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 was good in 2015, despite an increase in chlorophyll a concentration of 68.7%, compared to 2014. This increase caused it to exceed the threshold level. On a more positive note, mean nitrogen concentration decreased slightly. And while phosphorus concentration increased somewhat, it is still well below the target value of 0.23 mg/l.

Water Quality: Nitrogen and phosophorus retained their ratings from 2013 and 2014 (“Good” and “Excellent”, respectively). Mean nitrogen concentration was 0.4927 mg/l and mean phosphorus concentration was 0.1023 mg/l. While chlorophyll a concentration had improved in 2014 to an “Excellent” rating, its significant increase pushed its 2015 rating down two notches, to “Caution.” 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).

Biotic Indicator: Measurement of the biotic indicator, seagrass, was performed in 2014 by the Southwest Florida Water Management District. Total seagrass acreage in Roberts Bay increased slightly to 321 acres from its previous value and is creeping toward 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
2015
Historical
period of record
High 0.032 0.048
Mean 0.011 0.008 0.00753171 0.0082000000
Low 0.002 0.000
No. of Samples 66 1139

Five-year Rolling Average

  • Annual Mean
Good

Nitrogen, Total

Units: mg/l Year
2015
Historical
period of record
High 0.997 1.376
Mean 0.493 0.17152141 0.4500000000
Low 0.235 0.065
No. of Samples 66 1131

Five-year Rolling Average

  • Annual Mean
Excellent

Phosphorus, Total

Score: Excellent How was this determined?
Units: mg/l Year
2015
Historical
period of record
High 0.230 0.480
Mean 0.102 0.134 0.06632946 0.1900000000
Low 0.050 0.050
No. of Samples 66 1158

Five-year Rolling Average

  • Annual Mean

Dissolved Oxygen

Units: mg/l Year
2015
Historical
period of record
High 10.00 11.40
Mean 6.80 6.47 1.17697
Low 4.70 3.50
No. of Samples 60 1270

Five-year Rolling Average

  • Annual Mean

Apparent Color

Units: PCU Year
2015
Historical
period of record
High 110.00 150.00
Mean 23.62 22.98 17.59929
Low 5.00 2.00
No. of Samples 66 1158

Five-year Rolling Average

  • Annual Mean

BOD, Biochemical oxygen demand

Units: mg/l Year
2015
Historical
period of record
High 3.40 5.90
Mean 1.55 1.41 0.80641
Low 0.60 0.50
No. of Samples 66 1022

Five-year Rolling Average

  • Annual Mean

Dissolved oxygen saturation

Units: percent (%) Year
2015
Historical
period of record
High 154.00 173.00
Mean 100.05 94.12 15.46962
Low 75.00 50.00
No. of Samples 60 1286

Five-year Rolling Average

  • Annual Mean

Karenia brevis ("red tide")

Units: #/l Year
2015
Historical
period of record
High 813000.00 912000.00
Mean 32265.31 9500.00 64754.49444
Low 1000.00 1000.00
No. of Samples 49 634

Five-year Rolling Average

  • Annual Mean

Light Attenuation

Units: K(1/m) Year
2015
Historical
period of record
High 1.88 3.56
Mean 1.00 1.02 0.4238
Low 0.42 0.19
No. of Samples 60 1069

Five-year Rolling Average

  • Annual Mean

Nitrogen, Ammonia + Ammonium as N

Units: ug/l Year
2015
Historical
period of record
High 61.00 243.00
Mean 14.67 23.79 28.15914
Low 5.00 5.00
No. of Samples 66 1158

Five-year Rolling Average

  • Annual Mean

Nitrogen, Kjeldahl

Units: ug/l Year
2015
Historical
period of record
High 960.00 1320.00
Mean 508.64 446.84 167.06276
Low 230.00 60.00
No. of Samples 66 1158

Five-year Rolling Average

  • Annual Mean

Nitrogen, Nitrite + Nitrate as N

Units: ug/l Year
2015
Historical
period of record
High 67.00 339.00
Mean 8.26 11.60 19.13121
Low 5.00 5.00
No. of Samples 66 1590

Five-year Rolling Average

  • Annual Mean

pH

Units: None Year
2015
Historical
period of record
High 8.20 8.40
Mean 7.94 7.88 0.17598
Low 7.70 7.10
No. of Samples 60 1286

Five-year Rolling Average

  • Annual Mean

Salinity

Units: PSS Year
2015
Historical
period of record
High 34.80 38.80
Mean 29.46 30.59 4.94043
Low 12.10 1.80
No. of Samples 60 1270

Five-year Rolling Average

  • Annual Mean

Specific conductance

Units: umho Year
2015
Historical
period of record
High 52730.00 58320.00
Mean 45346.00 46839.04 7020.68198
Low 20270.00 3370.00
No. of Samples 60 1286

Five-year Rolling Average

  • Annual Mean

Temperature, water

Units: deg F Year
2015
Historical
period of record
High 90.14 92.48
Mean 79.81 77.70 8.94096
Low 62.60 48.56
No. of Samples 60 1286

Five-year Rolling Average

  • Annual Mean

Turbidity

Units: NTU Year
2015
Historical
period of record
High 8.40 24.00
Mean 4.30 4.26 2.11343
Low 1.70 0.85
No. of Samples 66 1140

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 (2015)

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 2015 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 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 2014 Trend
Urban & Built-up 32,90853.3% 37,84461.3% 37,98761.6%
Agriculture 6,33810.3% 2,4974% 2,3093.7%
Rangeland 5470.9% 1990.3% 4300.7%
Upland Forests 3,5885.8% 2,1093.4% 1,9233.1%
Water 13,35021.6% 14,22723.1% 14,13122.9%
Wetlands 2,8704.7% 2,2273.6% 2,3723.8%
Barren Land 290% 90% 1090.2%
Transportation and Utilities 1,8453% 2,6024.2% 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.