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 an ALERT 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” and Florida Administrative Code, Chapter 62-302.532, “Estuary-Specific Numeric Interpretations of the Narrative Nutrient Criterion”.
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.
The Five Year Trend 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, light attenuation, salinity 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.
The targets and thresholds to which sample data are compared are those recommended by the Sarasota Bay Estuary Program to the U.S. Environmental Protection Agency as numeric nutrient criteria for water quality standards. For details, refer to “Numeric Nutrient Values for Sarasota Bay,” published in August 2010: Executive Summary Full Report
Dona/Roberts Bay is located within the Charlotte Harbor National Estuary Program's study area; recommended targets and thresholds for the bay were developed with CHNEP's guidance and are defined in the document “Proposed Numeric Nutrient Criteria for the Charlotte Harbor National Estuary Program Estuarine System”, Sept. 2011.
Chlorophyll a: Chlorophyll is one of the most important pigment groups. Chlorophylls give a characteristic green color to plants, algae and photosynthetic bacteria, which use the energy captured by chlorophylls (and other pigments) along with carbon dioxide and water to assemble the molecules needed for growth, cellular repair and disease prevention.
Chlorophyll a is the most common type of chlorophyll, and its concentration in bay water is used to estimate the relative abundance of phytoplankton. When water contains excess nutrients, it may cause an overgrowth of phytoplankton, reducing the clarity of the water and inhibiting the penetration of sunlight to the bay bottom, thereby adversely affecting seagrass growth. Seagrass beds provide essential habitat for macroinvertebrates, juvenile fish, manatees, and other creatures and are a hallmark of healthy coastal ecosystems. (Learn more about coastal water clarity)
The chlorophyll a rating is found by comparing a mean of all chlorophyll a sample data recorded during the calendar year to the targets and thresholds below. (See Calculations section, below.)
*No target value has yet been established for chlorophyll a concentration in Lower Lemon Bay.
Nitrogen: Although it is an essential nutrient for plants and animals, an excess amount of nitrogen in a waterway can lead to low levels of dissolved oxygen and negatively affect various plant life and organisms. Sources of nitrogen include wastewater treatment plants, runoff from fertilized lawns and croplands, failing septic systems, runoff from animal manure storage areas, and industrial discharges that contain corrosion inhibitors.
Nitrogen availability is most often the limiting factor in the growth of algae and other plants in estuarine and coastal systems, and that is the case in Sarasota's bays, which are said to be “nitrogen-limited”. That is, the amount of nitrogen present in the water is the controlling factor in whether or not algae grows (and chlorophyll a levels increase). Higher concentrations of nitrogen are related to human population density and activity. Levels may spike temporarily after heavy rainfall due to atmospheric deposition of nitrogen and increased runoff from the watershed. Incidental releases of poorly-treated effluent or stormwater may also result in a temporary, locally restricted increase in nitrogen levels.
There are three forms of nitrogen that are commonly measured in water bodies: ammonia, nitrates and nitrites. Total Nitrogen (TN) is the sum of total kjeldahl nitrogen (organic and reduced nitrogen), ammonia, and nitrate-nitrite. It can be derived by monitoring for total kjeldahl nitrogen (TKN), ammonia and nitrate-nitrite individually and adding the components together.
The TN rating is found by comparing a mean of all TN sample data recorded during the calendar year to the targets and thresholds below. (See Calculations section, below.)
*No target value has yet been established for TN concentration in Lower Lemon Bay.
**TN target and threshold values are based on chlorophyll a targets and thresholds. The relationship between chlorophyll a and TN concentrations in Sarasota Bay is particularly complex, depending upon location, ambient water color, and season. Therefore the threshold and target values for TN are calculated on an annual basis, incorporating the most recent values for water color in the north and south bay. The calculation is a geometric mean that incorporates arithmetic mean values for color, grouped by region (north or south) and season (wet or dry). The precise formula for calculating the annual threshold value is defined in “Florida Administrative Code 62-302.532, Estuary-Specific Numeric Interpretations of the Narrative Nutrient Criterion”.
Phosphorus: Both nitrogen and phosphorus are necessary nutrients for plants and animals. Nitrogen is usually the most important limiting nutrient in estuaries, driving large increases of microscopic phytoplankton called “algal blooms” or increases of large aquatic bottom plants, but phosphorus can become limiting in coastal systems if nitrogen is abundant in a biologically available form. (A “limiting nutrient” is one whose lack of availability reduces the rate of growth or prevents the growth of phytoplankton or other primary producers.) Sarasota's bays are typical, in that they are nitrogen-limited.
Sources of phosphorus include soil and rocks, wastewater treatment plants, runoff from fertilized lawns and cropland, runoff from animal manure storage areas, disturbed land areas, drained wetlands, water treatment, decomposition of organic matter, and commercial cleaning preparations.
Total Phosphorus (TP) is a measurement of all forms of phosphorus in a water sample, including orthophosphate, condensed phosphate, and organic phosphate.
The TP rating is found by comparing a mean of all TP sample data recorded during the calendar year to the targets and thresholds below. (See Calculations section, below.)
*No target value has yet been established for TP concentration in Lower Lemon Bay.
In addition to nutrient and chlorophyll a concentrations, dissolved oxygen levels and water clarity are also objective indicators of bay health. In addition to basic nutrients, adequate sunlight and oxygen are necessary for good biological productivity in coastal bays. Both plants and animals in a bay need oxygen to survive, and the seagrasses which provide food and cover for bay creatures need light for photosynthesis. The ability of a coastal bay to provide these needs is determined by complex interactive cycles which are affected by rainfall, temperature, water clarity, and tidal action.
Dissolved Oxygen: Dissolved oxygen (DO) is one of the most important indicators of water quality. It is essential for the survival of fish and other aquatic organisms. Oxygen dissolves in surface water due to the aerating action of winds. Oxygen is also introduced into the water as a byproduct of aquatic plant photosynthesis. When dissolved oxygen becomes too low, fish and other aquatic organisms cannot survive.
The colder water is, the more oxygen it can hold. As the water becomes warmer, less oxygen can be dissolved in the water. Salinity is also an important factor in determining the amount of oxygen a body of water can hold; fresh water can absorb more oxygen than salt water.
Oxygen levels also may be reduced when there are too many bacteria or algae in water. After the algae complete their life cycle and die, they are consumed by bacteria. During this decay process the bacteria also consume the oxygen dissolved in the water. This can lead to decreased levels of biologically available oxygen, in some cases leading to fish kills and death to other aquatic organisms.
Light Attenuation: Adequate light is needed for good seagrass growth. Seagrass meadows provide critical habitat for recreationally and commercially important fish and invertebrate species and are also important feeding areas for the Florida manatee. They serve to improve water quality by reducing nutrients in the water column and are an important component of the energy and nutrient cycles in coastal environments.
Seagrasses typically grow at depths no deeper than six to eight feet, and while light requirements vary by seagrass species, the availability of light is one of the limiting factors on the depth at which seagrasses can be found. Light intensity at the water's surface varies by season, cloud cover and inversely with depth of water.
Light attenuation, the rate at which light decreases with depth, depends upon the amount of light-absorbing dissolved substances (mostly organic carbon compounds washed in from decomposing vegetation in the watershed) and the amount of absorption and scattering caused by suspended materials (soil particles from the watershed, algae and detritus). Thus light attenuation is related to turbidity (see below).
The percentage of the surface light absorbed or scattered in a 1 meter long vertical column of water, is called the vertical extinction coefficient. This parameter is symbolized by “k”.
Prior to 2012, Manatee County collected light attenuation data using a method that measured light availability in selected wavelengths that are used in photosynthesis (PAR). This data is not readily converted into the vertical extinction coefficient model used by Sarasota County, so for consistency we have chosen not to display it here. Beginning in 2012, vertical extinction coefficient data is available for all bays included in this report, and will be displayed.
Rainfall: Urban runoff is the number one cause of coastal pollution in populated areas. Heavy rainfall releases nutrients from soil and washes pollutants like oil, grease, pesticides and herbicides into stormwater drains. The pollution content of rainwater runoff is greatest during the first minutes of a storm as all standing deposits are washed away. High bacterial loads in urban runoff can also lead to beach closures, reducing recreational opportunities.
Shortly after a heavy rainstorm, it is common to see a spike in nutrient levels in water bodies, followed soon thereafter by a corresponding increase in chlorophyll a, as algae grows in response to the flush of nutrients entering the bay from the watershed.
Rainfall data shown on the Bay Conditions Report page are collected from within the watershed and reflects conditions in the area near the bay, but may not precisely match rainfall levels within the bay itself.
Salinity: Salinity is an important indicator for a number of reasons. Patterns of salinity in coastal bays reflect the relative influx of fresh water from rivers and of marine water supplied by exchange with the ocean. The ability of water to absorb oxygen is affected by its salinity; fresh water can hold more oxygen. Salinity determines what species are present in an area; most aquatic organisms, both plants and animals, function optimally within a narrow range of salinity. Salinity also impacts water clarity, affecting the tendency of particulate matter to settle to the bottom, and on the ability of bottom sediments to bind to pollutants.
Salinity is described as a ratio and has no units. The Practical Salinity Scale (PSS) compares the conductivity ratio of a sea water sample to a standard potassium chloride solution.
Turbidity: This measure of water clarity, or murkiness, is an optical property that expresses the degree to which light is scattered and absorbed by materials in suspension. These materials include colored dissolved organic matter (soils) and suspended sediment, organic detritus, and living organisms. Turbid water has reduced light penetration, inhibiting the growth of seagrasses, and can cause dissolved oxygen levels to be reduced, when bacteria consuming organic particles create an increase in biological oxygen demand.
Coastal erosion and sedimentation can contribute to turbid conditions and poor water quality. Erosion is the detachment of soil particles from the land surface by natural forces such as rain and wind storms and by anthropogenic influences. Sedimentation occurs when eroded soil from the watershed is deposited by runoff into surface waters. Sedimentation rates in estuaries are naturally high, but human activity (poor soil conservation practices and altering natural water circulation patterns) can accelerate the process. Human activity such as dredging and boat traffic also can cause increased turbidity, as bottom sediments are disturbed and resuspended.
Turbidity is shown in Nephelometric Turbidity Units (NTUs). A Nephelometer, also called a turbidimeter, estimates how light is scattered by suspended particulate material in the water. It uses a photocell set at 90 degrees to the direction of the light beam to estimate scattered, rather than absorbed, light. This measurement generally provides a very good correlation with the concentration of particles in the water that affect clarity.
A geometric mean is found by multiplying the n sample values together and then taking the nth root of the product, as follows:
The more familiar arithmetic mean is found simply by adding the sample values together and dividing by the number of samples:
The formula for calculating the annual threshold value for Total Nitrogen in Sarasota Bay is defined in “Florida Administrative Code 62-302.532, Estuary-Specific Numeric Interpretations of the Narrative Nutrient Criterion”.
Intent: The Bay Conditions Report is meant to give an overall picture of ecosystem health for the bay. 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). Assessment of whether a particular indicator has exceeded its regulatory limit is performed solely by the Florida Department of Environmental Protection, and depictions on the Atlas are for information purposes only. More information »
Method Detection Limit: A 2007 report by the National Water Quality Monitoring Council found that nutrient concentrations in many estuaries and coastal areas of the U.S. are near or below most laboratory detection limits. The Method Detection Limit (MDL) for a particular analytical process is the lowest concentration at which the substance being measured can be identified in the water column. Phosphorus, in particular, occurs at relatively low levels of concentration, and is therefore harder to measure precisely and more likely to fall below the MDL. The Bay Conditions Report for individual bays gives the MDL for those water quality indicators whose measured value is likely to fall below it. When this occurs, the MDL is substituted in the calculations used to determine bay and water quality ratings; the value stored in the water quality database is not changed.
Data sufficiency: The annual Total Nitrogen target and threshold values for Sarasota Bay are calculated using a formula that incorporates color data. When there is a data insufficiency (not enough color samples, or samples that use a mix of different methods), target and threshold values cannot be calculated. “Official” targets and thresholds may use only data collected by the Florida Department of Environmental Protection.