Conventional Chemistry Thresholds
Thresholds for conventional chemicals (such as nutrients, pH, dissolved oxygen, etc.) follow the selection approach for toxic chemicals in water unless noted below. For conventional chemistry, threshold selection involves special treatment of several analytes with unusual properties. For example, temperature, oxygen and pH all have normal distributions, not the typically skewed distributions of chemical contaminants for which zero is protective and any increasing concentrations over zero are steadily more damaging. Oxygen and pH also are “double ended thresholds” for which values either above or below an acceptable range can indicate adverse effects. This resulted in special considerations relative to the scoring approach.
Chlorophyll a (ug/L): The cold water aquatic life chlorophyll a threshold used for this project is 15 ug/L (NCAC, 2004; OAR, 2000). The benthic chlorophyll a threshold used in the Biostimulation Index for this project is 44 mg/m2, which represents the 95th percentile of reference sites statewide (Fletscher, et al., 2013). This number is applied both to measured benthic chlorophyll a concentrations (where available) and to modeled concentrations resulting as output from the Benthic Biomass Spreadsheet Tool (Tetratech Inc., 2007).
Dissolved Oxygen (mg/L): The Central Coast Basin Plan requires that dissolved oxygen concentrations in cold water habitat remain above 7.0 mg/L. Increased photosynthesis from excessive plant and algal material during daylight hours can result in supersaturated oxygen concentrations over 13 mg/l (Worcester et al., 2010). For this project, oxygen MEQ scores are calculated for both an upper and lower threshold, where measurements under 7.0 mg/L or over 13.0 mg/L contribute to MEQ calculations of Exceedance and Magnitude. Because we do not consider values that fall between these two numbers to be of concern, the calculation for Magnitude only includes excursions outside of these thresholds.
Dissolved Oxygen Deficit (mg/L): The California Numeric Nutrient Endpoint (NNE) Benthic Biomass Spreadsheet Tool (Tetratech Inc., 2007) calculates a modeled estimate of dissolved oxygen deficit resulting from biostimulatory substances. The model inputs include nutrient concentrations, temperature, and several other parameters. A modeled deficit of 1.25 mg/L has been determined to be associated with field conditions of excessive nutrient enrichment in the Central Coast Region (Worcester et al., 2010). This project uses a threshold of 1.25 mg/L for aquatic life protection.
Water Temperature (°C): Moyle (2002) describes the upper range of optimum temperatures for steelhead growth at 18°C and temperatures above 23°C as potentially lethal without acclimatization. The threshold for this project is set at 18°C for aquatic life protection. Since this is the upper range of optimum, temperatures approaching 18°C are not of increasing concern. Therefore, only excursions above this threshold are considered in calculation of magnitude.
Nitrate as N (mg/l): This project utilizes the Central Coast aquatic life guideline value of 1.0 mg/L as N as the threshold for aquatic life protection (Worcester et al., 2010).
Ortho-phosphate (as Phosphorus) (mg/L): Williamson (1994) established a guideline value for orthophosphate-P in the Pajaro River at 0.12 mg/L, below which waters are at low risk for eutrophication. This project utilizes 0.12 mg/L as its ortho-phosphate threshold for aquatic life protection.
pH: The pH score is calculated both on upper and lower thresholds, based on the Central Coast Basin Plan cold water objectives of 7.0 to 8.5 pH units. Measurements under 7.0 or over 8.5 contribute to calculations of exceedance and magnitude. Because we do not consider values that fall between these two numbers to be of concern, the calculation for magnitude only includes excursions outside of these thresholds.
Total Suspended Solids (TSS) (mg/L): This project utilizes a threshold of 30 mg/L for aquatic life protection. This is a threshold used as an aquatic life criterion by several other states (USEPA, 2006).
Turbidity (NTU): This project utilizes a threshold of 25 NTU for aquatic life protection, based on levels that are of concern for visual feeders like steelhead trout (Sigler, et al., 1984).
