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 health of Dona/Roberts Bay has declined from the 2012 assessment, based on water quality indicators. All three measures increased in value, with only one of the three earning a passing rating.

Water Quality: All three water quality parameters used in scoring – chlorophyll a, nitrogen and phosphorus – showed marked increases during the latter part of 2013. This increase caused the annual mean concentration of both chlorophyll a and nitrogen to exceed the threshold values established for a “Good” rating. The mean chlorophyll a concentration was 0.0066 mg/l, which exceeds the threshold value of 0.0049 mg/l. Mean nitrogen concentration was 0.543 mg/l, which is higher than the threshold of 0.420 mg/l. While the annual mean concentration level of phosphorus also increased (to a mean value of 0.1013 mg/l), it remained below the threshold of 0.180 mg/l. The means for chlorophyll a, nitrogen and phosphorus were calculated as arithmetic means (per the Numeric Nutrient Criteria outlined in the Florida Administrative Code, section 62-302.532). With two water quality measures exceeding threshold values, Dona/Roberts Bay is therefore rated as a Caution for 2013.

Biotic Indicator: Measurement of the biotic indicator, seagrass, is performed at two-year intervals and is due for monitoring again in 2014. When last assessed in 2012, total seagrass acreage in Dona/Roberts Bay was 79 acres, 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
2013
Historical
period of record
High 0.061 0.061
Mean 0.007 0.005 0.00558206 0.0043000000
Low 0.001 0.000
No. of Samples 48 543

Five-year Rolling Average

  • Annual Mean
Caution

Nitrogen, Total

Score: Caution How was this determined?
Units: mg/l Year
2013
Historical
period of record
High 1.715 1.715
Mean 0.543 0.422 0.26673616 0.4000000000
Low 0.155 0.055
No. of Samples 48 536

Five-year Rolling Average

  • Annual Mean
Excellent

Phosphorus, Total

Score: Excellent How was this determined?
Units: mg/l Year
2013
Historical
period of record
High 0.420 0.470
Mean 0.101 0.128 0.07434207 0.1700000000
Low 0.050 0.050
No. of Samples 48 550

Five-year Rolling Average

  • Annual Mean

Dissolved Oxygen

Units: mg/l Year
2013
Historical
period of record
High 7.70 9.20
Mean 6.06 6.22 1.02908
Low 3.70 2.90
No. of Samples 48 512

Five-year Rolling Average

  • Annual Mean

Apparent Color

Units: PCU Year
2013
Historical
period of record
High 400.00 400.00
Mean 54.96 36.49 59.7851
Low 5.00 2.00
No. of Samples 48 549

Five-year Rolling Average

  • Annual Mean

BOD, Biochemical oxygen demand

Units: mg/l Year
2013
Historical
period of record
High 5.70 8.50
Mean 1.10 1.05 0.79064
Low 0.50 0.50
No. of Samples 47 446

Five-year Rolling Average

  • Annual Mean

Dissolved oxygen saturation

Units: percent (%) Year
2013
Historical
period of record
High 114.00 129.00
Mean 86.79 90.74 12.60001
Low 58.00 37.00
No. of Samples 48 512

Five-year Rolling Average

  • Annual Mean

Karenia brevis ("red tide")

Units: #/l Year
2013
Historical
period of record
High 2460000.00 2460000.00
Mean 101250.00 29356.10 172775.58695
Low 1000.00 1000.00
No. of Samples 48 410

Five-year Rolling Average

  • Annual Mean

Light Attenuation

Units: K(1/m) Year
2013
Historical
period of record
High 5.38 9.04
Mean 1.20 1.08 0.8721
Low 0.25 0.06
No. of Samples 48 504

Five-year Rolling Average

  • Annual Mean

Nitrogen, Ammonia + Ammonium as N

Units: ug/l Year
2013
Historical
period of record
High 180.00 326.00
Mean 35.00 27.30 35.72047
Low 5.00 5.00
No. of Samples 48 550

Five-year Rolling Average

  • Annual Mean

Nitrogen, Kjeldahl

Units: ug/l Year
2013
Historical
period of record
High 1680.00 1680.00
Mean 526.46 410.31 254.8318
Low 150.00 50.00
No. of Samples 48 550

Five-year Rolling Average

  • Annual Mean

Nitrogen, Nitrite + Nitrate as N

Units: ug/l Year
2013
Historical
period of record
High 108.00 108.00
Mean 16.58 11.96 15.66476
Low 5.00 5.00
No. of Samples 48 637

Five-year Rolling Average

  • Annual Mean

pH

Units: None Year
2013
Historical
period of record
High 8.10 8.30
Mean 7.80 7.82 0.25218
Low 7.00 6.70
No. of Samples 48 512

Five-year Rolling Average

  • Annual Mean

Salinity

Units: PSS Year
2013
Historical
period of record
High 34.70 38.50
Mean 27.97 30.89 7.91426
Low 0.70 0.10
No. of Samples 48 512

Five-year Rolling Average

  • Annual Mean

Specific conductance

Units: umho Year
2013
Historical
period of record
High 52550.00 57710.00
Mean 42864.94 47137.20 11565.43379
Low 1400.00 280.00
No. of Samples 48 512

Five-year Rolling Average

  • Annual Mean

Temperature, water

Units: deg F Year
2013
Historical
period of record
High 89.60 94.46
Mean 77.47 77.77 9.51196
Low 64.22 50.90
No. of Samples 48 512

Five-year Rolling Average

  • Annual Mean

Turbidity

Units: NTU Year
2013
Historical
period of record
High 12.00 23.00
Mean 4.23 4.19 2.70054
Low 1.10 0.35
No. of Samples 48 541

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 (2013)

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 2013 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.