay and its tidal tributaries to meet Clean Water Act standards for dissolved oxygen.

Long-term Trend (1987-2012)

Statistically rigorous long-term trend analyses have not recently been conducted. Goal achievement has averaged 41 percent and has ranged from 27 percent to 68 percent.

Short-term Trend (2003-2012)

Statistically rigorous short-term trend analyses have not recently been conducted.

Change from Previous Year (2011 to 2012)

There was no significant change. Goal achievement during the 2010-2012 assessment period was 34.9 percent compared to 34.1 percent during the 2009-2011 assessment period.

Additional Information

Water Quality Standards

States have adopted water quality standards that reflect the oxygen needs of the Bay’s aquatic life. The standards vary with water depth, season and duration of exposure.

To meet state regulations, all data gathered within each tidal river and mainstem Bay segment must meet required dissolved oxygen concentrations, based on a combination of interpolation and cumulative frequency distribution (CFD) analyses. These analyses allow for some temporal and spatial exceedences of dissolved oxygen criteria.  However, if the designated use of a segment of the Bay has concentrations that exceed the permitted spatial and temporal allowances, the entire volume of water for that designated use in the given segment is considered out of attainment.

When assessing water quality, regulators examine conditions from the past three years to adjust for annual weather-driven fluctuations.

Water Quality Assessment Modifications

Refinements have been made in the water quality standards assessment methodology: 

  • In 2009-10, as part of the development of the Total Maximum Daily Load (TMDL), Bay Program scientists reviewed the methodology for assessing dissolved oxygen levels. They found instances where the science underlying the assessment tools had advanced.
  • The methodology was updated to incorporate these advances, which had the effect of changing the reference against which open water areas are measured.  As a result, some areas that were previously thought to have unacceptable amounts of hypoxia are now understood to have achieved our dissolved oxygen standards for the given time period. 
  • The technical description of the methodological changes is available in the Technical Addendum published in May 2010.
  • These modifications took effect for the 2010 Bay Barometer (published spring 2011) and are reflected in revised historic data (from 1985-2009).

Dissolved Oxygen and Nutrients

The settling and subsequent decomposition of algae are largely responsible for the Bay’s low dissolved oxygen levels. Thus, dissolved oxygen concentrations are an important indicator of nutrient loadings to the Bay and the capacity of the Bay to support aquatic life.

Over time, large-scale reductions in the amount of nutrients flowing into the Bay will help improve low oxygen conditions.

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