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:

Water quality in Roberts Bay was adversely affected by Hurricane Ian in Sept. 2022, when torrential rain washed pollutants into the bay and caused a temporary increase in Chlorophyll a levels. In 2023, Chlorophyll a levels dropped, and levels of Total Nitrogen and Total Phosphorus, already relatively low, also decreased, so that by all measures water quality in Roberts Bay in 2023 was excellent.

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
2023
Historical
period of record
High 16.43 58.47
Mean 5.01 8.52 8.05104 8.2000000000
Low 1.84 0.09
No. of Samples 56 1118
  • Six-month
    Moving Average
  • Annual
    Mean
Excellent

Nitrogen, Total

Score: Excellent How was this determined?
Units: mg/l Year
2023
Historical
period of record
High 0.545 1.376
Mean 0.370 0.436 0.1668311 0.4500000000
Low 0.135 0.065
No. of Samples 56 1609
  • Six-month
    Moving Average
  • Annual
    Mean
Excellent

Phosphorus, Total

Score: Excellent How was this determined?
Units: mg/l Year
2023
Historical
period of record
High 0.120 0.480
Mean 0.066 0.121 0.06505021 0.1900000000
Low 0.050 0.050
No. of Samples 56 1566
  • Six-month
    Moving Average
  • Annual
    Mean

Dissolved Oxygen

Units: mg/l Year
2023
Historical
period of record
High 9.35 11.60
Mean 6.60 6.59 1.20773
Low 4.71 1.35
No. of Samples 55 1253
  • Six-month
    Moving Average
  • Annual
    Mean

Apparent Color

Note: The latest available sample for this parameter is from December 2019
Units: PCU Year
2023
Historical
period of record
High 150.00
Mean 22.98 17.6616
Low 2.00
No. of Samples 0 1331
  • Six-month
    Moving Average
  • Annual
    Mean

BOD, Biochemical oxygen demand

Units: mg/l Year
2023
Historical
period of record
High 2.80 5.90
Mean 1.11 1.36 0.78505
Low 0.50 0.50
No. of Samples 56 1440
  • Six-month
    Moving Average
  • Annual
    Mean

Dissolved oxygen saturation

Units: percent (%) Year
2023
Historical
period of record
High 159.00 173.00
Mean 101.90 96.14 15.83497
Low 75.00 20.00
No. of Samples 52 1377
  • Six-month
    Moving Average
  • Annual
    Mean

Karenia brevis ("red tide")

Units: #/l Year
2023
Historical
period of record
High 86000.00 1832000.00
Mean 3410.71 10333.84 79342.13558
Low 0.00 0.00
No. of Samples 56 993
  • Six-month
    Moving Average
  • Annual
    Mean

Light Attenuation

Units: K(1/m) Year
2023
Historical
period of record
High 1.04 3.56
Mean 0.66 1.00 0.41667
Low 0.30 0.15
No. of Samples 56 1544
  • Six-month
    Moving Average
  • Annual
    Mean

Nitrogen, Ammonia + Ammonium as N

Units: ug/l Year
2023
Historical
period of record
High 56.00 243.00
Mean 17.75 23.47 27.52106
Low 5.00 5.00
No. of Samples 56 1569
  • Six-month
    Moving Average
  • Annual
    Mean

Nitrogen, Kjeldahl

Units: ug/l Year
2023
Historical
period of record
High 540.00 1320.00
Mean 370.71 451.87 162.07044
Low 130.00 60.00
No. of Samples 56 1605
  • Six-month
    Moving Average
  • Annual
    Mean

Nitrogen, Nitrite + Nitrate as N

Units: ug/l Year
2023
Historical
period of record
High 24.00 339.00
Mean 7.48 13.61 21.14018
Low 5.00 5.00
No. of Samples 56 1440
  • Six-month
    Moving Average
  • Annual
    Mean

pH

Units: None Year
2023
Historical
period of record
High 8.32 8.40
Mean 7.86 7.88 0.1788
Low 7.45 7.10
No. of Samples 52 532
  • Six-month
    Moving Average
  • Annual
    Mean

Rainfall

Units: inches/yr Year
2023
Historical
period of record
High 25.5 57.0
Mean 37.6 13.4346155420208
Low 6.4
No. of Samples 365 7217
  • Six-month
    Moving Average
  • Annual
    Mean

Salinity

Units: PSS Year
2023
Historical
period of record
High 37.50 38.70
Mean 34.65 29.72 5.31874
Low 29.90 1.80
No. of Samples 50 1185
  • Six-month
    Moving Average
  • Annual
    Mean

Specific conductance

Units: umho Year
2023
Historical
period of record
High 56300.00 58320.00
Mean 52458.00 46396.29 7024.33672
Low 46000.00 3370.00
No. of Samples 50 1581
  • Six-month
    Moving Average
  • Annual
    Mean

Temperature, water

Units: deg F Year
2023
Historical
period of record
High 91.40 92.30
Mean 82.03 78.75 8.61567
Low 69.80 48.56
No. of Samples 49 1072
  • Six-month
    Moving Average
  • Annual
    Mean

Turbidity

Units: NTU Year
2023
Historical
period of record
High 6.00 24.00
Mean 2.69 4.07 1.98091
Low 1.00 0.85
No. of Samples 56 1598
  • Six-month
    Moving Average
  • Annual
    Mean

Annual Averages

Indicator Units 2019 2020 2021 2022 2023 Trend
Dissolved Oxygen mg/l 6.67 6.87 6.97 6.51 6.60
Dissolved oxygen saturation percent (%) 96.97 100.00 99.38 96.02 101.90
Light Attenuation K(1/m) 1.03 0.92 0.83 0.88 0.66
Rainfall inches/yr 37.42 48.81 44.63 50.74 25.53
Salinity PSS 28.97 29.60 29.81 30.72 34.65
Turbidity NTU 3.25 2.94 3.47 3.93 2.69

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 (2023)

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 2023 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 ug/l
  • 1.0 - 5.9 ug/l
  • 6.0 - 10.9 ug/l
  • 11.0 - 17.9 ug/l
  • Greater than 18 ug/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.