This report proposes methods and approaches for deriving numeric nutrient criteria for Florida’s estuaries to replace the existing narrative criteria. The primary management goal is to protect, maintain, and restore a balanced natural population of aquatic flora and fauna. To accomplish this goal EPA evaluated several candidate assessment endpoints and selected three for consideration in the derivation of the numeric nutrient criteria:
- Healthy seagrass communities—as measured by the depth of seagrass colonization. Water clarity affects colonization depth and is linked to nutrient loading, ambient nutrient concentrations, and algal biomass (measured as chlorophyll a).
- Balanced algal biomass and production—as measured by chlorophyll a, which is linked to nutrient loading and ambient nutrient concentrations.
- Balanced faunal communities—as measured by dissolved oxygen levels, which is linked to nutrient loading, ambient nutrient concentrations, chlorophyll a, and the decomposition of organic material in the benthic zone and hypoxia.
A conceptual model was developed by EPA that documents pathways for nutrient effects on aquatic life. The model shows the linkages between nitrogen and phosphorus criteria; chlorophyll a criteria; and the key conditions that affect the three assessment endpoints.
Three basic categories of analytical approaches were considered by EPA for deriving the numeric nutrient criteria: reference condition, stressor response relationships, and water quality simulation models. Each approach was evaluated based on their strengths, weaknesses, and applicability (e.g., availability of data). EPA assembled a large diverse set of environmental data to support this effort. The approaches could be applied independently or in combination depending on the estuarine region.
The reference condition approach involves defining a reference condition based on historical data from a group of minimally-impacted waterbodies known to fully support designated uses. A percentile of the water quality measurements is selected as the numeric criteria. Because essentially no water quality data exists for impaired Florida estuaries, however, the reference condition approach considered by EPA focuses on current water quality conditions that support balanced natural populations of flora and fauna. Sources of these data included peer-reviewed literature, state and local reports, and Florida’s Impaired Waters Rule database. Statistical reference points under consideration include upper percentile concentrations and the median or average of the water quality condition distribution.
The stressor response approach involves developing regression models to document relationships between one or more explanatory variables and a single response variable. EPA considered several stressor response relationships including using nitrogen and phosphorus concentration as explanatory variables and chlorophyll a as the response variable. Other analyses included the development of regressions to quantify the relationship between light attenuation and chlorophyll a and the relationship between chlorophyll a and nitrogen loading.
The water quality simulation modeling approach involves linking together watershed nutrient loading models, hydrodynamic models, and water quality models in order to simulate physical, chemical, and biological processes in a waterbody. EPA developed an inventory of candidate watershed and estuary models that have been previously applied to estuaries in Florida. Based on a review, the Agency is considering the Loading Simulation Program for simulating freshwater flows and nutrient loading from the watershed, the Environmental Fluid Dynamics Code for simulation of estuarine hydrodynamics, and the Water Quality Analysis Simulation Programs for simulation of estuarine water quality.
EPA is also deriving numerical nutrient criteria, known as Downstream Protection Values (DPVs), for the terminal reaches of streams flowing into estuaries. The approach begins by estimating the limits of phosphorus and nitrogen loading rates that protect a balanced natural population of aquatic flora and fauna. DPVs are then derived based on protective loading rates and average streamflow. DPVs can be also be determined for upstream reaches of contributing watersheds by accounting for expected loss or permanent retention of nitrogen and phosphorus within the stream network (USEPA 2010a).
Reference:
USEPA. 2010a. Methods and Approaches for Deriving Numeric Criteria for Nitrogen/Phosphorus Pollution in Florida’s Estuaries, Coastal Waters, and Southern Inland Flowing Waters. U.S. Environmental Protection Agency, Office of Water. Accessed October 2016.