6.1 Estimate and communicate annual nutrient loads from stormwater

Chapter 6. Stormwater Design and Maintenance
Science Solutions
Activities to Remove Nutrients

Key Message: Robust, long-term monitoring and modeling programs that account for surface and subsurface runoff, streamflow, and the composition of nitrogen species (organic and inorganic) are essential to accurately estimate and manage nutrient loads in watersheds.

Importance

Surface and subsurface runoff carries excess nutrients and other pollutants to downstream waterbodies, including creeks, estuaries, and bays. Identifying the locations of primary outfalls and quantifying their pollutant loads is critical for building data-driven nutrient budgets and targeted management strategies. Monitoring nutrient concentrations and streamflow volumes enables the calculation of nutrient loads and can help identify pollution hotspots—particularly when focusing on dissolved inorganic nitrogen (DIN), a form of nitrogen that is readily available to aquatic organisms and can rapidly fuel algae growth. The fraction of DIN within total nitrogen (TN) can provide a useful indicator of pollution sources and help prioritize areas for management action. Higher DIN fractions often signal human-related sources such as wastewater, fertilizers, and urban development.

Overview

Sarasota County’s Spatially Integrated Model for Pollutant Loading Estimates (SIMPLE), updated in 2021 (Janicki and JEA 2021), uses rainfall, land use, and known pollution sources to identify nutrient hotspots. Model outputs guide development of watershed management plans and priority BMP projects. These include the Sarasota Bay Watershed Management Plan Update (Stantec 2022) and pending plans for Little Sarasota Bay and Lemon Bay (in prep 2025) (see Chapter 6.1, Chapter 6.2).

However, the most direct and accurate method for estimating average annual nutrient loads to the bays is through measurement. Nutrient loads are calculated as the product of water volume and nutrient concentration, so both water quality and water quantity data are needed. Reliable estimates depend on long-term data records, which improve precision by accounting for year-to-year variability.

Sarasota County Stormwater Environmental Utility has mapped the major primary outfalls that discharge to the six bay segments and the numerous smaller outfalls located along barrier islands and other coastal fringes (Figure 6.1.1).

Figure 6.1.1. Major stormwater outfalls discharging to canals, creeks, and bay waters. Watersheds corresponding to 6 primary bay segments and the Myakka River indicate receiving waters. Source: Sarasota County Government

Sarasota County has a comprehensive monitoring program in its primary watersheds (Figure 6.1.2). The program was phased in from 2004–2007 (see Chapter 10.1), allowing for more than a decade of nutrient load estimates. Data from Southwest Florida Water Management District (SWFWMD), Florida Department of Environmental Protection (FDEP), and U.S. Geological Survey (USGS) further enhance historical coverage. For instance, a USGS streamflow gage has operated on the Myakka River since 1939.

Figure 6.1.2. Water quality and water flow monitoring stations in Sarasota County. Source: Sarasota County Water Atlas

A key challenge is that many stream gage stations still require the development or verification of rating curves to estimate flow. A rating curve defines the relationship between stream height (stage) and water discharge, specific to the hydraulics at each gage site (Fondriest Environmental, Inc. 2015). For meaningful flow calculations, stream stages must be recorded upstream of tidal influence. While Sarasota’s Automated Rainfall Monitoring System (ARMS) stations provide stage data, they do not measure discharge. Some USGS stations, however, have well-established rating curves.

Water quality monitoring across Southwest Florida is coordinated through the Regional Ambient Monitoring Program (RAMP), which standardizes sampling and lab procedures to ensure data quality. These datasets are submitted to FDEP’s public Watershed Information Network (WIN). Sarasota County data are published on the Sarasota County Water Atlas (see Chapter 9.4).

The Water Atlas includes multiple tools and dashboards for tracking nutrient concentrations, water clarity, red tide events, salinity, bacteria, and dissolved oxygen. It also publishes Bay Condition Index Scores that summarize the status of chlorophyll-a, nitrogen, and phosphorus in Sarasota Bay. This kind of public-facing data visualization is essential for transparency and community awareness about local water health.

Approach

To demonstrate the process of estimating annual nutrient loads, monitoring data from 2006–2018 were analyzed for two watersheds: urban/suburban Whitaker Bayou and more natural Deer Prairie Slough.

Deer Prairie Slough Source: Sarasota Water Atlas

Whitaker Bayou Source: Sarasota Water Atlas

Water quantity data for Whitaker Bayou was collected continuously by a USGS station beginning in 1995 (Figure 6.1.3), while monthly water quality data collection began in 2006. This analysis focused on TN and DIN, with DIN defined as the sum of ammonium (NH4), nitrate (NO3), and nitrite (NO2)—the most bioavailable forms of nitrogen. Elevated concentrations of DIN can be indicative of anthropogenic sources such as wastewater by-products, synthetic fertilizers, and possibly areas where natural landscapes have been removed and replaced with compacted material (sands, silts, and clays).

Figure 6.1.3. Observed rainfall and calculated runoff (USGS gage 02299861) for the Whitaker Bayou watershed 1995-2018. In some years more than 50% of rainfall runs off the land into the bayou. Source: Sarasota County Water Atlas.

Nitrogen loads were computed for each month and totaled to obtain the annual load. Results showed that the average annual TN unit load for Whitaker Bayou from 2006–2018 was 3.43 pounds/acre, with DIN making up 28% of the total nitrogen load (Table 6.1.1). A comparison with the more natural Deer Prairie Slough found that although total nitrogen loads were only about 6% lower, the DIN percentage in Whitaker Bayou was nearly four times higher—suggesting human influence on nutrient composition (Table 6.1.2 and Figure 6.1.4). Projects in urbanized watersheds may therefore benefit from focusing on reducing DIN through targeted interventions like denitrification.

Table 6.1.1. Annual nitrogen loads 2006-2018 for Whitaker Bayou as calculated from runoff volume and water quality sampling data.

Table 6.1.2. Comparison of nitrogen loading between a suburban (Whitaker Bayou) and a rural (Deer Prairie Slough) watershed.

Figure 6.1.4. Comparison of organic nitrogen versus dissolved inorganic nitrogen in urban (Whitaker Bayou) and rural (Deer Prairie Slough) watersheds.

Further analysis (2006–2021) of 72 monitoring sites countywide showed DIN as a percentage of TN ranging from 8% to 66%. Higher DIN percentages tended to correspond to areas with more anthropogenic modification (Figure 6.1.5). Conversely, watershed areas with limited anthropogenic alterations or certain management strategies exhibit low DIN percentages, generally below 10% (provisional methods and data, Suau communication).

Figure 6.1.5. Dissolved inorganic nitrogen (DIN) as a fraction of total nitrogen (TN) expressed as a management grade ranging from A+ (less than 10% DIN) to F (greater than 50% DIN).

Percent DIN could be used as a management metric to help prioritize locations that may require detailed site evaluations to identify potential sources of elevated DIN and suggest potential management actions (Table 6.1.3). Sites with confirmed sources of DIN that are also sources of elevated levels of total nitrogen on or near a waterway/estuary could be further prioritized.

Table 6.1.3. An example of the DIN management metric applied to select monitoring locations with a subjective initial suggestion of potential DIN sources. A more thorough review and site evaluation will likely reveal additional and more direct sources of elevated DIN.

The analysis also compared annual TN and DIN unit loads (pounds/acre) across 11 sites (provisional methods and data, Suau communication). While TN loads did not correlate strongly with land use, DIN loads were consistently higher in more urbanized watersheds such as Whitaker Bayou and Pillippi Creek, reinforcing the importance of targeting DIN in management strategies (Table 6.1.4).

Table 6.1.4. Average annual total nitrogen and dissolved inorganic bioavailable nitrogen loads (pounds per acre) for select monitoring locations in Sarasota County.

Analyzing available data for concurrent periods of record for Deer Prairie Slough and other watersheds could yield average annual nutrient loads for a range of hydrologic and land use conditions, as well as provide additional guidance on the management of specific nitrogen constituents, such as nitrate.

The availability of a decade’s worth of water quantity and quality monitoring data available is important for doing these analyses (see Chapter 10.1). Since nutrient loads vary annually with natural variations in rainfall and runoff, multiple years of data are necessary to compute statistical averages precisely and reliably. Most critically, these data provide a basis for calibrating and verifying nutrient load models to assure they accurately reflect actual observations for scenario planning. Model calibration could and should be performed at the watershed or watercourse scale but could also be scaled for bay management (Figure 6.1.1). Once calibrated, nutrient load models could be used to predict and test load reductions and evaluate the costs and benefits of public policy and investments (e.g., regulatory, BMPs, capital projects).

Recommendations

  • Use relative DIN percentage as a metric to prioritize strategic reduction investigations, management actions, and investments. Consider an area’s proximity to waterways for prioritization.
  • Where available, utilize measured nitrogen loads as the basis for planning purposes and loading model calibration/verification (see Chapter 10.2).
  • Reinstate discontinued monitoring locations where DIN percentages for their period of record exceeded 15%.
  • The following sampling locations should be added to address significant watershed area gaps:
    • South Creek at U.S.41
    • Fox Creek at I-75
    • Fox Creek upstream of confluence with Shakett Creek
    • Forked Creek at S.R. 776
    • Wares Creek
    • Palma Sola Creek
    • Unnamed drainage ditches at El Conquistador Parkway.
  • Leverage partnerships and coordination opportunities to augment Sarasota County Government monitoring with other municipalities, estuary programs, and entities such as the Suncoast Waterkeepers (see Chapter 9.2).
  • Develop unit DIN loads for watershed areas as a metric for relative comparison and management of DIN.
  • Use measured nitrogen loads as the basis for planning purposes as well as pollutant load model calibration/verification.
  • Engage the USGS or other qualified hydrologists to develop rating curves and manage the ARMS database (see Chapter 10.1).

Resources

  • Sarasota County Water Atlas
  • Sarasota County Stormwater Environmental Utility
  • Southwest Florida Water Management District
  • Florida Department of Environmental Protection
  • United States Geological Survey

Status

Preliminary analysis

Performance Measure

  • Quality control and assurance of water quantity flow data to identify and address data gaps or anomalies. Rating curves applied or developed to convert water elevations into average daily discharges and runoff volumes
  • Quality control and assurance of nutrient water quality data to identify and address data gaps or anomalies and reduce and render the data usable
  • Monthly and annual total phosphorus and total nitrogen loads, as well as dissolved inorganic nitrogen and nitrate loads, for each primary outfall and their watershed

Experts or Leads

USGS, John Coffin; Sarasota County Environmental Stormwater Utility; New College of Florida Data Science Program; University of South Florida Water Institute Water Atlas; Stephen Suau

Cost Estimate

$100,000-$1,000,000

Related Activities

Chapter 9.4, Chapter 10.1

 

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Other Stormwater System Activities