Data gathered from 2009 to 2011 indicate that 34 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. This is a decrease of about 4 percent from the 2010 assessment.
What is dissolved oxygen? Laura Fabian from the Maryland Department of Natural resources explains how, just like humans, all underwater life in the Chesapeake Bay needs oxygen to survive.
Produced by Matt Rath
Music: “A Moment of Jazz” by Ancelin
When oxygen is in water, it is in a dissolved form. Just like animals on land, the Bay’s fish and shellfish need oxygen to survive. Dissolved oxygen concentrations are an important indicator of nutrient pollution and the Bay’s capacity to support life.
The necessary amount of dissolved oxygen varies by species, season and location in the Bay.
The Bay’s low dissolved oxygen levels are primarily the result of excess nutrient pollution, which fuels the growth of algae blooms. Algae eventually die and sink to the Bay’s bottom, where they are decomposed by bacteria in a process that depletes oxygen.
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 (1987-2011)
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 (2002-2011)
Statistically rigorous short-term trend analyses have not recently been conducted.
Change from Previous Year (2010 to 2011)
Goal achievement decreased from 38 percent (2008-2010 assessment period) to 34 percent (2009-2011 assessment period).
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
Recent refinements have been made in the water quality standards assessment methodology:
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.