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Seeing the Light: A Water Clarify Index for Integrated Water Quality Assessments

Lisa M. Smith and Linda C. Harwell

United States Environmental Protection Agency
Gulf Ecology Division
1 Sabine Island Drive
Gulf Breeze, Florida 32561

The Environmental Protection Agency Environmental Monitoring and Assessment Program (EMAP) uses water clarity as an indicator in integrated water quality assessments. After the publication of the first National Coastal Condition Report, the national water clarity reference value of 10% light at one meter was reevaluated and modified to reflect expected differences in regional reference light conditions. These regional differences range from naturally turbid estuaries like those found in Mississippi and Louisiana areas, to clear water estuaries expected to support extensive beds of submerged aquatic vegetation, for example, Florida and Tampa Bays. For the second National Coastal Condition Report, water clarity was assessed based on regional reference values. Not all states collected transmissivity at one meter; however, all states reported secchi depth. Different regional water clarity reference values and data collection initiated the development of a water clarity index based on light attenuation coefficients (k). This index incorporates reference conditions and is interchangeable with secchi depth measurements and transmissivity. Evaluation of the water clarity index shows that k values based on transmissivity at 1 meter can be estimated from secchi depth measurements and successfully used as a surrogate for transmissivity data collected using light meters. The development and application of this index are presented at both a local and regional scale.

Keywords: Water clarity, water quality, EMAP, transmissivity, secchi, light attenuation coefficient

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