As a result of a lawsuit between the federal government and the state of Florida over phosphorus- (P-) enriched discharges and impacts to the Everglades, the state enacted the Everglades Forever Act, which required the Florida Department of Environmental Protection (FDEP) to derive a numeric water quality criterion for P that would “prevent ecological imbalances in natural populations of flora or fauna.” Legislation provided the basis for extensive research and monitoring efforts designed to better understand the effects of P enrichment and to determine levels of enrichment that produced undesirable ecosystem changes.
FDEP established a P criterion to protect the Everglades from ecological impacts of P enrichment, which was set as an annual maximum concentration not to exceed 15 µg/L and a 5-year average concentration not to exceed 10 µg/L. Historical data were used to set reference conditions for P, and researchers identified how P enrichment impacts the ecology of the system and the concentration at which impacts occur. The narrative nutrient standard for class III Florida waters such as the Everglades states that “in no case shall nutrient concentrations of a body of water be altered so as to cause an imbalance in natural populations of aquatic flora or fauna.” The FDEP approach to detecting violations of that standard with respect to surface water P concentrations in the Everglades was to test for statistically significant departures in ecological conditions from those at reference sites (i.e., interior sampling locations with background P concentrations). Water column and soil P levels were both considered in all four major hydrologic units to account for spatial variability.
Monitoring was conducted along gradients in different parts of the Everglades to assess ecological responses to P enrichment. Fixed sampling stations were located along the full extent of each gradient to document ecological conditions associated with increasing levels of P enrichment. Monthly surface-water sampling and less frequent soil sampling was conducted to quantify P gradients in each area. Diel DO regimes, periphyton, and benthic macroinvertebrates were sampled quarterly when surface water was present.
Based on available information and preliminary studies, researchers chose five ecological features as biotic response variables. Structural indicators included the periphyton community, dominant macrophyte populations, and benthic macroinvertebrate community. Diel fluctuations in water column DO provided an important indicator of shifts in aquatic metabolism. The loss of open-water slough-wet prairie habitats was chosen as an indicator of changes to the landscape, specifically because they are areas of high natural diversity and productivity. Researchers found significant departures from reference conditions based on the response variables at P concentrations significantly greater than 10 µg/L. Multiple changes in each of the major indicator groups—periphyton, DO, macrophytes, and macroinvertebrates—all occurred at or near the same concentration.
FDEP adopted annual maximum concentrations at a given sampling location of 15 µg/L TP and 5-year average concentrations of 10 µg/L TP. FDEP proposed the criterion in 2001, and it was approved in 2003. The final criteria limits are applied to reference areas to ensure no further degradation and to areas already impacted by P enrichment to assess recovery in response to P control measures, including agricultural BMPs and treatment wetlands to remove P from surface runoff before it was discharged into the Everglades (USEPA 2008c).
Reference:
USEPA. 2008c. Nutrient Criteria Technical Guidance Manual: Wetlands. EPA-822-B-08-001. U.S. Environmental Protection Agency, Office of Water. Accessed October 2016. https://nepis.epa.gov/Exe/ZyPDF.cgi/P1002DY6.PDF?Dockey=P1002DY6.PDF.