The Maine Department of Environmental Protection (DEP) used a weight-of-evidence approach to determine total phosphorus (TP) and chlorophyll a criteria for fresh surface waters. Maine DEP used data in the process that were primarily collected over several years by the agency between June and September, as well as some data from various literature sources to support their findings. The lines of evidence include analyses of TP and chlorophyll a relationships with macroinvertebrate assemblages, relative algae richness, and reference conditions as determined by a percentile approach and conditional probability analyses.
Reference sites were rivers and streams that had no dams or point source discharges, and had 95 percent or more of upstream watershed land use consisting of forest or wetlands. The U.S. Environmental Protection Agency recommends use of the 75th percentile of reference sites to establish reference conditions, but Maine DEP found that the 86th percentile corresponded with the observed impacts to designated uses for the state’s waters. Therefore, for classes AA and A, Maine DEP used the 90th percentile of reference sites to determine where TP was less likely to negatively impact environmental response. The researchers also used between a 75th and 90th percentile on a case-by-case basis for the relationship between TP and the protection of aquatic life. To determine chlorophyll a thresholds for class AA and A streams, rivers, and impoundments, Maine DEP also used a percentile approach. They used 115 control samples and took the 90th percentile of the control data to determine the threshold.
In addition, Maine DEP used changepoint analyses to determine nutrient thresholds. Maine has an existing standard for identifying algal blooms in lakes via chlorophyll a (8 μg/L) in class B and C waters and impoundments. Maine DEP examined that limit using a changepoint analysis of log-transformed chlorophyll a and Secchi transparency data. They used a total of 1,153 samples in their analysis and applied a chi-squared test of Pearson correlation to determine confidence. Maine DEP also used changepoint analysis on 1,153 paired samples of TP and Secchi depth collected in August over several years to determine TP thresholds for lakes (Maine DEP 2009).
Reference:
Maine DEP (Maine Department of Environmental Protection). 2009. Description of Nutrient Criteria for Fresh Surface Waters (Chapter 583). DEPLW-0974A. Accessed October 2016. http://www.maine.gov/dep/water/nutrient-criteria/091202_description_of_nutrient_criteria.pdf.