Pass

3 out of 3 indicators were rated as PASS.

All three indicators must pass for the bay to be rated as PASS.

Learn more about how this report is created

Summary:

The overall water quality in Blackburn Bay continued to be healthy in 2021. The concentration of Total Nitrogen, ammonia, ammonium, and nitrate-nitrite all decreased from 2020, but mean Kjeldahl Nitrogen concentration rose very slightly. The mean concentration of Chlorophyll a increased significantly due to a spike in value late in the year. Total Phosphorus concentrations continued to be very low and not of concern.

Note: Beginning in 2020, Sarasota County switched from measuring apparent color to true color. The latter will be added to Bay Conditions reports in the near future.

Blackburn Bay

Blackburn Bay

Water Chemistry Ratings

Total nitrogen, total phosphorus, and chlorophyll a levels are monitored carefully by water resource managers and used by regulatory authorities to determine whether a bay meets the water quality standards mandated by the Clean Water Act. The trend graphs for these indicators are shown below, along with their target and threshold values. A target value is a desirable goal to be attained, while a threshold is an undesirable level which is to be avoided. 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. Learn More about these ratings and how they are calculated »

The charts below illustrate the general trend of water quality parameters. They show a six-month running average, which moderates high and low values in the data.

Chart Legend
  • Six-month Moving Average
  • Annual Mean
  • Target
  • Threshold
Good

Chlorophyll a

Units: ug/l Year
2021
Historical
period of record
High 42.32 59.70
Mean 6.14 5.44 5.91409 6.0000000000
Low 0.80 0.20
No. of Samples 48 2124
  • Six-month
    Moving Average
  • Annual
    Mean
Excellent

Nitrogen, Total

Score: Excellent How was this determined?
Units: mg/l Year
2021
Historical
period of record
High 1.105 2.205
Mean 0.355 0.335 0.18422774 0.3600000000
Low 0.085 0.055
No. of Samples 48 1283
  • Six-month
    Moving Average
  • Annual
    Mean
Excellent

Phosphorus, Total

Score: Excellent How was this determined?
Units: mg/l Year
2021
Historical
period of record
High 0.230 0.530
Mean 0.067 0.093 0.06375065 0.1700000000
Low 0.050 0.050
No. of Samples 48 1294
  • Six-month
    Moving Average
  • Annual
    Mean

Dissolved Oxygen

Units: mg/l Year
2021
Historical
period of record
High 9.14 11.90
Mean 6.78 6.44 1.13126
Low 4.31 1.27
No. of Samples 48 1308
  • Six-month
    Moving Average
  • Annual
    Mean

Apparent Color

Note: The latest available sample for this parameter is from December 2019
Units: PCU Year
2021
Historical
period of record
High 250.00
Mean 19.86 26.58372
Low 2.00
No. of Samples 0 1194
  • Six-month
    Moving Average
  • Annual
    Mean

BOD, Biochemical oxygen demand

Units: mg/l Year
2021
Historical
period of record
High 3.60 7.10
Mean 1.03 1.06 0.67096
Low 0.50 0.50
No. of Samples 48 1179
  • Six-month
    Moving Average
  • Annual
    Mean

Dissolved oxygen saturation

Units: percent (%) Year
2021
Historical
period of record
High 144.00 198.00
Mean 96.67 93.93 15.56341
Low 67.00 20.00
No. of Samples 48 1308
  • Six-month
    Moving Average
  • Annual
    Mean

Karenia brevis ("red tide")

Units: #/l Year
2021
Historical
period of record
High 922000.00 2280000.00
Mean 90358.33 59129.23 226185.44018
Low 0.00 0.00
No. of Samples 48 845
  • Six-month
    Moving Average
  • Annual
    Mean

Light Attenuation

Units: K(1/m) Year
2021
Historical
period of record
High 2.74 5.03
Mean 0.84 0.89 0.4931
Low 0.26 0.02
No. of Samples 48 1165
  • Six-month
    Moving Average
  • Annual
    Mean

Nitrogen, Ammonia + Ammonium as N

Units: ug/l Year
2021
Historical
period of record
High 55.00 132.00
Mean 14.40 15.48 14.081
Low 5.00 5.00
No. of Samples 48 1292
  • Six-month
    Moving Average
  • Annual
    Mean

Nitrogen, Kjeldahl

Units: ug/l Year
2021
Historical
period of record
High 1100.00 2200.00
Mean 401.67 367.82 182.7847
Low 80.00 50.00
No. of Samples 48 1295
  • Six-month
    Moving Average
  • Annual
    Mean

Nitrogen, Nitrite + Nitrate as N

Units: ug/l Year
2021
Historical
period of record
High 18.00 65.00
Mean 6.44 6.08 4.32593
Low 5.00 5.00
No. of Samples 48 1616
  • Six-month
    Moving Average
  • Annual
    Mean

pH

Units: None Year
2021
Historical
period of record
High 8.19 8.40
Mean 8.00 7.94 0.16958
Low 7.73 6.10
No. of Samples 48 1308
  • Six-month
    Moving Average
  • Annual
    Mean

Salinity

Units: PSS Year
2021
Historical
period of record
High 36.10 39.30
Mean 31.97 32.80 4.14204
Low 25.00 5.70
No. of Samples 48 1308
  • Six-month
    Moving Average
  • Annual
    Mean

Specific conductance

Units: umho Year
2021
Historical
period of record
High 54500.00 58760.00
Mean 48800.00 49939.47 5753.08442
Low 39300.00 10130.00
No. of Samples 48 1308
  • Six-month
    Moving Average
  • Annual
    Mean

Temperature, water

Units: deg F Year
2021
Historical
period of record
High 88.16 92.84
Mean 75.66 76.73 8.83787
Low 57.74 47.84
No. of Samples 48 1308
  • Six-month
    Moving Average
  • Annual
    Mean

Turbidity

Units: NTU Year
2021
Historical
period of record
High 6.00 39.00
Mean 2.59 3.91 3.24666
Low 1.00 0.20
No. of Samples 48 1290
  • Six-month
    Moving Average
  • Annual
    Mean

Annual Averages

Indicator Units 2017 2018 2019 2020 2021 Trend
Dissolved Oxygen mg/l 6.37 6.00 6.26 6.34 6.78
Dissolved oxygen saturation percent (%) 93.27 87.46 90.96 91.10 96.67
Light Attenuation K(1/m) 0.99 0.99 0.95 0.83 0.84
Salinity PSS 32.04 32.58 31.38 31.39 31.97
Turbidity NTU 4.03 2.81 5.11 3.61 2.59

Other Measures of Bay Health

In addition to nutrient levels and chlorophyll concentration, dissolved oxygen levels, and water clarity are also objective indicators of bay health. These have complex interactive cycles which are affected by rainfall, temperature, and tidal action, as well as other factors. High nutrient levels (nitrogen and phosphorus) can stimulate excessive growth of marine algae (indicated by chlorophyll a level), resulting in reduced water clarity (and increased light attenuation) and depleted oxygen levels. 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.

Bay Contour Maps (2021)

Contour mapping is one of the best ways to visualize spatial differences in coastal water quality. The interactive map shown below presents monthly data for one selected water quality indicator atop an aerial view of the bay. Choose a different water quality parameter from the list at the top to change the map. Learn More about Water Quality Contour Mapping »

Showing 2021 Monthly Contour Maps for: January
Loading interactive contour map viewer...
Contour Visibility:

Visit the Water Quality Contour Mapping Tool to view and compare monthly water quality contour maps for ten different water quality indicators. In addition, you can generate your own custom maps.

Contour Legend:

  • Less than 1 mg/l
  • 1.0 - 5.9 mg/l
  • 6.0 - 10.9 mg/l
  • 11.0 - 17.9 mg/l
  • Greater than 18 mg/l

Seagrasses

Among the most important habitats in Florida's estuarine environments, seagrass beds are indispensable for the role they play in cycling nutrients, supplying food for wildlife, stabilizing sediments, and providing habitat for juvenile and adult finfish and shellfish. Use the interactive map below to observe the size, density and location of seagrass beds from year to year. The graph shows how the total amount of seagrass in the bay has changed over time. Seagrass calculations are aggregates of patchy and continuous seagrass measurements only. Recordings of attached algae are not included in these summaries.
Learn More about Seagrasses »

Showing Seagrass Coverage for :
Loading interactive seagrass coverage viewer...
Loading interactive chart...
  • Target 447 acres

Visit our Seagrass page to discover the beauty and importance of seagrass habitats, and sign up to help monitor their health.

Impervious Features

Blackburn Bay is located within the Little Sarasota Bay Watershed. View details about the Little Sarasota Bay Watershed »

Rain that falls on land that is in a natural state is absorbed and filtered by soils and vegetation as it makes it way into underground aquifers. However, in developed areas, "impervious surfaces" impede this process and contribute to polluted urban runoff entering surface waters. These surfaces include human infrastructure like roads, sidewalks, driveways and parking lots that are covered by impenetrable materials such as asphalt, concrete, brick and stone, as well as buildings and other permanent structures. Soils that have been disturbed and compacted by urban development are often impervious as well. Learn more about Impervious Features »

The Sarasota County Stormwater Environmental Utility (SEU) mapped impervious surfaces in the County in 2014. A map showing impervious surfaces can be viewed using the interactive Sarasota NPDES Viewer.

14% of the land area within the Little Sarasota Bay Watershed is covered by impervious surfaces

Impervious Surface Coverage by Type

Land Use / Land Cover

Land use within a bay's watershed has a major effect on its water quality. In general, less development means better water quality. Land Cover/Land Use classifications categorize land in terms of its observed physical surface characteristics (upland or wetland, e.g.), and also reflect the types of activity that are taking place on it (agriculture, urban/built-up, utilities, etc.). Florida uses as its standard a set of statewide classifications which were developed by the Florida Department of Transportation. Learn More about Land Use and Land Cover »

Blackburn Bay is located within the Little Sarasota Bay Watershed. The chart below shows the land use / land cover characteristics for Little Sarasota Bay Watershed within the boundary of this Water Atlas. View details about the Little Sarasota Bay Watershed »

Acreage and Percentage within each Land Use / Land Cover Category for Little Sarasota Bay Watershed
Land Use Classification 1990 2005 2011 2014 2017 2020 Trend
Urban & Built-up 8,94331.9% 11,83442.2% 12,10243.1% 12,16243.3% 12,77745.5% 13,34347.5%
Agriculture 3,55012.6% 3,22811.5% 3,25811.6% 4,22315% 3,12411.1% 2,83710.1%
Rangeland 8252.9% 1,8226.5% 1,4745.3% 5792.1% 1,2334.4% 8773.1%
Upland Forests 7,09825.3% 3,06610.9% 2,98110.6% 2,7259.7% 2,6879.6% 2,6039.3%
Water 3,42912.2% 4,12314.7% 4,14714.8% 4,17514.9% 4,22715.1% 4,33715.5%
Wetlands 3,49012.4% 3,13311.2% 3,19111.4% 3,22711.5% 3,12111.1% 3,12011.1%
Barren Land 620.2% 180.1% 190.1% 200.1% 60% 60%
Transportation and Utilities 6752.4% 8413% 8923.2% 9523.4% 8983.2% 9483.4%
Land Use Chart

Data Sources

The data sources listed below provided water quality data used to create the report on this page. Not all data sources provided data for every bay, and not every Bay Conditions Report used data from all listed data sources. While some data sources have no data for the scored year, they provided period-of-record (historical high, mean, low) data. Click on a data source name to review its metadata.