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 Roberts Bay improved in 2017, with two of the three water quality indicators improving over the previous year. Despite improvements in both nitrogen and phosphorus concentrations, chlorophyll a levels increased slightly, keeping its rating at "Caution". Nitrogen concentration showed significant improvement, resulting in its rating being upgraded to "Good". Phosphorus concentration continued to be "Excellent",  its concentration was already quite low, but it decreased from the previous year.

Water Quality: Chlorophyll a concentration increased only slightly, from 0.0117 to 0,014 mg/l, but its annual mean value was already above the threshold. Mean nitrogen concentration fell from 0.554 mg/l in 2016 to 0.4978 mg/l, bringing it below the desired target value. Already very good, phosphorus concentrations improved slightly, falling to 0.111 mg/l, well below the target value of 0.19 mg/l. The mean for chlorophyll a 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). 

Biotic Indicator: Measurement of the biotic indicator, seagrass, was performed in 2016 by the Southwest Florida Water Management District. Total seagrass acreage in Roberts Bay increased slightly to 356 acres from its previous value exceeding the desired target of 348 acres.

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 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
2017
Historical
period of record
High 0.058 0.058
Mean 0.014 0.008 0.00792411 0.0082000000
Low 0.002 0.000
No. of Samples 64 2234

Five-year Rolling Average

  • Annual Mean
Good

Nitrogen, Total

Units: mg/l Year
2017
Historical
period of record
High 1.195 1.376
Mean 0.498 0.437 0.17524609 0.4500000000
Low 0.265 0.065
No. of Samples 64 1275

Five-year Rolling Average

  • Annual Mean
Excellent

Phosphorus, Total

Score: Excellent How was this determined?
Units: mg/l Year
2017
Historical
period of record
High 0.260 0.480
Mean 0.112 0.132 0.06534318 0.1900000000
Low 0.050 0.050
No. of Samples 84 1310

Five-year Rolling Average

  • Annual Mean

Dissolved Oxygen

Units: mg/l Year
2017
Historical
period of record
High 9.60 11.60
Mean 6.86 6.49 1.18053
Low 5.35 3.50
No. of Samples 60 1406

Five-year Rolling Average

  • Annual Mean

Apparent Color

Units: PCU Year
2017
Historical
period of record
High 70.00 150.00
Mean 24.25 23.50 18.20883
Low 8.00 2.00
No. of Samples 79 1305

Five-year Rolling Average

  • Annual Mean

BOD, Biochemical oxygen demand

Units: mg/l Year
2017
Historical
period of record
High 4.10 5.90
Mean 1.74 1.43 0.82869
Low 0.50 0.50
No. of Samples 79 1169

Five-year Rolling Average

  • Annual Mean

Dissolved oxygen saturation

Units: percent (%) Year
2017
Historical
period of record
High 136.00 173.00
Mean 101.91 94.59 15.49306
Low 79.00 50.00
No. of Samples 80 1426

Five-year Rolling Average

  • Annual Mean

Karenia brevis ("red tide")

Units: #/l Year
2017
Historical
period of record
High 96000.00 912000.00
Mean 5622.81 9599.32 60871.28079
Low 0.00 0.00
No. of Samples 57 730

Five-year Rolling Average

  • Annual Mean

Light Attenuation

Units: K(1/m) Year
2017
Historical
period of record
High 2.48 3.56
Mean 1.04 1.02 0.43252
Low 0.15 0.15
No. of Samples 60 1207

Five-year Rolling Average

  • Annual Mean

Nitrogen, Ammonia + Ammonium as N

Units: ug/l Year
2017
Historical
period of record
High 50.00 243.00
Mean 8.97 23.43 28.97336
Low 5.00 5.00
No. of Samples 79 1305

Five-year Rolling Average

  • Annual Mean

Nitrogen, Kjeldahl

Units: ug/l Year
2017
Historical
period of record
High 1190.00 1320.00
Mean 502.74 457.01 170.02914
Low 260.00 60.00
No. of Samples 84 1310

Five-year Rolling Average

  • Annual Mean

Nitrogen, Nitrite + Nitrate as N

Units: ug/l Year
2017
Historical
period of record
High 24.00 339.00
Mean 6.30 11.78 19.24129
Low 5.00 5.00
No. of Samples 84 1742

Five-year Rolling Average

  • Annual Mean

pH

Units: None Year
2017
Historical
period of record
High 8.20 8.40
Mean 7.89 7.88 0.17314
Low 7.63 7.10
No. of Samples 80 1426

Five-year Rolling Average

  • Annual Mean

Salinity

Units: PSS Year
2017
Historical
period of record
High 35.50 38.80
Mean 29.30 30.34 5.08674
Low 14.90 1.80
No. of Samples 60 1406

Five-year Rolling Average

  • Annual Mean

Specific conductance

Units: umho Year
2017
Historical
period of record
High 53600.00 58320.00
Mean 44527.50 46498.78 7156.7632
Low 24500.00 3370.00
No. of Samples 80 1426

Five-year Rolling Average

  • Annual Mean

Temperature, water

Units: deg F Year
2017
Historical
period of record
High 91.76 92.48
Mean 80.89 77.81 8.84313
Low 70.52 48.56
No. of Samples 75 1371

Five-year Rolling Average

  • Annual Mean

Turbidity

Units: NTU Year
2017
Historical
period of record
High 8.00 24.00
Mean 3.85 4.24 2.05816
Low 1.30 0.85
No. of Samples 64 1290

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 (2017)

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 2017 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 1995 1999 2005 2011 2014 Trend
Urban & Built-up 32,90853.3% 34,51755.9% 35,33457.3% 37,84461.3% 38,34362.1% 37,98761.6%
Agriculture 6,33810.3% 5,9459.6% 4,6467.5% 2,4974% 2,2153.6% 2,3093.7%
Rangeland 5470.9% 3370.5% 2970.5% 1990.3% 2250.4% 4300.7%
Upland Forests 3,5885.8% 2,8544.6% 2,6194.2% 2,1093.4% 1,8743% 1,9233.1%
Water 13,35021.6% 13,44521.8% 13,63322.1% 14,22723.1% 14,27823.1% 14,13122.9%
Wetlands 2,8704.7% 2,3793.9% 2,3523.8% 2,2273.6% 2,2293.6% 2,3723.8%
Barren Land 290% 1080.2% 7211.2% 90% 990.2% 1090.2%
Transportation and Utilities 1,8453% 2,1303.5% 2,1133.4% 2,6024.2% 2,4524% 2,4534%
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.