Creek Condition Reports

What does this mean?

The Creek Conditions Reports are designed to summarize water quality conditions in 17 tidal creek basins. These coastal creeks, many of which are fed by drainage canals, flow downstream to bays and eventually to the Gulf of Mexico. Each creek has upstream segments that are freshwater, and downstream segments that are predominantly marine. “Predominantly” because tidal influence causes the salinity in each segment to vary throughout the course of the day. “Predominantly freshwater” and “predominantly marine” streams are assessed differently by environmental regulators, with different parameters used to assess water quality, and different pollution limits assigned.

The Water Chemistry Ratings section shows sampling history and trend information for several primary water quality indicators. The rating for each is determined by the sampled values for that indicator for the year shown. (See “Methods” section below for more information about each indicator.)

The rating system used is based on Numeric Nutrient Criteria adopted by the Florida Department of Environmental Protection in accordance with the Clean Water Act. These rules may be updated over time; a summary of the current standards may be found in Florida Administrative Code, Chapter 62-302.530, "Table: Surface Water Quality Criteria".

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 for a given laboratory method. 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 “Download Data” link to see all the individual data samples that were used in creating the ratings, charts and graphs shown on the Creek Conditions Report.

Water quality data for predominantly marine stream segments are aggregated and rated separately from the data for predominantly freshwater segments. Each individual water quality parameter is rated for both the upstream (freshwater) and downstream (marine) portions of a creek, and based on these the entire creek is given a rating. Learn More about how these ratings are assigned »

In addition to the primary indicators used in establishing creek ratings, other parameters related to water quality also are displayed. These include salinity, turbidity, rainfall and oysters.

According to the Florida Administrative Code:

“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, or agricultural use). These rules are defined in F.A.C. 62-302.532 and 62-302.533 and explained more fully in “Implementation of Florida’s Numeric Nutrient Standards”, dated April 2013.


How are the data collected? (Methods)

Water Chemistry Ratings

Water quality data were collected by Sarasota County Environmental Services, Charlotte County, the Florida Department of Environmental Protection, the Florida Department of Agriculture and Consumer Services, and the Charlotte Harbor Estuaries Water Quality Monitoring Network. Total Nitrogen, Total Phosphorus, Chlorophyll a, Dissolved Oxygen Saturation, Turbidity and Salinity were measured for freshwater and marine stream segments.

In addition to considering “numeric” standards for water's chemical and physical properties, regulators also apply a “narrative” standard. This standard states that “in no case shall nutrient concentrations of a body of water be altered so as to cause an imbalance in natural populations of aquatic flora or fauna.” (See F.A.C. 62-302.531(2)(c) and F.A.C. 62-303.530(47)(b)). Complete 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 ecological picture of stream health.

Chlorophyll a: This is a specific form of chlorophyll used in photosynthesis. Measuring Chlorophyll a is a way to indirectly determine the amount of photosynthesizing plants found in a water sample. “Corrected” measurements of Chlorophyll a adjust for the presence of a substance called pheophytin, a natural degradation product of chlorophyll which may interfere with the accurate measurement of Chlorophyll a.

The rating for Chlorophyll a is determined by comparing the annual geometric mean of all sampled values to a standard. A geometric mean is consistent with the Surface Water Quality Standards, Florida Administrative Code 62-302.531, Numeric Interpretations of Narrative Nutrient Criteria, and Florida Administrative Code 62-303.351, Nutrients in Freshwater Streams.

Chlorophyll a standards are expressed in micrograms per liter (µg/l). For freshwater streams, the maximum allowable concentration of Chlorophyll a is 20 µg/L. For predominantly marine streams, it is 11 µg/L.

The graph shows a monthly geometric mean for Chlorophyll a, along with its regulatory standard.

Nitrogen: There are three forms of nitrogen that are commonly measured in water bodies: ammonia (NH3), nitrates (NO3) and nitrites (NO2). “Total nitrogen” is the sum of total kjeldahl nitrogen (ammonia and its reaction products), nitrates, and nitrites. While nitrogen occurs naturally in water, too much of it can contribute to the excessive growth of algae, which in turn can decrease the amount of oxygen available to aquatic life or lead to toxic algal blooms.

For freshwater stream segments, the rating for TN is determined by comparing the annual geometric mean of its sampled values to a standard of 1.65 milligrams per liter (mg/L). (See Calculations section below.) A geometric mean is less affected by data anomalies, dampening the influence of very high or very low values. A geometric mean is consistent with the Surface Water Quality Standards, Florida Administrative Code 62-302.531, Numeric Interpretations of Narrative Nutrient Criteria, and Florida Administrative Code 62-303.351, Nutrients in Freshwater Streams.

There is no corresponding Total Nitrogen numeric standard for predominantly marine streams. Instead, the Creek Conditions Report uses a trend analysis of all TN samples within a creek’s marine stream segments. For a creek to receive a “Pass” condition, the trend for the creek must show that TN is decreasing, or that there is no discernible trend. More information about the trend analysis used can be found in the Calculations section, below.

The graph shows a monthly geometric mean for TN. For freshwater stream segments, the regulatory standard is shown. For marine stream segments, a trend line is shown.

Phosphorus: Total Phosphorus (TP) is a measure of all the forms of phosphorus in a water sample (orthophosphate, condensed phosphate, and organic phosphate). Some Florida soils are naturally high in phosphorus, but when too much of this essential element enters surface waters, it can contribute to the excessive growth of algae, which in turn can decrease the amount of oxygen available to aquatic life or lead to toxic algal blooms.

For freshwater stream segments, the rating for TP is determined by comparing the annual geometric mean of its sampled values to a standard of 0.49 milligrams per liter (mg/L). (See Calculations section below.) A geometric mean is less affected by data anomalies, dampening the influence of very high or very low values. A geometric mean is consistent with the Surface Water Quality Standards, Florida Administrative Code 62-302.531, Numeric Interpretations of Narrative Nutrient Criteria, and Florida Administrative Code 62-303.351, Nutrients in Freshwater Streams.

There is no corresponding numeric standard for predominantly marine streams. Instead, the Creek Conditions Report uses a trend analysis of all TP samples within a creek’s marine stream segments. For a creek to receive a “Pass” condition, the trend for the creek must show that TP is decreasing, or that there is no discernible trend. More information about the trend analysis used can be found in the Calculations section, below.

The graph shows a monthly geometric mean for TP. For freshwater stream segments, the regulatory standard is shown. For marine stream segments, a trend line is shown.

Dissolved Oxygen Saturation: Dissolved oxygen (DO) in surface water is used by all forms of aquatic life and is therefore an important measurement for assessing the health of streams. Low dissolved oxygen is the most common cause for impairment of streams in Florida. A confounding factor in understanding dissolved oxygen is that some healthy waterbodies have naturally low oxygen levels, such as wetlands or springs. Inflows from groundwater or adjacent wetlands can cause low oxygen conditions to occur, even in streams that are biologically sound. Salinity and temperature affect the amount of oxygen that water can hold; freshwater can absorb more oxygen than saltwater, and cold water can absorb more oxygen than warm water. Low dissolved oxygen is not a direct pollutant; rather, it is a response to either anthropogenic (human-originated) pollutants or to specific natural conditions (e.g., natural decomposition of leaf litter, stagnant flow, etc.).

For waters such as tidal creeks that fluctuate between fresh and marine, the applicable regulatory criteria are determined by the location of the creek (its “bioregion”, see map) and its conductivity/salinity at the time of the DO 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 mg/L or as percent saturation (%), for regulatory purposes, percent saturation is used.

Regulatory criteria for assessing dissolved oxygen levels in tidal streams is complex because of fluctuating salinity. For freshwater streams in the peninsula bioregion where Sarasota is located, the regulatory standard2 for DO is a daily average of 38% (not to be exceeded more than 10% of the time). The regulatory criteria for 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% standard, and an annual geometric mean of all DO measurements in a basin’s predominantly marine segments is calculated and compared to the 56% standard.

The graph shows a monthly geometric mean for DO saturation, as well as the regulatory standard.

1Marine criteria are used to assess streams when the specific conductance of water is above 4,580 µmhos/cm, or chloride concentration is greater than 1,500 mg/L.

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.


Other Measures of Stream Health

Other factors affecting water quality in streams are listed below. These parameters are included in the Creek Conditions Report in order to place the water quality parameters above in a more comprehensive environmental context.

Salinity: Salinity is an important indicator for a number of reasons. Patterns of salinity in coastal water bodies 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, as many aquatic 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.

The salinity measure used here 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. It is important to realize that salinity in coastal creeks fluctuates as a result of tidal influence.

Turbidity: This measure of water clarity (or the lack of it) 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 submerged vegetation, and can cause dissolved oxygen levels to be reduced when bacteria consuming organic particles create an increase in biological oxygen demand.

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 (human) 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.

Oysters: Oysters have long been recognized as key bio-indicators of the ecological health of marine and estuarine ecosystems. Changes in oyster health can provide an early warning of potential adverse impacts associated with hydrological alterations occurring throughout the watershed. Monitoring those changes is a simple, cost-effective tool to document those changes and allows watershed managers to quickly identify and minimize those impacts. The Sarasota County Oyster Monitoring Program was initiated in 2003, and expanded in 2006, to examine the role of Crassostrea virginica (Eastern Oyster) as an environmental indicator of watershed health. Oysters are monitored at the end of the dry season (spring) and again at the end of the wet season (fall). At each site, oysters are collected and placed in five gallon buckets for counting, and the number of live oysters, dead oysters, and spat (small juveniles) are recorded. Learn more about oysters and how they are monitored in Sarasota County.

Watershed Factors Affecting Water Quality

Rainfall: Urban runoff is a major cause of coastal pollution in populated areas. Heavy rainfall carries nutrients, bacteria, fertilizer, grease, pesticides and herbicides into stormwater drains, which flow to streams, lakes, bays and oceans. 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 water body from its watershed.

Rainfall data shown in the Creek Conditions Report are collected from within the watershed and are accurate data but cannot precisely match rainfall in a specific location because rainfall varies throughout the watershed. The source of the rainfall data shown in the graphs is the Sarasota County Automated Rain Monitoring Sensors (ARMS). Data from individual rainfall monitoring stations can be viewed/downloaded using the Water Atlas Real-Time Data Mapper.

Annual and monthly Radar-based Rainfall Estimates for individual basins and watersheds are also available on the Sarasota County Water Atlas.

LULC

Land Use/Land Cover: Land use within a watershed has a major effect on the water quality, hydrology and ecology of the waterbodies within it, including creeks. Runoff from agriculture and the built environment may adversely affect water quality due to non-point source pollution such as sediments and nutrients. Wetlands have a positive impact by serving to control flooding and to filter pollutants. When vegetation is removed from the landscape during development, the watershed's ability to absorb nutrients and trap sediments is diminished because so many surfaces are impervious to rain — such as roofs, sidewalks, parking lots and roads. The built landscape has more pollutants than a natural landscape and the stormwater runoff carries the pollutants to surface waters.

Land Use/Land Cover GIS data for Sarasota County is provided by the Southwest Florida Water Management District. Land use assignments are based upon examination of aerial photographs, according to statewide classifications set forth in the Florida Land Use, Land Cover Classification System.

The Land Use/Land Cover Chart shows the composition of the creek’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 Water Atlas Map Viewer tool.

permeability-icon

Impervious Surfaces: Soil permeability is closely related to land use. When rain falls on natural, undeveloped land, water is readily absorbed into the soil. Plants take up the water and nutrients, and soil filters the water before it reaches waterways. By contrast, in an urban landscape, paved surfaces absorb little water and it flows quickly to surface water bodies. Urbanization also short circuits the natural process rainfall recharging groundwater.

An impervious surface is a hard surface area which prevents or retards the entry of water into the soil mantle as under natural conditions prior to development; and/or a hard surface area which causes water to run off the surface in greater quantities, or at a greater rate of flow, from that present under natural conditions prior to development.

Common impervious surfaces include roof tops, walkways, patios, driveways, parking lots or storage areas, concrete or asphalt paving, gravel roads, packed earthen materials, and oiled, macadam, or other surfaces which similarly impede the natural infiltration of surface and stormwater runoff. Open, uncovered flow control or water quality treatment facilities are not considered impervious surfaces for determinations of standards.​

The Sarasota County Stormwater Environmental Utility (SEU) mapped impervious surfaces in the County in 2013. The Impervious Surface Graph shows the cumulative amount of each impervious surface type within the watershed where a creek is located. A map showing impervious surfaces can be viewed using the Water Atlas Map Viewer tool.


Calculations

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

Trends for Total Phosphorus and Total Nitrogen 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 or TP) changes in a consistent direction over time. The time period used in the analysis for the Creek Conditions Reports is from the earliest data available through the year being scored. 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 to show the direction and degree of change:

+2 strong increasing trend
+1   weak increasing trend
0 stable trend, or no trend
-1 weak decreasing trend
-2 strong decreasing trend


Caveats and Limitations

Intent: The Creek Conditions Report is 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 and are not intended to imply regulatory compliance (or non-compliance).

In addition to considering “numeric” standards for water's chemical and physical properties, regulators also apply a “narrative” standard. This standard states that “in no case shall nutrient concentrations of a body of water be altered so as to cause an imbalance in natural populations of aquatic flora or fauna.” (See F.A.C. 62-302.531(2)(c) and F.A.C. 62-303.530(47)(b)). The Creek Conditions Report is a numerical analysis that does not include narrative criteria or biological sampling results; for a complete ecological picture of stream health, both of these should be considered.

Method Detection Limit: The Method Detection Limit (MDL) is the lowest concentration that can be detected by a given laboratory method. For calculations of the water quality ratings, the Creek Conditions Report uses the MDL for those values below the MDL. The value in the database and the data download function of the Water Atlas is not changed by this calculation method.


Additional Information