Caution

2 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 health of Blackburn Bay deteriorated slightly in 2016. Chlorophyll a increased significantly with the mean value increasing to 0.0108 mg/l, exceeding the threshold value. Nitrogen concentration also increased remaining in the “Good” range, between target and threshold levels. The mean value for phosphorus concentration increased only very slightly, and is still well below the target.

Water Quality: While two of the three water quality indicators ( nitrogen, and phosphorus) were rated as pass, chlorophyll a's rating changed from “Good” to “Caution” because of its continued increase in value. The mean for chlorophyll a was calculated as an arithmetic mean and the means for nitrogen and phosphorus were calculated as geometric means (per the Numeric Nutrient Criteria outlined in the Florida Administrative Code, section 62-302.532). The mean chlorophyll a level was 0.0108 mg/l, exceeding the threshold concentration of 0.0082 mg/l . The mean nitrogen level increased to 0.4276 mg/l, exceeding the target value of 0.36 mg/l. Phosphorus concentrations in the bay showed an increase as well, with a mean in 2016 of 0.0821 mg/l, well below the target (0.170 mg/l) and threshold (0.210 mg/l) levels and earning a score of “Excellent”.

Biotic Indicator: Measurement of the biotic indicator, seagrass, was performed in 2016 by the Southwest Florida Water Management District. Total seagrass acreage in Blackburn Bay was measured to be 314 acres, a decrease in coverage from 344 acres in 2014. Seagrass acreage remains below the target level of 447 acres.

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 Five-year Rolling Average Graphs 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
  • Trend Line
  • Annual Mean
  • Target
  • Threshold
Caution

Chlorophyll a

Score: Caution How was this determined?
Units: mg/l Year
2016
Historical
period of record
High 0.060 0.060
Mean 0.011 0.005 0.0060042 0.0060000000
Low 0.002 0.000
No. of Samples 57 1016

Five-year Rolling Average

  • Annual Mean
Good

Nitrogen, Total

Units: mg/l Year
2016
Historical
period of record
High 1.185 2.205
Mean 0.428 0.325 0.18556374 0.3600000000
Low 0.205 0.055
No. of Samples 57 1010

Five-year Rolling Average

  • Annual Mean
Excellent

Phosphorus, Total

Score: Excellent How was this determined?
Units: mg/l Year
2016
Historical
period of record
High 0.220 0.530
Mean 0.082 0.099 0.06654854 0.1700000000
Low 0.050 0.050
No. of Samples 57 1026

Five-year Rolling Average

  • Annual Mean

Dissolved Oxygen

Units: mg/l Year
2016
Historical
period of record
High 8.70 11.90
Mean 6.25 6.46 1.12825
Low 4.40 1.60
No. of Samples 48 1052

Five-year Rolling Average

  • Annual Mean

Apparent Color

Units: PCU Year
2016
Historical
period of record
High 200.00 250.00
Mean 29.72 19.86 27.41843
Low 2.00 2.00
No. of Samples 57 1026

Five-year Rolling Average

  • Annual Mean

BOD, Biochemical oxygen demand

Units: mg/l Year
2016
Historical
period of record
High 3.70 7.10
Mean 1.15 1.07 0.68602
Low 0.50 0.50
No. of Samples 57 915

Five-year Rolling Average

  • Annual Mean

Dissolved oxygen saturation

Units: percent (%) Year
2016
Historical
period of record
High 117.00 198.00
Mean 90.88 94.41 15.92401
Low 73.00 26.00
No. of Samples 48 1052

Five-year Rolling Average

  • Annual Mean

Karenia brevis ("red tide")

Units: #/l Year
2016
Historical
period of record
High 2240000.00 2280000.00
Mean 262625.00 57466.78 235216.59164
Low 1000.00 1000.00
No. of Samples 32 602

Five-year Rolling Average

  • Annual Mean

Light Attenuation

Units: K(1/m) Year
2016
Historical
period of record
High 3.27 5.03
Mean 1.11 0.87 0.50495
Low 0.28 0.16
No. of Samples 44 886

Five-year Rolling Average

  • Annual Mean

Nitrogen, Ammonia + Ammonium as N

Units: ug/l Year
2016
Historical
period of record
High 132.00 132.00
Mean 16.16 15.42 14.12505
Low 5.00 5.00
No. of Samples 57 1028

Five-year Rolling Average

  • Annual Mean

Nitrogen, Kjeldahl

Units: ug/l Year
2016
Historical
period of record
High 1180.00 2200.00
Mean 461.58 357.17 184.53785
Low 200.00 0.05
No. of Samples 57 1027

Five-year Rolling Average

  • Annual Mean

Nitrogen, Nitrite + Nitrate as N

Units: ug/l Year
2016
Historical
period of record
High 27.00 65.00
Mean 7.30 6.04 4.4999
Low 5.00 5.00
No. of Samples 57 1348

Five-year Rolling Average

  • Annual Mean

pH

Units: None Year
2016
Historical
period of record
High 8.30 8.40
Mean 7.89 7.94 0.1778
Low 7.50 6.10
No. of Samples 48 1052

Five-year Rolling Average

  • Annual Mean

Salinity

Units: PSS Year
2016
Historical
period of record
High 35.70 39.30
Mean 30.74 33.05 4.27893
Low 10.90 5.70
No. of Samples 48 1052

Five-year Rolling Average

  • Annual Mean

Specific conductance

Units: umho Year
2016
Historical
period of record
High 53900.00 58760.00
Mean 47038.75 50289.67 5950.02914
Low 18400.00 10130.00
No. of Samples 48 1052

Five-year Rolling Average

  • Annual Mean

Temperature, water

Units: deg F Year
2016
Historical
period of record
High 68.90 92.84
Mean 67.37 76.43 8.93612
Low 64.76 47.84
No. of Samples 8 1012

Five-year Rolling Average

  • Annual Mean

Turbidity

Units: NTU Year
2016
Historical
period of record
High 25.00 39.00
Mean 4.81 3.98 3.43301
Low 1.20 0.20
No. of Samples 57 1017

Five-year Rolling Average

  • Annual Mean

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 (2016)

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 2016 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.
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 2014 Trend
Urban & Built-up 8,94331.9% 11,83442.2% 12,16243.3%
Agriculture 3,55012.6% 3,22811.5% 4,22315%
Rangeland 8252.9% 1,8226.5% 5792.1%
Upland Forests 7,09825.3% 3,06610.9% 2,7259.7%
Water 3,42912.2% 4,12314.7% 4,17514.9%
Wetlands 3,49012.4% 3,13311.2% 3,22711.5%
Barren Land 620.2% 180.1% 200.1%
Transportation and Utilities 6752.4% 8413% 9523.4%
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.