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 health of Blackburn Bay has remained in good condition with all water quality parameters below their associated target levels. However, the biotic indicator, seagrass, has continued to decrease.

Water quality: All three water quality indicators (chlorophyll a, nitrogen, and phosphorus) were rated as pass (below the threshold).The mean for chlorophyll a was calculated as an arithmetic mean and the means for nitrogen and phosphorus were calculated as geometric means (Numeric Nutrient Criteria Recommendations).The mean chlorophyll a level (0.0051 mg/l) in Blackburn Bay has decreased and was scored as “Excellent” (scored as good in 2010), below the target (0.006 mg/l) and threshold (0.0082 mg/l) levels. The mean nitrogen level (0.3513 mg/l) has slightly decreased and was scored as excellent, below the target (0.360 mg/l) and threshold (0.430 mg/l) levels. The mean phosphorus level (0.0556 mg/l) has remained constant and was also scored as excellent, below the target (0.170 mg/l) and threshold (0.210 mg/l) levels.

Biotic Indicator:  The total acreage of seagrass has remained relatively constant since 1988 but in 2010 the mean level of seagrass (323 acres) was still below the target 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
Excellent

Chlorophyll a

Score: Excellent How was this determined?
Units: mg/l Year
2011
Historical
period of record
High 0.012 0.043
Mean 0.004 0.005 0.00497711 0.0060000000
Low 0.001 0.000
No. of Samples 58 723

Five-year Rolling Average

  • Annual Mean
Excellent

Nitrogen, Total

Score: Excellent How was this determined?
Units: mg/l Year
2011
Historical
period of record
High 0.675 1.189
Mean 0.348 0.16900195 0.3600000000
Low 0.215 0.055
No. of Samples 59 711

Five-year Rolling Average

  • Annual Mean
Excellent

Phosphorus, Total

Score: Excellent How was this determined?
Units: mg/l Year
2011
Historical
period of record
High 0.140 0.530
Mean 0.055 0.113 0.06989435 0.1700000000
Low 0.050 0.050
No. of Samples 60 735

Five-year Rolling Average

  • Annual Mean

Dissolved Oxygen

Units: mg/l Year
2011
Historical
period of record
High 8.90 11.90
Mean 6.41 6.55 1.14963
Low 4.50 1.60
No. of Samples 48 810

Five-year Rolling Average

  • Annual Mean

Apparent Color

Units: PCU Year
2011
Historical
period of record
High 48.00 250.00
Mean 13.38 19.44 27.68677
Low 5.00 2.00
No. of Samples 60 735

Five-year Rolling Average

  • Annual Mean

BOD, Biochemical oxygen demand

Units: mg/l Year
2011
Historical
period of record
High 1.60 7.10
Mean 0.78 1.13 0.7403
Low 0.50 0.50
No. of Samples 60 624

Five-year Rolling Average

  • Annual Mean

Dissolved oxygen saturation

Units: percent (%) Year
2011
Historical
period of record
High 115.00 198.00
Mean 92.48 95.83 16.73963
Low 74.00 26.00
No. of Samples 48 812

Five-year Rolling Average

  • Annual Mean

Karenia brevis ("red tide")

Units: #/l Year
2011
Historical
period of record
High 1000.00 2280000.00
Mean 1000.00 37585.23 190942.02998
Low 1000.00 1000.00
No. of Samples 60 352

Five-year Rolling Average

  • Annual Mean

Light Attenuation

Units: K(1/m) Year
2011
Historical
period of record
High 1.61 5.03
Mean 0.75 0.86 0.51698
Low 0.16 0.16
No. of Samples 48 652

Five-year Rolling Average

  • Annual Mean

Nitrogen, Ammonia + Ammonium as N

Units: ug/l Year
2011
Historical
period of record
High 44.00 120.00
Mean 13.93 14.78 13.53602
Low 5.00 5.00
No. of Samples 60 737

Five-year Rolling Average

  • Annual Mean

Nitrogen, Kjeldahl

Units: ug/l Year
2011
Historical
period of record
High 670.00 1150.00
Mean 353.00 326.79 166.91719
Low 210.00 0.05
No. of Samples 60 735

Five-year Rolling Average

  • Annual Mean

Nitrogen, Nitrite + Nitrate as N

Units: ug/l Year
2011
Historical
period of record
High 7.00 65.00
Mean 5.28 6.02 4.83512
Low 5.00 5.00
No. of Samples 60 1057

Five-year Rolling Average

  • Annual Mean

pH

Units: None Year
2011
Historical
period of record
High 8.40 8.40
Mean 8.05 7.93 0.19196
Low 7.80 6.10
No. of Samples 48 812

Five-year Rolling Average

  • Annual Mean

Rainfall

Units: inches/yr Year
2011
Historical
period of record
High 42.8 52.2
Mean 33.6 17.078760538706
Low 2.4
No. of Samples 363 4615

Five-year Rolling Average

  • Annual Mean

Salinity

Units: PSS Year
2011
Historical
period of record
High 36.40 39.30
Mean 33.44 33.36 4.21857
Low 26.00 5.70
No. of Samples 48 804

Five-year Rolling Average

  • Annual Mean

Specific conductance

Units: umho Year
2011
Historical
period of record
High 54850.00 58760.00
Mean 50830.63 50736.34 5847.4379
Low 40490.00 10130.00
No. of Samples 48 812

Five-year Rolling Average

  • Annual Mean

Temperature, water

Units: deg F Year
2011
Historical
period of record
High 87.80 92.84
Mean 75.50 76.39 9.16347
Low 61.70 47.84
No. of Samples 48 812

Five-year Rolling Average

  • Annual Mean

Turbidity

Units: NTU Year
2011
Historical
period of record
High 21.00 39.00
Mean 4.20 3.97 3.4951
Low 1.40 0.20
No. of Samples 59 717

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 (2011)

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 2011 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.