In its development of numeric nutrient criteria (NNC) for lakes, the Minnesota Pollution Control Agency (MPCA) has drawn upon existing studies and data sets to inform their decisions. MPCA used data sets that were previously defined and include Minnesota’s ecoregion reference lake data set (originally assembled in 1985–1987); MPCA’s 2004 305(b) assessment of trophic status data for Minnesota lakes; and the U.S. Environmental Protection Agency’s (EPA’s) ambient water quality criteria recommendations for aggregated nutrient ecoregions VI, VII, and VIII (EPA assessed lakes) (USEPA 2000d, 2000e, 2000f). In addition, MPCA conducted independent sampling of selected lakes, including Secchi depth transparency, dissolved oxygen and temperature profile, chlorophyll a, total suspended solids, pH, alkalinity, color, turbidity, conductance, nitrogen, Total Kjeldahl Nitrogen, and phosphorus.
MPCA combined the collected data into four primary databases:
- The assessment database—which consists of monitored lakes for which summer measurements were taken from 1993 to 2002 and lakes for which measurements were taken (not necessarily in the summer) from 1970 to 1992. Carlson’s Trophic State Index was calculated for each lake in the assessment database.
- The reference lake database—which consists of 90 minimally impacted and representative lakes across four ecoregions.
- A diatom-inferred phosphorus database—which was used for lake-sediment core, diatom-reconstructed total phosphorus data from three separate but related studies.
- A database containing data from EPA criteria documents—which primarily consist of STORET data from 1990 to 999—and that was primarily used to fill in gaps left by limited data for certain ecoregions.
The researchers based their proposed criteria on comparisons of percentiles between assessed lakes and reference lakes and considered values within the interquartile range to be typical (i.e., 25–75th percentile). In addition to the percentile approach, the researchers also used relationships between variables and case studies from independent lakes to inform their recommendations for NNC (MPCA 2005).
References:
MPCA (Minnesota Pollution Control Agency). 2005. Minnesota Lake Water Quality Assessment Report: Developing Nutrient Criteria. 3rd Ed. Accessed October 2016.
https://www.pca.state.mn.us/sites/default/files/lwq-a-nutrientcriteria.pdf Exit .
USEPA. 2000d. Ambient Water Quality Criteria Recommendations: Lakes and Reservoirs in Nutrient Ecoregion VI. EPA-822-B-00-008. U.S. Environmental Protection Agency, Office of Water, Washington DC. Accessed November 2016. https://www.epa.gov/sites/production/files/documents/lakes6.pdf.
USEPA. 2000e. Ambient Water Quality Criteria Recommendations: Lakes and Reservoirs in Nutrient Ecoregion VII. EPA-822-B-00-009. U.S. Environmental Protection Agency, Office of Water. Washington DC. Accessed November 2016. https://www.epa.gov/sites/production/files/documents/lakes7.pdf.
USEPA. 2000f. Ambient Water Quality Criteria Recommendations: Lakes and Reservoirs in Nutrient Ecoregion VIII. EPA-822-B-00-010. U.S. Environmental Protection Agency, Office of Water. Washington DC. Accessed November 2016. https://www.epa.gov/sites/production/files/documents/lakes8.pdf.