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 three main measures of water quality, Chlorophyll a, Total Nitrogen, and Total Phosphorus concentrations, all improved in Roberts Bay in 2021. While Keldahl Nitrogen and Nitrate-Nitrite rose slightly, Ammonia-Ammonium concentration was lower. 

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.

Roberts Bay

Roberts 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
Excellent

Chlorophyll a

Score: Excellent How was this determined?
Units: ug/l Year
2021
Historical
period of record
High 20.52 58.47
Mean 5.44 8.33 7.94173 8.2000000000
Low 0.09 0.09
No. of Samples 56 2481
  • 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 0.730 1.376
Mean 0.436 0.441 0.17135558 0.4500000000
Low 0.245 0.065
No. of Samples 56 1528
  • 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 0.220 0.480
Mean 0.081 0.126 0.06461052 0.1900000000
Low 0.050 0.050
No. of Samples 56 1542
  • Six-month
    Moving Average
  • Annual
    Mean

Dissolved Oxygen

Units: mg/l Year
2021
Historical
period of record
High 10.20 11.60
Mean 6.97 6.54 1.19322
Low 4.09 1.35
No. of Samples 56 1658
  • 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 150.00
Mean 23.31 17.92683
Low 2.00
No. of Samples 0 1425
  • Six-month
    Moving Average
  • Annual
    Mean

BOD, Biochemical oxygen demand

Units: mg/l Year
2021
Historical
period of record
High 1.80 5.90
Mean 0.93 1.38 0.79784
Low 0.50 0.50
No. of Samples 56 1401
  • Six-month
    Moving Average
  • Annual
    Mean

Dissolved oxygen saturation

Units: percent (%) Year
2021
Historical
period of record
High 133.00 173.00
Mean 99.38 95.20 15.50208
Low 63.00 20.00
No. of Samples 56 1658
  • Six-month
    Moving Average
  • Annual
    Mean

Karenia brevis ("red tide")

Units: #/l Year
2021
Historical
period of record
High 170000.00 1832000.00
Mean 4964.29 10483.89 80503.60157
Low 0.00 0.00
No. of Samples 56 962
  • Six-month
    Moving Average
  • Annual
    Mean

Light Attenuation

Units: K(1/m) Year
2021
Historical
period of record
High 1.41 3.56
Mean 0.83 1.02 0.41899
Low 0.39 0.15
No. of Samples 56 1459
  • Six-month
    Moving Average
  • Annual
    Mean

Nitrogen, Ammonia + Ammonium as N

Units: ug/l Year
2021
Historical
period of record
High 53.00 243.00
Mean 16.86 23.35 27.9583
Low 5.00 5.00
No. of Samples 56 1537
  • Six-month
    Moving Average
  • Annual
    Mean

Nitrogen, Kjeldahl

Units: ug/l Year
2021
Historical
period of record
High 720.00 1320.00
Mean 443.39 458.05 166.15897
Low 240.00 60.00
No. of Samples 56 1542
  • Six-month
    Moving Average
  • Annual
    Mean

Nitrogen, Nitrite + Nitrate as N

Units: ug/l Year
2021
Historical
period of record
High 58.00 339.00
Mean 10.80 11.71 18.48366
Low 5.00 5.00
No. of Samples 56 1974
  • Six-month
    Moving Average
  • Annual
    Mean

pH

Units: None Year
2021
Historical
period of record
High 8.21 8.40
Mean 7.92 7.88 0.17034
Low 7.72 7.10
No. of Samples 56 1658
  • Six-month
    Moving Average
  • Annual
    Mean

Salinity

Units: PSS Year
2021
Historical
period of record
High 36.20 38.80
Mean 29.81 30.18 4.96889
Low 21.00 1.80
No. of Samples 56 1658
  • Six-month
    Moving Average
  • Annual
    Mean

Specific conductance

Units: umho Year
2021
Historical
period of record
High 54600.00 58320.00
Mean 45830.36 46340.93 6994.44877
Low 33600.00 3370.00
No. of Samples 56 1658
  • Six-month
    Moving Average
  • Annual
    Mean

Temperature, water

Units: deg F Year
2021
Historical
period of record
High 90.14 92.48
Mean 77.51 78.03 8.8389
Low 59.00 48.56
No. of Samples 56 1658
  • Six-month
    Moving Average
  • Annual
    Mean

Turbidity

Units: NTU Year
2021
Historical
period of record
High 8.60 24.00
Mean 3.47 4.11 1.98987
Low 1.10 0.85
No. of Samples 56 1537
  • Six-month
    Moving Average
  • Annual
    Mean

Annual Averages

Indicator Units 2017 2018 2019 2020 2021 Trend
Dissolved Oxygen mg/l 6.86 6.77 6.67 6.87 6.97
Dissolved oxygen saturation percent (%) 101.91 99.57 96.97 100.00 99.38
Light Attenuation K(1/m) 1.04 1.13 1.03 0.92 0.83
Salinity PSS 29.30 29.55 28.97 29.60 29.81
Turbidity NTU 3.85 3.69 3.25 2.94 3.47

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 348 acres

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

Impervious Features

Roberts Bay is located within the Sarasota Bay Watershed. View details about the 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.

19% of the land area within the 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 »

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

Acreage and Percentage within each Land Use / Land Cover Category for Sarasota Bay Watershed
Land Use Classification 1990 2005 2011 2014 2017 2020 Trend
Urban & Built-up 32,90853.3% 37,84461.3% 38,34362.1% 37,98761.6% 38,74962.8% 56,97059.1%
Agriculture 6,33810.3% 2,4974% 2,2153.6% 2,3093.7% 1,8223% 2,9863.1%
Rangeland 5470.9% 1990.3% 2250.4% 4300.7% 2080.3% 2610.3%
Upland Forests 3,5885.8% 2,1093.4% 1,8743% 1,9233.1% 1,7562.8% 2,0752.2%
Water 13,35021.6% 14,22723.1% 14,27823.1% 14,13122.9% 14,25523.1% 25,36026.3%
Wetlands 2,8704.7% 2,2273.6% 2,2293.6% 2,3723.8% 2,3273.8% 4,8895.1%
Barren Land 290% 90% 990.2% 1090.2% 1000.2% 760.1%
Transportation and Utilities 1,8453% 2,6024.2% 2,4524% 2,4534% 2,5114.1% 3,7833.9%
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.