Learn More: Creek Conditions Overview

The Creek Conditions Reports on the Sarasota County Water Atlas are designed to summarize water quality conditions in each of 17 creek basins. These creeks carry rainwater, along with any pollutants it has picked up, downstream to coastal bays. Each creek has one or more upstream segments that are primarily freshwater and one or more downstream segments that are predominantly marine. “Predominantly” because tidal influence causes the salinity in each segment to vary throughout the day. “Predominantly freshwater” and “predominantly marine” streams are assessed differently by environmental regulators, with different parameters used to evaluate water quality, and different pollution limits assigned.

Each Creek Conditions Report provides an annual evaluation for a single creek based upon a comparison of the results of water quality testing for each of its segments and established regulatory limits (see Caveats). These limits are also referred to as “Numeric Nutrient Criteria.” For all segments, the concentration of chlorophyll a, phosphorus, nitrogen and dissolved oxygen are used as quantitative measures of water quality. In addition to meeting Numeric Nutrient Criteria, both fresh and marine streams also must meet or exceed regulatory standards related to biological assessment (See Florida Administrative Code F.A.C. 62-302.531(2)(c) and F.A.C. 62-303.530(47)(b) ). The flowchart shown here summarizes the regulatory assessment process for streams in Florida. The reference to “site specific analysis” refers to a provision in state law allowing special assessment criteria for situations that are deemed exceptional. The “thresholds” referred to in the flowchart are the regulatory limits for pollutants.

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 Creek Conditions Reports provide detailed information about these primary water quality indicators, as well as other important measures of ecosystem health.

The rating system developed for the Creek Conditions Reports is based on water quality standards as required by the Clean Water Act. In Florida, the Department of Environmental Protection enforces the Act, and the criteria for streams are defined in Florida Statutes. 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”. The rating for each creek is assigned based on whether sample data are within Numeric Nutrient Criteria limits. Lower levels of phosphorus, nitrogen and chlorophyll a are desirable, whereas higher levels of dissolved oxygen are desirable. Ratings used in the Creek Conditions Reports are for information only and do not imply regulatory compliance or non-compliance. (See Caveats).

“Numeric nutrient criteria” are the pollutant concentration limits used by environmental regulators to determine whether a water body is “impaired” for its established uses (such as potable water supply, shellfish harvesting, recreation, navigation, agricultural use) These rules are defined in F.A.C. 62-302.532 and F.A.C. 62-302.533 and explained more fully in “Implementation of Florida’s Numeric Nutrient Standards”, dated April 2013.

Each creek receives a “Pass” or “Caution” rating based upon its water quality status. A creek receives a “Pass” rating only if the water quality in both its freshwater and marine stream components is within limits as described below. The standards used are based on limits defined in the Florida Administrative Code, referenced in the paragraphs shown.

To receive a “Pass” rating, measures of water quality in a creek’s freshwater stream components must meet all of the following conditions:

• Total Nitrogen (TN): The annual geometric mean of all sample values from predominantly freshwater stream segments must be below the regulatory standard of 1.65 mg/l. [F.A.C. 62-302.531(2)]
• Total Phosphorus (TP): The annual geometric mean of all sample values from predominantly freshwater stream segments must be below the regulatory standard of 0.49 mg/l. [F.A.C. 62-302.531(2)]
• Chlorophyll a (Chl a): The annual geometric mean of all sample values from predominantly freshwater stream segments must be below the regulatory standard of 0.020 mg/l. [F.A.C. 62-303.351(4)]
• Dissolved Oxygen Saturation (DO): The annual geometric mean of all sample values from freshwater stream segments must be above a minimum standard of 38%.* [F.A.C. 62-302.533(1)]

To receive a “Pass” rating, measures of water quality in a creek’s marine stream components must meet all of the following conditions:

• Total Nitrogen (TN): A numeric standard for nitrogen is not used in the assessment of predominantly marine streams. Instead, the Creek Conditions Reports use a trend analysis of all samples within a creek's marine stream segments. This trend analysis must show that TN is decreasing, or that there is no discernible trend. Specifics of the trend analysis are shown in the Calculations section below.
• Total Phosphorus (TP): A numeric standard for phosphorus is not used in the assessment of predominantly marine streams. Instead, the Creek Conditions Reports use a trend analysis of all samples within a creek's marine stream segments. This trend must show that TP is decreasing, or that there is no discernible trend. Specifics of the trend analysis are shown in the Calculations section below.
• Chlorophyll a (Chl a): The annual geometric mean of all sample values from predominantly marine stream segments must be below the regulatory limit of 0.011 mg/l. [F.A.C. 62-303.353(2)]
• Dissolved Oxygen Saturation (DO): The annual geometric mean of all sample values from predominantly marine stream segments must be above a limit of 56%. [F.A.C. 62-302.533(2)] *

*The regulatory limits for dissolved oxygen in streams are complex and related to salinity. See Caveats, below.

See the Calculations section below for how means and trends are computed.

Learn More about the evaluation methods and significance of these and other water quality measures »

The numeric nutrient standards to which sample data are compared were developed by the Florida Department of Environmental Protection. For a discussion of the empirical methods used in their development, see “Technical Support Document: Development of Numeric Nutrient Criteria for Florida Lakes, Spring Vents and Streams” (2012).

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

Trends for Total Phosphorus, Total Nitrogen and Dissolved Oxygen Saturation are determined by applying the seasonal Kendall Tau trend test (Hirsch et al., 1982; Hirsch and Slack, 1984; Reckhow, 1993) which performs a trend analysis for individual seasons of the year. It then combines the individual results into one overall test for whether the dependent variable (in this case TN, TP or Chl a) changes in a consistent direction over time. The implementation of the seasonal Kendall Tau trend test algorithm was performed by Janicki Environmental, Inc. The algorithm tests whether a statistically significant trend exists. If so, it returns a value of -2, -1, 0, 1, or 2 to show the degree of decrease/increase.

Intent: The Creek Conditions Reports are meant to give an overall picture of ecosystem health. While ratings are based on the official water quality standards used by resource managers, they are for information and education only. Regulatory compliance is determined by the Florida Department of Environmental Protection which incorporates additional factors, including biological health, into its assessment. Biological data regarding floral/faunal populations are not available at this time, and thus are not included in the Creek Conditions Report. If available, those data would give a more complete picture of a stream’s ecological health.

Dissolved Oxygen Saturation Regulations:

For waters such as tidal creeks that fluctuate between fresh and marine, the applicable regulatory criteria are complex. The applicable standard is determined by the location of the creek (its “bioregion”) and its salinity at the time of the DO saturation measurement. This means that freshwater criteria may apply at some times while marine criteria will apply at other times¹. While DO may be measured in milligrams/liter (mg/l) or as percent saturation (%), for regulatory purposes, percent saturation is used.

For freshwater streams in the peninsula bioregion where Sarasota is located, the regulatory minimum standard² for DO saturation is a daily average of 38% (not to be exceeded more than 10% of the time). The regulatory standard for DO saturation in marine waters is comprised of three parts:

1. a daily average of 42% (not to be exceeded more than 10% of the time),
2. a 7-day average of 51% (not to be exceeded more than once in any 12-week period), and
3. a 30-day average of 56% (not to be exceeded more than once per year).

For simplicity’s sake in creating the Creek Conditions Reports, we have elected to characterize each stream segment as either predominantly freshwater or predominantly marine¹. An annual geometric mean of all DO measurements in a basin’s predominantly freshwater segments is calculated and compared to the 38% minimum, and an annual geometric mean of all DO measurements in a basin’s predominantly marine segments is calculated and compared to the 56% minimum.

¹By Florida statute, marine criteria are used to assess streams when the chloride concentration of water is above 1,500 mg/l; this value is equivalent to specific conductance of 4,580 µmhos/cm, or salinity of 2.7 Practical Salinity Units (PSU).

²Note that because the DO standard is a minimum requirement, the limit is “exceeded” when the percentage falls below the limit. In other words, low oxygen conditions fail to meet the standard. Reference: F.A.C. 62-302.533

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• A Nonparametric Trend Test for Seasonal Data with Serial Dependence
• Effects of changing land use on the microbial water quality of tidal creeks
• Evolution of Tidal Creek Networks in a High Sedimentation Environment
• Final Report on the Development of a Tidal Creek Condition Index for Sarasota County
• Linkages between tidal creek ecosystems and the landscape and demographic attributes
• Naples Bay Tidal Creek Restoration Project Description
• Nutrients in the Nation’s Streams and Groundwater, 1992-2004
• Peconic Estuary Program Tidal Creek Study
• Report on Benthic Sampling for the 2007 Implementation of the Sarasota Tidal Creek Condition Index
• Report on Graphical Data Analysis for the 2007 Implementation of the Sarasota Tidal Creek Index
• Report on Preparatory Tasks 1-4 for Implementation of the Sarasota Tidal Creek Condition Index
• Responses of Tidal Creek Macrobenthic Communities to the Effects of Watershed Development
• Sarasota Bay Numeric Nutrient Criteria: Tidal Creeks (Letter Memorandum)
• Southwest Florida Tidal Creeks: Nutrient Management Framework and Indicator Development
• Streams in the Urban Landscape
• Techniques of Trend Analysis for Monthly Water Quality Data
• The Importance of Tidal Creek Ecosystems (preface)
• Tidal Creek & Salt Marsh Sediments in South Carolina Coastal Estuaries: Dist of Organic Contaminants
• Tidal Creek & Salt Marsh Sediments in South Carolina Coastal Estuaries: Distribution of Trace Metals
• Tidal Creek Ecosystems: Sentinel Habitats for Assessing Consequences of Rapid Coastal Development