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 Water Chemistry section shows sample history and trend information for each of the primary water quality indicators. The rating for each is determined by the most recent sample values for that indicator, 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. (See “Methods” section below for more information about each indicator.)
On each Bay Condition Report page, an individual indicator receives an EXCELLENT rating if its mean value is below the target, a GOOD rating if its mean value is above the target but does not exceed the threshold, and a CAUTION rating if the mean value exceeds the threshold.
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:
- objectively defined
- scientifically defensible
- geographically specific
- related to a valued natural resource
- linked to a human activity that can be managed
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.
The Five-year Rolling Average Graph illustrates the general tendency of each water quality indicator, using a six-month moving average. The graph plots the arithmetic mean of sample values collected during the previous six-month period. A moving average tends to moderate temporary spikes and dips in a graph and instead shows a general trend. The Method Detection Limit is the lowest concentration which can be measured by the sampling method used to monitor the indicator. The chart accompanying the graph gives an idea of the historic range of values for the indicator, and where data in the scored year fall with respect to that range. You can use the Data Download link to see all the individual data samples that were used in creating the ratings, charts and graphs shown on the Bay Condition Report.
In addition to the primary indicators used in establishing bay ratings, other measures of water quality are also displayed for the bay. These include dissolved oxygen, apparent color, biochemical oxygen demand, dissolved oxygen saturation, red tide,
light attenuation, ammonia, Kjeldahl nitrogen, nitrate/nitrite, pH, salinity, specific conductance, water temperature and turbidity. Rainfall, though not a measure of water quality, has an important influence on it. A heavy rain can cause a temporary spike in nutrient levels due to runoff from the adjacent watershed, followed shortly thereafter by a flush of algal growth.
The Bay Contour Maps help you to visualize spatial patterns in the water quality data. For a more detailed map, use the Water Quality Contour Mapping tool.
The Seagrass Map shows where seagrass beds are located, and how their extent has changed over time. Use the slider to view current and past mapping results.
The types of land use in a watershed have a major influence on the water quality in the lakes, streams and bays that receive runoff from it. The Land Use/Land Cover Chart shows the composition of the bay's watershed in terms of the types of activities that take place there. A map showing land use/land cover can be viewed using the Advanced Mapping tool.