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Water Quality Standards Achievement for Water Clarity/Underwater Bay Grasses

Results of the 2013 to 2015 assessment period indicate that 5.2 percent of the 91 tidal water segments of the Chesapeake Bay that contain the shallow-water habitat designated use met water quality standards for water clarity/underwater grasses.

July 22, 2011

Clear water is critical to underwater life. Bay grasses need sunlight to grow, and fish need sunlight to see. But what factors cause water clarity to fluctuate? Adam Davis from the Chesapeake Research Consortium explains, and uses a secchi disc to measure water clarity in Spa Creek.


Excess nutrients and sediment are among the leading causes of the Chesapeake Bay’s poor health. Both can impact the clarity of water and the amount of algae and oxygen it contains. This indicator measures the achievement of water quality standards for water clarity in the Bay.

Water quality standards for the Bay were developed in 2003 by the U.S. Environmental Protection Agency (EPA). Within these standards, five aquatic habitats—also called “designated uses”—were identified, each with its own criteria for dissolved oxygen, water clarity/underwater grasses and chlorophyll a. If the Bay and its tidal tributaries are to function as a healthy ecosystem, all designated uses must meet all clean water criteria.


In 2014, the Chesapeake Bay Program adopted a goal to reduce pollution and achieve the water quality necessary to protect human health and support the region’s underwater resources.

Additional Information

Collecting data

Since the 2006-2008 assessment period, water clarity assessments are only conducted on a biennial basis. Furthermore, these assessments are only conducted for one set of segments in the Bay, which rotates every three years. When water clarity assessments are not available for any segment, the attainment of the shallow-water bay grasses designated use is assessed using the measured underwater grass acres meeting the segment-specific restoration acre goals. When water clarity assessment data are available, the shallow-water bay grasses designated use is considered in attainment if sufficient acres of underwater grasses are observed within the segment or if enough acres of shallow-water habitat meet the water clarity criteria to support the restoration of the desired underwater grass acreage for that segment.

Analyzing data

The methodology used to calculate this indicator considers the achievement or non-achievement of water quality standards for water clarity/underwater grasses in the 91 tidal water segments that contain the shallow-water habitat designated use.
This indicator uses a surface area-weighted approach, which takes the relative size of each segment into account and ensures we report the best available measure of water quality standards achievement in the Bay.

Reporting data

The University of Maryland Center for Environmental Science reports a water clarity indicator that tracks mid-channel data in all tidal waters, not in relation to Bay water quality standards. The indicator is updated annually and is available online.

Source of Data

Chesapeake Bay Program

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