Learn More: Creek Condition Reports

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 six-month simple moving average 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” (abbreviated TN) 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 six-month simple moving average for TN. For freshwater stream segments, the regulatory standard is shown. For marine stream segments, if an increasing or decreasing trend is detected, the 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 six-month simple moving average for TP. For freshwater stream segments, the regulatory standard is shown. For marine stream segments, if an increasing or decreasing trend is detected, the 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 six-month simple moving average 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.

Biochemical Oxygen Demand: This measure is an indicator of the amount of organic matter in water. It describes the amount of dissolved oxygen needed by aerobic biological organisms (i.e., bacteria) to break down the organic material that is present in a water sample. If the rate of oxygen consumption exceeds that which is available, the water may become anoxic, containing too little oxygen to support fish and other aquatic organisms.

Apparent Color: Apparent color is the color of the water as seen by the human eye. Apparent colors are caused by substances that are either suspended or dissolved in the water column. Other factors can also affect the apparent color of a water body, including the color of the bay bottom, water depth, reflections from the sky or nearby structures; and the presence or absence of seagrass or algae.

Escherichia coli (E. coli): One of the most common species of coliform bacteria, E. coli is a normal component of the large intestines in humans and other warm-blooded animals. If is found in human sewage in high numbers. It is used as an indicator organism because it is easily cultured, and its presence in water in defined amounts indicates that sewage may be present. If sewage is present in water, pathogenic or disease-causing organisms may also be present.

Nitrogen, Ammonia + Ammonium as N: Unlike other forms of nitrogen, which can cause nutrient over-enrichment of a water body at elevated concentrations and have indirect effects on aquatic life, ammonia causes direct toxic effects on aquatic life. It is toxic to fish at relatively low concentrations in pH-neutral or alkaline water. It is difficult for aquatic organisms to sufficiently excrete it, leading to toxic buildup in internal tissues and blood, and potentially death. Among the sources for this pollutant are wastewater treatment plants, agricultural and residental fertilizer runoff, leaking septic tanks, pet wastes, and atmospheric deposition.

Nitrogen, Kjeldahl: This water quality measure gives the total concentration of organic nitrogen and ammonia.

Nitrogen, Nitrite + Nitrate as N: Nitrates and nitrites occur normally in nature from the breakdown of ammonia in the nitrogen life cycle. Nitrates in nature cause plant and algae growth that may affect the balance of water-based ecosystems. Nitrate is found in fertilizers and animal waste. Rain tends to wash fertilizers containing nitrates into nearby natural water systems and ground water.

pH: This measure indicates the acidity or alkalinity of water. pH stands for ‘potential of Hydrogen’ because it is a measure of the hydrogen ion concentration in a solution. The range of values is from 1 to 14, with 7 as the middle (neutral) point. Values below 7 indicate acidity, above 7 alkalinity, with a value of 1 being the most acidic.

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, the Practical Salinity Scale (PSS), is a ratio and therefore has no units. It compares the conductivity of a water sample to a standard potassium chloride solution. It is important to realize that salinity in coastal creeks fluctuates constantly as a result of tidal influence.

Specific Conductance: This is the standard measurement of conductivity, or the capability of water to pass electrical flow, which is related to the number of ions in the water. These conductive ions come from dissolved salts and inorganic materials such as alkalis, chlorides, sulfides and carbonate compounds, collectively known as electrolytes. Water’s conductivity increases with temperature. Specific conductance is a standard measure of conductivity, measured at, or converted to, a temperature of 25°C.

Temperature, Water: Temperature impacts both the chemical and biological characteristics of surface water. It affects the ability of the water to absorb oxygen, the photosynthetic rate of aquatic plants, metabolic rates of aquatic organisms, and the sensitivity of these organisms to pollution, parasites and disease.

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 aerobic bacteria consume organic particles and thereby 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.

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, while accurate, 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.

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 Advanced Mapping tool.

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 NPDES Viewer tool.