This case study presents the Florida Department of Environmental Protection’s (FDEP’s) background and methodology for developing estuarine nutrient criteria for Tampa Bay as well as proposed numeric TN loading criteria for the four mainstem segments of Tampa Bay. Those criteria were derived using a stressor-response approach that included developing a series of regression models linking nitrogen loading to chlorophyll a and light requirements needed to support seagrass growth at depths that equate to 1950 levels of seagrass colonization in the bay.
These numeric nutrient criteria support FDEP’s overarching goal of managing Tampa Bay nutrient loadings and concentrations at levels that protect and maintain healthy, well-balanced aquatic communities. Successful seagrass growth and reproduction require that adequate levels of light penetrate the water column. Because phytoplankton biomass in the water column affects light attenuation and nitrogen loading affects biomass, quantitative relationships and targets must be established that link light attenuation, phytoplankton biomass (as measured by chlorophyll a concentration), and nitrogen loading. The stressor-response approach was chosen because of the availability of data to accurately model relationships between nutrient loading and key response variables.
FDEP selected a healthy seagrass community as measured by the areal extent of colonization as an assessment endpoint for protecting and maintaining healthy, well-balanced aquatic communities in Tampa Bay. Historical levels of seagrass colonization were chosen to be the indicator variable that supports the endpoint and 95 percent of the areal extent of seagrasses found in the 1950s was chosen as the target value. The seagrass health and areal colonization assessment endpoint is linked to numeric nutrient criteria development because water clarity, which affects the depth and extent of seagrass colonization, is affected by the amount of phytoplankton biomass found in the water column which, in turn, is affected by nutrient loading. The incident light and seagrass depth relationship was modeled to develop a light attenuation target. The chlorophyll a and incident light relationship was modeled to develop the chlorophyll a target. Nitrogen loading and chlorophyll a were modeled to develop the nitrogen loading target.
The mainstem of Tampa Bay is divided into four segments for management purposes and to reflect differences in nutrient sources, freshwater inflows, tidal processes, and so forth. Seagrass, light attenuation, chlorophyll a, and nitrogen loading targets as well as numeric nutrient criteria were developed for each segment. Since 1972, 52 stations in Tampa Bay have been monitored by the Environmental Protection Commission of Hillsborough County, Florida. The analytical database used to derive the nutrient criteria included mean monthly and annual chlorophyll a, light attenuation, nutrient concentrations, and annual TN loadings.
Given the minimum light requirement of 20.5 percent of incident light, predictions of chlorophyll a levels, and Secchi disk depths (light penetration necessary to restore seagrass to average depths observed in each of the bay segments during the 1950s) were used in developing annual targets for these parameters. TN loads from both external and intersegment transport were related to observed chlorophyll a using a regression model.
Estuarine numeric TN loading criteria were derived for four segments of Tampa Bay based on 1992–1994 conditions. Rainfall and subsequent hydrologic loading were determined to significantly influence residence time in the bay and algal biomass production. For that reason, nitrogen criteria are expressed as a nitrogen delivery ratio (i.e., tons TN per million cubic meters of freshwater delivered) (Janicki Environmental 2011).
Reference:
Janicki Environmental (Janicki Environmental, Inc.). 2011. Proposed Numeric Nutrient Criteria for Tampa Bay. Prepared for Tampa Bay Estuary Program. Accessed October 2016. https://www.tbeptech.org/attachments/article/97/TBEP_Estuarine_NNC_Recommendation_September132011.pdf Exit