Bay Conditions Index Scores

What does this mean?

Each bay receives an evaluation for the previous calendar year based upon the sampling values for three important indicators: chlorophyll a, nitrogen, and phosphorus. These measures of water quality were chosen because nutrient pollution is such an important focus of water resource management, especially where wastewater discharges or runoff from urban or agricultural areas is a concern. The Bay Conditions Report provides detailed information about these three primary water quality indicators, as well as other important measures of ecosystem health.

The rating for each bay is determined by the most recent sample values for these indicators, as compared to target and threshold levels defined for the individual bay. A threshold is an undesirable concentration which should not be exceeded; by contrast, a target is a desired level which, if achieved, should produce optimal seagrass growth and a productive coastal ecosystem.

The rating system used here was established by a team of local water resource management professionals and is based on work done by Janicki Environmental Inc. for the Sarasota Bay Estuary Program (SBEP) and the Charlotte Harbor National Estuary Program (CHNEP), to establish benchmarks for numeric nutrient criteria as required by the U.S. Environmental Protection Agency’s enforcement of the Clean Water Act. The criteria (thresholds) thus developed are now part of Florida Law. These rules may be updated over time; the current standards may be found in Florida Administrative Code, Chapter 62-302.530, “Table: Surface Water Quality Criteria”.

Specific goals defined in the scope of work for this project directed that the benchmarks established be:

The benchmark-setting strategy used by Janicki Environmental involved applying numerical analysis and models to a wealth of empirical data collected within Sarasota County's estuarine waters, including data on watershed characteristics, water chemistry, clarity, seagrass, and bathymetry.

For more information on the development of the targets and thresholds used here, read “Numeric Nutrient Criteria Recommendations for the Sarasota Bay Estuary Program” (Mar. 2011) and “Proposed Numeric Nutrient Criteria for the Charlotte Harbor National Estuary Program Estuarine System” (Sept. 2011) prepared for the SBEP and CHNEP, respectively, by Janicki Environmental, Inc.


How are the data collected? (Methods)

For each indicator the “average value” of samples taken during the previous year is compared to a threshold that has been defined for it by water resource managers. Depending on the bay and the indicator, this average is either an arithmetic or a geometric mean. See the “Calculations” section below for definitions.
» Learn More about the specific indicators, targets and thresholds used for each bay.

Sarasota County collects water quality samples from multiple depths, but Manatee County collects its Palma Sola Bay samples from near the surface. So, to make the bay comparisons in this section consistent, surface water quality samples are used for determining all bay ratings.


Calculations

The thresholds to which sample data are compared are those recommended to the U.S. Environmental Protection Agency as numeric nutrient criteria for water quality standards.

For a discussion of the empirical method used in their development, see “Empirical Approaches to Establishing Numeric Nutrient Criteria for Southwest Florida Estuaries”.

A geometric mean is found by multiplying the n sample values together and then taking the nth root of the product, as follows:

geometric mean formula

The more familiar arithmetic mean is found simply by adding the sample values together and dividing by the number of samples:

arithmetic mean formula


Caveats and Limitations

Intent: The Sarasota Bay Conditions Overview is meant to give an overall picture of ecosystem health for the area. While ratings are based on the official water quality standards used by resource managers, they are for information and education only and are not intended to imply regulatory compliance (or non-compliance).


Additional Information