In wadeable streams, nutrient enrichment, in concert with other site-specific factors, can result in the overabundance of algal biomass, low dissolved oxygen and altered biotic communities. These changes can have adverse effects on stream ecosystem services. Scientifically-based water quality objectives (WQO) and tools that relate these objectives to nutrient management are needed in California to prevent eutrophication from occurring and to provide targets to restore waterbodies where adverse effects have already occurred.
The California State Water Resources Control Board (SWRCB) is developing nutrient water quality objectives for the State’s surface waters. USEPA guidance on nutrient objective development generally recommends three means to set nutrient objectives (USEPA 2000): 1) a reference approach, based on a statistical percentile of nutrient or biotic response indicators in minimally-disturbed waterbodies; 2) an empirical stress-response approach, based on statistical analyses of field data on nutrients, algal abundance and indicators of aquatic life; or 3) a process-based approach, involving identification of ecological responses of concern and mechanistically modeling the linkage back to nutrient loads and other co-factors controlling response. Among the approaches that the SWRCB staff is considering is the process-based approach, known as the Nutrient Numeric Endpoint (NNE) framework (Tetra Tech 2006).
The NNE framework is intended to serve as numeric guidance to translate narrative WQO. It consists of two tenets: 1) assessment and recommended numeric (regulatory) endpoints based on the ecological response of an aquatic waterbody to eutrophication (e.g., algal abundance, dissolved oxygen [DO]) to assess waterbody condition and 2) scoping-level models that link the response indicator endpoints to nutrient inputs and other site-specific factors and management controls. These scoping models were intended to be used to establish default nutrient targets for point source discharge and municipal stormwater permits and total maximum daily loads (Tetra Tech 2006). Tetra Tech (2006) developed the benthic biomass spreadsheet tool (BBST) for use in streams. As the SWRCB prepares to propose nutrient objectives for wadeable streams, scientific analyses of improved data from California statewide stream probabilistic and targeted bioassessment surveys can strengthen the scientific basis for policy decisions. In the context of this study, “endpoints” refer to policy decisions on levels at which point management action should be taken; “thresholds” refer to the output of scientific analyses.
The objectives of this project are three-fold:
• Estimate the natural background and ambient concentrations of nutrients and candidate indicators of primary producer abundance in California wadeable streams;
• Explore relationships and identify thresholds of adverse effects of nutrient concentrations and primary producer abundance on aquatic life indicators in California wadeable streams;
• Evaluate the Benthic Biomass Spreadsheet Tool for California wadeable streams using existing data sets and recommend avenues for refinement.