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)
- [Chlorophyll a] is the predicted chlorophyll a concentration for the bay.
- color is the arithmetic mean of annual values by region.
- 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.
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 a “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 below:
Figure 3. 62-302.532 Estuary-Specific Numeric Interpretations of the Narrative Nutrient Criterion for Sarasota Bay