The California State Water Resources Control Board (SWRCB) is developing nutrient water quality objectives (WQOs) for the state’s surface waters using the Nutrient Numeric Endpoints (NNE) assessment framework. The NNE assessment framework is comprised of two components: (1) a suite of numeric regulatory endpoints based on the ecological response of an aquatic water body to nutrient overenrichment (eutrophication [e.g., algal biomass, DO]); and 2) nutrient-response models linking the ecological response endpoints to site-specific nutrient targets and other potential management controls. The NNE assessment framework is intended to serve as numeric guidance to translate narrative WQOs.
The NNE assessment framework is a structured set of decision rules that specify how to use monitoring data to categorize specific segments of San Francisco Bay with respect to adverse effects on the bay’s beneficial uses from nutrient over-enrichment. While the decision on regulatory endpoints should be informed by science, it is ultimately a policy decision. The intention is that the SWRCB would propose regulatory endpoints for San Francisco Bay based on the synthesis of science represented in the NNE assessment framework and feedback from bay stakeholders.
Development of an assessment framework begins by choosing response indicators, which were reviewed using four criteria: (1) has a strong linkage to beneficial uses; (2) has a well-vetted means of measurement; (3) can model the relationship between the indicator, nutrient loads, and other management controls; and (4) has an acceptable signal-to-noise ratio to assess eutrophication. These criteria are based on McKee’s 2011 Numeric Nutrient Endpoint Development for San Francisco Bay Estuary: Literature Review and Data Gap Analysis.
Ecological response indicators provide a more direct risk-based linkage to beneficial uses than nutrient concentrations or loads alone. Thus the NNE assessment framework is based on the diagnosis of eutrophication or other adverse effects and its consequences rather than nutrient overenrichment per se.
Except in some cases (e.g., unionized ammonium toxicity), nutrients themselves do not impair beneficial uses. Rather, ecological response to nutrient loading causes adverse effects that impair uses. Instead of setting objectives solely in terms of nutrient concentrations, it is preferable to use an analysis that takes into account the risk of impairment of those uses. That is the reason the NNE assessment framework needs to target ecological response indicators such as DO, phytoplankton and harmful algal bloom (HAB) biomass (e.g., chlorophyll a, water clarity), macroalgal biomass and percent cover, benthic algal biomass (sediment chlorophyll a) and submerged aquatic vegetation (SAV) density and percent cover, and aesthetics (e.g., foul odors, unsightliness). Those indicators provide a more direct risk-based linkage to beneficial uses than the ambient nutrient concentrations or nutrient loads. With that approach, it is critical that tools be developed that link the response indicators back to nutrient loads and other cofactors and management controls (e.g., hydrology). Once the indicator is determined, a weight of evidence approach using the NNE assessment framework can assess the status of eutrophication and nutrient-response models will be used to convert response indicators to site-specific nutrient loads or concentrations.
For San Francisco Bay, indicators varied among four habitat types: unvegetated subtidal, seagrass and brackish SAV, intertidal flats, and tidally muted habitats (e.g., estuarine diked baylands). Two types of indicators were designated:
- Primary indicators, which met all evaluation criteria and would, therefore, be expected to be a primary line of evidence of the NNE assessment framework for San Francisco Bay.
- Supporting indicators, which fell short of meeting evaluation criteria, but might be used as supporting lines of evidence.
Note: This terminology is used to provide a level of confidence in how the indicators should be employed in the context of multiple lines of evidence.
The review found four types of indicators that met all evaluation criteria and were designated as the primary indicators: DO; phytoplankton biomass, productivity, and assemblage; cyanobacterial abundance and toxin concentration (all subtidal habitats); and macroalgal biomass and cover (intertidal habitat, tidally muted habitats, and seagrass habitats). Other indicators evaluated met up to three of the review criteria and were designated as the supporting indicators: HAB cell counts and toxin concentration, urea and ammonium (all subtidal), light attenuation, and epiphyte load (seagrass/brackish SAV). Ultimately, the real distinction between primary indicators and supporting indicators and how they would be used as multiple lines of evidence in an NNE assessment framework is entirely dependent on indicator group and particular applications to specific habitat types. Some primary indicators (e.g., DO) could be stand-alone, while others such as phytoplankton biomass, productivity and assemblage could be used as multiple lines of evidence, as use of any one alone is likely to be insufficiently robust (Sutula et al. 2013).
Reference:
Sutula, M., K. McLaughlin, S. Bricker, D. Senn, and E. Novick. 2013. DRAFT: A Review of Scientific Approaches Supporting NNE Assessment Framework Development for San Francisco Bay. Prepared for San Francisco Regional Water Quality Control Board. Accessed October 2016. http://www.waterboards.ca.gov/sanfranciscobay/water_issues/programs/planningtmdls/amendments/estuarineNNE/SAG-June-2013/NNE_Framework_White_Paper.pdf EXIT.