Sarasota Bay Nitrogen

What does this mean?

Nitrogen to Chlorophyll a Relationship for Sarasota Bay

This "Learn More" expands on and includes information found in the Numeric Nutrient Criteria For Sarasota Bay, Final Report, August 18, 2010, (SBEP NNC Final Report) prepared by Janicki Environmental, Inc. for the Sarasota Bay Estuary Program.

The Sarasota Bay Estuary Program (SBEP) submitted a proposal to the U.S. Environmental Protection Agency (EPA) for estuary numeric nutrient criteria for each bay within their study area. First, a model was developed that relates each nutrient of interest to an established chlorophyll a standard. Chlorophyll a provides a measure of algal biomass which is a key determinant of water quality in our water resources (bays, rivers, lakes and ponds). Algae absorb, scatter, and reflect sunlight, thus reducing the amount that is available to support photosynthesis by seagrass. The ability of a bay to produce abundant seagrass is a primary indicator of a bay's condition; thus the amount of algae, as measured by chlorophyll a concentration, is an important measure of bay health. (For Sarasota Bay, the desired concentration, or target, for chlorophyll a is 5.2 μg/L.) The SBEP also recommended a threshold concentration, above which seagrass growth may be negatively impacted, that should not be exceeded. (This recommended chlorophyll a threshold value for Sarasota Bay is 6.1 μg/L.) The SBEP's threshold recommendations for the bays within its study area were adopted as the legal limits for chlorophyll a, as stated in the Florida Administrative Code (FAC 62-302.532).


How are the data collected? (Methods)

Sarasota Bay is divided between the north and south bay segments (Figure 1). The north segment is monitored by Manatee County and the south segment is monitored for Sarasota County by Mote Marine Laboratory. The north segment has significantly higher levels of total nitrogen (TN) and apparent color and lower levels of chlorophyll a than are found in the southern segment (Table 1). Color is a measure of the presence of dissolved substances in water. Because color limits the amount of light that is available for plant growth (including algal growth), the two bay segments would be expected to have different relationships between chlorophyll a and nutrients. A SBEP study of the relationship between nitrogen concentration and chlorophyll a concentration for Sarasota Bay determined that bay color and season (wet season/dry season) were the determining factors in the concentration of nitrogen required for algal growth. Therefore, an accurate nitrogen model for the bay was possible only if color, bay segment and the seasonality of the chlorophyll a response to nitrogen were all considered.

map showing north and south bay segments
Figure 1. Sarasota Bay segmentation. The segment dividing line is the Manatee/Sarasota political border (SBEP NNC Final Report, p 61).


Table 1. Comparison of water quality between the north and south segments of Sarasota Bay. Values represent medians for the period 1998-2009 (SBEP NNC Final Report, p 61)

Table VI-1. Comparison of water quality within Northern and Southern Sarasota Bay. Values represent medians for the period 1998-2009.

Variable Northern Sarasota Bay Southern Sarasota Bay
Chlorophyll a (μg/L) 5.2 3.4
TN (mg/L) 0.56 0.28
Color (PtCo units) 20 10


Calculations

A model (Equation 1) was developed by regressing chlorophyll a and total nitrogen with corrections for seasonality (dry/wet), color, and region (north/south), as stated below. This equation explained 67% of the variability (R2 = 0.67) of nitrogen and chlorophyll a for Sarasota Bay (Figure 2).

Equation 1: [Chlorophyll a] = –1.06 + (3.58 × [TN]) + (0.32 × color) + (2.03 × season) – (4.84 × region) (SBEP NNC Final Report, p. 53-55)

Where:

  1. [Chlorophyll a] is the predicted chlorophyll a concentration for the bay.
  2. color is the arithmetic mean of annual values by region.
  3. season is the wet season (July-October) and dry season (all other months) and a dummy variable of 1 for wet season and 0 for dry season is used.

This model was then used to develop the target and threshold concentrations for TN in the bay (see statement below). Because the nitrogen target is based on color and season, and color concentrations change from year to year, the target and threshold must be determined annually using sample results by season and region.

scatter plot of sampled chlorophyll a values
Figure 2. Chlorophyll a concentration predicted from model versus those observed between 1998 and 2009 in Sarasota Bay (R2=0.679). (See SBEP NNC Final Report, p. 61)

The regression model was then converted into an numeric nutrient standard for nitrogen for the whole bay by first calculating the nitrogen threshold based on the arithmetic mean of the color for both bay segments by month (CNi and CSi), and then selecting the dry and wet season results to determine the monthly north segment dry and wet values (NWi and NDi) and monthly south segment dry and wet values (SWi and SDi), and then determining the geometric mean of the summed seasonal values for the north and south bay sections. Please see the excerpt from the legal numeric nutrient standard FAC 62-302.532[1] below:

excerpt from FAC showing algorithm to calculate TN threshold
Figure 3. 62-302.532 Estuary-Specific Numeric Interpretations of the Narrative Nutrient Criterion for Sarasota Bay

 


Caveats and Limitations

The regulatory threshold for nitrogen in Sarasota Bay is not constant. Its value is calculated using annual sampling data and the legal threshold may be changed periodically at the discretion of the regulatory authority, the Florida Department of Environmental Protection (FDEP). The corresponding "target" value is advisory (non-regulatory), is recommended by the Sarasota Bay Estuary Program, and is based on the threshold value for the same time period.


Additional Information