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Project: Technical Support for Nutrient Numeric Endpoint and TMDL Tool Development

Background and Objective

The State Water Resources Control Board (SWRCB), with the support of USEPA Region IX, is working to develop nutrient numeric endpoints (NNEs) to regulate nutrient levels in the state’s waters. Nutrient criteria must be developed within the context of the Clean Water Action Plan, with a primary goal of maintaining nutrient levels that support the health of aquatic systems. The criteria should also limit excessive growth of macrophytes or phytoplankton, potentially harmful algal blooms leading to oxygen declines, imbalance of aquatic species, public health threats, and general degradation of aquatic resources. The effort to develop NNEs was initiated in 1999 and has focused primarily on freshwater systems including streams, rivers, lakes, and reservoirs. Because estuaries and coastal lagoons are an important resource, defining NNEs for these water bodies has more recently become a priority for the SWRCB. Because the San Francisco Bay-Delta encompasses approximately 80% of the state's estuarine habitat, but differs from many small estuaries in other areas of California, a site-specific NNE assessment framework and nutrient algal-response model will be developed for this area.

The NNEs will consist of narrative nutrient objectives accompanied by numeric guidance. The NNE framework is founded on the concept that biological response indicators provide a more direct risk-based linkage to beneficial uses than nutrient concentrations alone. A "weight of evidence" approach with multiple indicators (e.g., dissolved oxygen, algae, or invertebrate communities) will produce NNEs with greater scientific validity and a more robust link to actual impairment of use, as opposed to an approach that relies on nutrient concentration data alone. The NNE framework has two components: a) response indicators and regulatory endpoints that specify how to assess water body condition, and b) nutrient-response models that can be used to link response indicators to nutrients and other management controls (e.g., hydrology) on a water body-specific basis. To overcome challenges associated with a best professional judgment approach, the SWRCB preliminarily offered “benthic biomass spreadsheet models” as scoping tools to relate ambient nutrient concentrations to algal biomass while accounting for physical factors such as stream flow velocity and canopy cover. However, early validation efforts indicated these spreadsheet tools require refinement. Stakeholders also need technical assistance in working through how to implement the NNE in management programs such as Total Maximum Daily Loads, 303(d) listing, and discharge permits.

The goal of this project is to provide technical support for development of NNEs in California streams, lakes, and estuaries.


This project was initiated in 2006 with anticipated completion in 2015.

Diagram showing the variety of estuarine beneficial uses for which NNEs will be developed.


This project is phased, with three major phases: 1) conceptual framework development, 2) development of an implementation plan defining the technical support required to develop NNEs, and 3) research activities to support NNE development.

Primary technical tasks for streams include a) documenting statistical thresholds in the dose-response relationships between proposed NNE indicators and metrics of aquatic life use in streams (algae and benthic macroinvertebrate indices of biological integrity); b) documenting “reference levels” of proposed NNE indicators and the percent of stream miles exceeding statistical or proposed regulatory thresholds; c) validating stream NNE spreadsheet models, identifying sources of error, and proposing refinements to NNE scoping tools; d) developing calibrated, site-specific, dynamic models to better understand factors controlling algal responses to nutrients; and e) using tools and data to assist with decisions about how to implement the NNE to set watershed-based nutrient targets. Given the diversity of lakes in California, model validation requires compilation of a substantial data set reflecting the range of conditions. To address these needs, researchers will a) compile existing data on phytoplankton biomass, nutrient concentrations, cyanobacterial dominance, and cyanotoxins in California lakes; and b) validate the NNE spreadsheet model for lakes, identifying sources of error and proposed refinements.

To address technical needs for nutrient objectives in estuaries, SCCWRP will help develop indicators, conduct dose-response studies, and hold expert workshops to support selection of regulatory endpoints for intertidal flats, seagrass habitats, and unvegetated subtidal habitat across the range of estuarine classes in the state. An ambient survey of eutrophication in Southern California estuaries was conducted in conjunction with the Southern California Bight '08 Coastal Wetlands and Estuaries study. In addition, SCCWRP will develop nutrient-algal response models including statistical stress-response models, "pilot" mechanistic models for Southern California Bight estuaries, calibrate dynamic simulation models for individual estuaries, and a conceptual model/modeling strategy for the San Francisco Bay.


This project is being conducted in collaboration with the SWRCB, USEPA Region IX, Tetra Tech Inc., and numerous other regulated, regulatory and non-governmental organizations.


Review of Science Supporting Dissolved Oxygen Objectives in Estuaries (Video) - January 2012 presentation to SCCWRP member agencies on approaches to developing consistent dissolved oxygen criteria.

Science to Support Nutrient Criteria Development (Video) - January 2010 overview presentation to SCCWRP member agencies
For more information on Technical Support for Nutrient Numeric Endpoint and TMDL Tool Development, contact Martha Sutula at (714) 755-3222.
This page was last updated on: 6/25/2014