Nutrient Criteria

Nutrient enrichment can cause negative environmental impacts to surface waters, such as algal blooms, low dissolved oxygen concentrations, fish kills, excessive growths of filamentous algae or bacteria, and generation of cyanotoxins. To better manage nutrient enrichment, the U.S. Environmental Protection Agency (USEPA) recommends that states develop and adopt numeric criteria for nitrogen and phosphorus for all jurisdictional waters and requires states to report annually on progress toward this goal. The Department has been developing nutrient criteria to incorporate into Maine's water quality standards since 2001.

Freshwater Nutrient Criteria

The Department intends to start a public rulemaking process late 2021 or early 2022 to formerly adopt nutrient criteria for Class AA, A, B, and C waters of the State of Maine. Stakeholder meetings were held December 11, 2020 and January 22, 2021 to get feedback and recommendations about the draft rule. For more information contact Tom Danielson (207) 441-7430.

Maine's water quality standards include five classes of fresh surface waters. Lakes are in Class GPA and all other fresh surface waters are either Class AA, A, B, or C. The proposed nutrient criteria would apply to Classes AA, A, B, or C but not Class GPA. If adopted, the nutrient criteria would be based on TP concentrations in the water and a combination of environmental response indicators, which are described in Section 3 of the proposed rule. Table 1 in Section 4 of the proposed rule establishes the nutrient criteria. Section 5 establishes a decision framework for determining attainment of nutrient criteria. Section 5 also describes the process of setting site-specific values for TP or another nutrient. In some circumstances, it may be necessary to establish a site-specific TP value that is lower than the default TP value of the Class to achieve attainment of water quality standards. In other circumstances, it may be reasonable to establish a site-specific TP value that is higher than the default TP value of the Class if a waterbody consistently attains environmental response indicators despite being enriched with nutrients. Finally, the Department may set site-specific values for another nutrient, such as nitrogen, if it is necessary to attain water quality standards. Currently, there are no site-specific criteria in the proposed Chapter 583 but new ones would be added to Section 6 of the rule at some later date through a separate rulemaking process.

Maine's proposed nutrient rule determines if a waterbody attains nutrient criteria by simultaneously evaluating nutrient concentrations and environmental response indicators. A combination of nutrient and environmental response indicators are needed to fully evaluate the impacts of nutrient enrichment. Nutrient enrichment does not always lead to negative environmental responses. Shading, scouring, grazing, substrate instability, and water chemistry could limit the growth of algae and plants despite abundant nutrients. Further, some forms of nutrients are not readily available to aquatic life. Thus, water quality standards focused only on nutrient concentrations could lead to false positives; some waterbodies could be called impaired even though there are no deleterious environmental responses. Similarly, low nutrient concentrations do not always indicate good environmental conditions. A large portion of nutrients in some streams and rivers is associated with episodic spikes in nutrient concentrations following storms. Although a series of water samples collected during low flow conditions could miss the peak nutrient concentrations, resident algae and plants could uptake and store some of the nutrients. Further, substantial growths of algae and plants can strip nutrients from the water. As a result, water samples could underestimate the amount of nutrients because the nutrients are no longer in the water. Thus, water quality standards focused only on nutrient concentrations could lead to false negatives; some waterbodies would attain criteria even despite substantial ecological impacts.

By incorporating both concentration limits and response indicators, the Department's proposed criteria are different from other water quality criteria which traditionally rely solely on water concentration limits. As a consequence, the proposed rules have had considerable scrutiny, most notably from the USEPA, who must assure that any criteria are consistent with requirements of the Clean Water Act. The Department has been working cooperatively with the USEPA to find an acceptable design that will work for Maine and could be adapted for use by other states. USEPA based their Guiding Principles for Integrated Nutrient Criteria (Bioconfirmation) in part on Maine's approach to nutrient criteria and promotes this to states as an alternative approach of developing nutrient criteria.

Marine Nutrient Criteria

In the marine environment, the impacts of excess nutrients are similar to those seen in fresh waters. However, while phosphorus is often the limiting nutrient for growth of algae in fresh surface waters, nitrogen is typically the limiting nutrient in marine waters. In estuaries and coastal areas, excess nitrogen can cause:

  • blooms of phytoplankton and macroalgae that can smother organisms living on and within mudflats
  • loss of marine vegetation like seagrasses that are important for carbon sequestration, shoreline protection, and provide important habitat and feeding grounds for juvenile fish and invertebrates
  • declines in oxygen concentrations in bottom waters that can cause losses to the biological community, even fish or shellfish kills
  • loss of recreational uses like swimming

The Department is currently receiving support from the EPA's Nutrient Scientific Technical Exchange Partnership & Support (N-STEPS) program to conduct data analyses that will lead to recommendations for nitrogen targets in the Class SC area in the vicinity of Portland. The N-STEPS process is occurring alongside stakeholder meetings of the Portland Area Nitrogen Group, which is meeting quarterly in 2021. Pertinent documents for each of these processes are available at the following links:

Related marine nutrient criteria documents:


Freshwater Nutrient Criteria: Tom Danielson (207) 441-7430
Marine Nutrient Criteria: Angela Brewer (207) 592-2352