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 continued to deteriorate in 2016. Chlorophyll a increased significantly to a mean concentration of 0.0094 mg/l, above the threshold concentration of 0.0049 mg/l. Nitrogen concentration also increased, reaching  the “Caution” range, exceeding target and threshold levels. The mean value for phosphorus concentration increased but is still well below the target.

Water Quality: All three nutrient measures used in scoring water quality increased in 2016, a worsening trend. The mean chlorophyll a concentration in 2016 was 0.0094 mg/l, exceeding the threshold value of 0.0049 mg/l. Mean nitrogen concentration was 0.5533 mg/l, higher than the threshold of 0.420 mg/l. The annual mean concentration of phosphorus, although higher than in 2016, remained well below the threshold of 0.180 mg/l, at 0.1022 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
2016
Historical
period of record
High 0.059 0.061
Mean 0.009 0.006 0.0064483 0.0043000000
Low 0.002 0.000
No. of Samples 49 690

Five-year Rolling Average

  • Annual Mean
Caution

Nitrogen, Total

Score: Caution How was this determined?
Units: mg/l Year
2016
Historical
period of record
High 1.310 1.715
Mean 0.553 0.447 0.27604783 0.4000000000
Low 0.165 0.055
No. of Samples 49 686

Five-year Rolling Average

  • Annual Mean
Excellent

Phosphorus, Total

Score: Excellent How was this determined?
Units: mg/l Year
2016
Historical
period of record
High 0.290 0.470
Mean 0.102 0.122 0.07205125 0.1700000000
Low 0.050 0.050
No. of Samples 49 697

Five-year Rolling Average

  • Annual Mean

Dissolved Oxygen

Units: mg/l Year
2016
Historical
period of record
High 7.90 9.20
Mean 6.16 6.19 1.05196
Low 3.80 2.10
No. of Samples 48 656

Five-year Rolling Average

  • Annual Mean

Apparent Color

Units: PCU Year
2016
Historical
period of record
High 340.00 400.00
Mean 61.51 39.55 64.31801
Low 2.00 2.00
No. of Samples 49 696

Five-year Rolling Average

  • Annual Mean

BOD, Biochemical oxygen demand

Units: mg/l Year
2016
Historical
period of record
High 3.90 8.50
Mean 0.99 1.03 0.74074
Low 0.50 0.50
No. of Samples 49 593

Five-year Rolling Average

  • Annual Mean

Dissolved oxygen saturation

Units: percent (%) Year
2016
Historical
period of record
High 117.00 129.00
Mean 89.00 90.17 13.21234
Low 60.00 33.00
No. of Samples 52 660

Five-year Rolling Average

  • Annual Mean

Karenia brevis ("red tide")

Units: #/l Year
2016
Historical
period of record
High 526000.00 2460000.00
Mean 84275.86 30853.33 161971.8293
Low 1000.00 1000.00
No. of Samples 29 525

Five-year Rolling Average

  • Annual Mean

Light Attenuation

Units: K(1/m) Year
2016
Historical
period of record
High 6.46 9.04
Mean 1.57 1.14 0.94835
Low 0.29 0.06
No. of Samples 42 642

Five-year Rolling Average

  • Annual Mean

Nitrogen, Ammonia + Ammonium as N

Units: ug/l Year
2016
Historical
period of record
High 120.00 326.00
Mean 28.69 27.88 35.63475
Low 5.00 5.00
No. of Samples 49 697

Five-year Rolling Average

  • Annual Mean

Nitrogen, Kjeldahl

Units: ug/l Year
2016
Historical
period of record
High 1270.00 1680.00
Mean 535.71 434.82 263.88619
Low 160.00 50.00
No. of Samples 49 697

Five-year Rolling Average

  • Annual Mean

Nitrogen, Nitrite + Nitrate as N

Units: ug/l Year
2016
Historical
period of record
High 67.00 108.00
Mean 17.61 12.80 16.25464
Low 5.00 5.00
No. of Samples 49 784

Five-year Rolling Average

  • Annual Mean

pH

Units: None Year
2016
Historical
period of record
High 8.30 8.30
Mean 7.77 7.82 0.25077
Low 7.10 6.70
No. of Samples 52 660

Five-year Rolling Average

  • Annual Mean

Rainfall

Units: inches/yr Year
2016
Historical
period of record
High 57.8 66.5
Mean 43.1 16.6892850730641
Low 9.6
No. of Samples 364 5444

Five-year Rolling Average

  • Annual Mean

Salinity

Units: PSS Year
2016
Historical
period of record
High 36.00 38.50
Mean 26.18 30.24 8.27409
Low 0.16 0.10
No. of Samples 48 656

Five-year Rolling Average

  • Annual Mean

Specific conductance

Units: umho Year
2016
Historical
period of record
High 54270.00 57710.00
Mean 41075.50 46224.35 12082.27369
Low 326.00 280.00
No. of Samples 52 660

Five-year Rolling Average

  • Annual Mean

Temperature, water

Units: deg F Year
2016
Historical
period of record
High 73.94 94.46
Mean 70.66 77.82 9.3252
Low 66.92 50.90
No. of Samples 8 616

Five-year Rolling Average

  • Annual Mean

Turbidity

Units: NTU Year
2016
Historical
period of record
High 14.00 23.00
Mean 4.34 4.24 2.64557
Low 1.10 0.35
No. of Samples 49 689

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 (2016)

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 2016 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 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 2005 2014 Trend
Urban & Built-up 11,51920.6% 17,10130.6% 17,84131.9%
Agriculture 14,25725.5% 14,43825.8% 13,17023.5%
Rangeland 3,9897.1% 1,7153.1% 2,3094.1%
Upland Forests 12,68822.7% 7,90614.1% 7,70113.8%
Water 1,9313.5% 2,8655.1% 2,8665.1%
Wetlands 10,34918.5% 9,80817.5% 9,79517.5%
Barren Land 220% 100% 510.1%
Transportation and Utilities 1,2022.1% 2,0993.8% 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.