Criteria Derivation

Deriving numeric nutrient criteria (NNC) is a deliberative process that builds on the three analytical approaches—the reference condition, stressor-response models, and mechanistic models. Through this process, water quality scientists and managers define the levels of nutrients that ensure protection of their waters’ designated uses.

This section describes the elements and key technical considerations associated with estimating nutrient conditions (e.g., nitrogen or phosphorus concentrations) and water quality indicators of those conditions (e.g., chlorophyll a concentration, water clarity), that will protect designated uses. The deliberative process closely follows EPA’s science-based, peer reviewed framework for characterizing risk, EPA’s Guidelines for Ecological Risk Assessment (USEPA 1998). Adhering to a structured, deliberative process strengthens confidence that the data and analyses are used in a clear, transparent, reasonable, and consistent manner. The process draws on the analytical results from the three approaches and contains specific elements—quantification, documentation, and communication—that result in a scientifically rigorous and transparent outcome (Figure 1).

A diagram of the process of estimating nutrient conditions.

Figure 1. Conceptual diagram of the deliberative process for deriving NNC


Case Studies

Yaquina Estuary, OR

  • Used 25th, median, and 75th percentiles from CDFs as input variables for the SRM
  • SRM results indicated that median values could be used as criteria for most parameters

Proposed Criteria for Tampa Bay

  • Numeric TN loading criteria were derived for the four segments
  • Due to rainfall and hydrologic loadings, criteria were expressed as a nitrogen delivery ratio

Florida Everglades Phosphorus

  • Analyzed biological and chemical data along P gradients
  • Found that TP concentrations greater than 10 ppb exhibited unhealthy ecological conditions
  • Additional analyses of P data were conducted to set longer and shorter term concentrations
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