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


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 would 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. 

The Department’s proposed rule would incorporate a decision framework based on phosphorus and multiple response indicators for rivers, streams, lakes, and impoundments into Maine’s water quality standards.  Phosphorus and some response indicators are tiered with different expectations for lakes (Class GPA) and several classes of streams and rivers (AA, A, B, and C).  The rule provides the ability to set site-specific criteria for nitrogen and carbon as needed.  The proposed rule also describes how the Department would decide if nutrient discharge limits in National Pollutant Discharge Elimination System permits are warranted.  The proposed rule provides flexibility in implementation of water quality management programs to minimize the costs of remedial actions. 

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. 

Related freshwater nutrient criteria documents:


Marine Nutrient Criteria

In the marine environment, the impacts of excess nutrients are similar to those seen in freshwaters.  However, while phosphorus is often the limiting nutrient for growth of algae in fresh 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 communities living within mudflats
  • loss of marine vegetation like seagrasses that are important for sediment stabilization 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
  • declines of commercial fisheries, especially shellfisheries
  • loss of recreational uses like swimming








As of January 2012, the Department’s first priority for marine waters is development of nitrogen-only criteria.  Throughout the coming years, the Department will continue to collect phosphorus data to determine if phosphorus would be considered limiting within State estuarine and marine waters, and therefore, if phosphorus criteria development is warranted.

Related marine nutrient criteria documents:


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