Caution

1 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 Dona/Roberts Bay was slightly improved in 2017, but chlorophyll a and nitrogen levels are still a cause for concern. Chlorophyll a decreased but is still above the threshold concentration. Nitrogen concentration also decresed, but likewise is still in the “Caution” range, exceeding threshold levels. The mean value for phosphorus concentration was already well below the target value but decreased further from its 2016 mean concentration.

Water Quality: The mean chlorophyll a concentration in 2017 was 0.0074 mg/l, an improvement over the previous year but still exceeding the threshold value of 0.0049 mg/l. Mean nitrogen concentration was 0.4626 mg/l; as with chlorophyll a, this is an improvement over the 2016 value but the concentration is higher than the threshold of 0.420 mg/l. The annual mean concentration of phosphorus was 0.0971 mg/l, well below the threshold of 0.180 mg/l. 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). In 2016, Dona/Roberts Bay retains its "Caution" rating, with two out of three water quality measures exceeding threshold values.

Biotic Indicator: Measurement of the biotic indicator, seagrass, was performed in 2016 by the Southwest Florida Water Management District. The increase in total seagrass acreage in Dona/Roberts Bay was slight. Between 2014 and 2016 it increased from 99 to 101 acres, remaining below the target level of 112 acres.

Dona/Roberts Bay

Dona/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 0.033 0.061
Mean 0.007 0.005 0.00612082 0.0043000000
Low 0.001 0.000
No. of Samples 49 1425

Five-year Rolling Average

  • Annual Mean
Caution

Nitrogen, Total

Score: Caution How was this determined?
Units: mg/l Year
2017
Historical
period of record
High 1.314 1.715
Mean 0.463 0.448 0.27429587 0.4000000000
Low 0.195 0.055
No. of Samples 49 735

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 0.290 0.470
Mean 0.094 0.120 0.07111834 0.1700000000
Low 0.050 0.050
No. of Samples 65 762

Five-year Rolling Average

  • Annual Mean

Dissolved Oxygen

Units: mg/l Year
2017
Historical
period of record
High 8.20 9.20
Mean 6.31 6.20 1.05411
Low 3.38 2.10
No. of Samples 48 708

Five-year Rolling Average

  • Annual Mean

Apparent Color

Units: PCU Year
2017
Historical
period of record
High 320.00 400.00
Mean 38.10 39.44 63.61265
Low 5.00 2.00
No. of Samples 61 757

Five-year Rolling Average

  • Annual Mean

BOD, Biochemical oxygen demand

Units: mg/l Year
2017
Historical
period of record
High 2.60 8.50
Mean 1.08 1.03 0.72059
Low 0.50 0.50
No. of Samples 61 654

Five-year Rolling Average

  • Annual Mean

Dissolved oxygen saturation

Units: percent (%) Year
2017
Historical
period of record
High 122.00 129.00
Mean 92.61 90.38 13.28488
Low 46.00 33.00
No. of Samples 64 724

Five-year Rolling Average

  • Annual Mean

Karenia brevis ("red tide")

Units: #/l Year
2017
Historical
period of record
High 627000.00 2460000.00
Mean 36522.22 31300.88 158309.71541
Low 0.00 0.00
No. of Samples 45 570

Five-year Rolling Average

  • Annual Mean

Light Attenuation

Units: K(1/m) Year
2017
Historical
period of record
High 4.32 9.04
Mean 1.11 1.13 0.93352
Low 0.12 0.06
No. of Samples 48 696

Five-year Rolling Average

  • Annual Mean

Nitrogen, Ammonia + Ammonium as N

Units: ug/l Year
2017
Historical
period of record
High 133.00 326.00
Mean 18.59 27.13 35.1038
Low 5.00 5.00
No. of Samples 61 758

Five-year Rolling Average

  • Annual Mean

Nitrogen, Kjeldahl

Units: ug/l Year
2017
Historical
period of record
High 1240.00 1680.00
Mean 434.00 434.75 260.41411
Low 190.00 50.00
No. of Samples 65 762

Five-year Rolling Average

  • Annual Mean

Nitrogen, Nitrite + Nitrate as N

Units: ug/l Year
2017
Historical
period of record
High 74.00 108.00
Mean 13.75 12.88 16.2373
Low 5.00 5.00
No. of Samples 65 849

Five-year Rolling Average

  • Annual Mean

pH

Units: None Year
2017
Historical
period of record
High 8.10 8.30
Mean 7.82 7.82 0.24666
Low 7.20 6.70
No. of Samples 64 724

Five-year Rolling Average

  • Annual Mean

Salinity

Units: PSS Year
2017
Historical
period of record
High 36.00 38.50
Mean 29.61 30.21 8.20674
Low 3.19 0.10
No. of Samples 48 708

Five-year Rolling Average

  • Annual Mean

Specific conductance

Units: umho Year
2017
Historical
period of record
High 54300.00 57710.00
Mean 45203.44 46134.11 11924.87647
Low 5770.00 280.00
No. of Samples 64 724

Five-year Rolling Average

  • Annual Mean

Temperature, water

Units: deg F Year
2017
Historical
period of record
High 92.84 94.46
Mean 81.25 78.12 9.13991
Low 70.70 50.90
No. of Samples 60 676

Five-year Rolling Average

  • Annual Mean

Turbidity

Units: NTU Year
2017
Historical
period of record
High 11.00 23.00
Mean 4.10 4.24 2.61132
Low 1.80 0.35
No. of Samples 49 745

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 112 acres

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

Impervious Features

Dona/Roberts Bay is located within the Dona and Roberts Bay Watershed. View details about the Dona and Roberts 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.

7% of the land area within the Dona and Roberts 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 »

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

Acreage and Percentage within each Land Use / Land Cover Category for Dona and Roberts Bay Watershed
Land Use Classification 1990 1995 1999 2005 2011 2014 Trend
Urban & Built-up 11,51920.6% 13,67024.4% 15,33327.4% 17,10130.6% 17,69731.6% 17,84131.9%
Agriculture 14,25725.5% 15,00626.8% 14,65226.2% 14,43825.8% 13,41024% 13,17023.5%
Rangeland 3,9897.1% 3,0395.4% 2,6374.7% 1,7153.1% 2,1043.8% 2,3094.1%
Upland Forests 12,68822.7% 10,64919% 8,90015.9% 7,90614.1% 7,74213.8% 7,70113.8%
Water 1,9313.5% 2,6164.7% 2,7845% 2,8655.1% 2,8135% 2,8665.1%
Wetlands 10,34918.5% 9,53517% 9,68217.3% 9,80817.5% 9,90717.7% 9,79517.5%
Barren Land 220% 260% 60% 100% 840.2% 510.1%
Transportation and Utilities 1,2022.1% 1,4012.5% 1,9493.5% 2,0993.8% 2,1853.9% 2,2093.9%
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.