Monitoring and Assessment of Coastal (Marine) Waters


Angela Brewer (207) 592-2352; James Stahlnecker (Toxics) (207) 215-6954
Maine Estuaries and Coastal Waters page

The Marine Unit monitors and assesses the health of marine waters (estuarine and coastal areas) through the collection of physical, chemical, and biological samples as related to attainment of Maine's water quality standards (38 M.R.S. § 465-B). For further information, see Maine's water classifications and Class SA marine waters.

The Marine Unit provides technical support to the DEP's Maine Pollutant Discharge Elimination System (MEPDES) program for wastewater discharges, as well as for Natural Resources Protection Act (NRPA) permits for coastal development projects. The Marine Unit supports volunteer water quality monitoring and other non-governmental organizations working within Maine's marine waters that engage in protection, maintenance and restoration of water and habitats (see list of partners).

Marine Environmental Monitoring Program (MEMP)

Maine's MEMP was established in 1991 (38 M.R.S. § 410-F) to monitor the "extent and effect of industrial contaminants and pollutants on marine and estuarine ecosystems and to determine compliance with and attainment of water quality standards". Monitoring efforts are focused on ambient water quality, nutrients, and eutrophication indicators, in particular near wastewater discharges. Eelgrass (Zostera marina) is a sensitive indicator of excess nitrogen in coastal waters, and is the subject of aerial mapping and long-term monitoring efforts in Casco Bay. Reports on Casco Bay eelgrass distribution and percent cover changes are available for 2013 and 2018. Southern Maine coast seagrass distribution is available for 2021. All mapped seagrass can be visualized through the Maine GeoLibrary (see “biota”).

Surface Water Ambient Toxics (SWAT) and Gulfwatch Programs

Maine's SWAT monitoring program was established in 1993 (34 M.R.S. § 420-B) to determine the nature, scope and severity of toxic contamination in the surface waters and fisheries of the State. The Marine Unit monitors for the presence of toxic contaminants in tissues from softshell clams (Mya arenaria), blue mussels (Mytilus edulis), and lobster (Homarus americanus), among other species.

Gulfwatch is a regional chemical contaminants monitoring program, organized and administered by the Gulf of Maine Council on Marine Environment (GOMC) since 1991. The Marine Unit coordinates the Maine sampling of blue mussels (Mytilus edulis) to assess the types and concentrations of contaminants in the nearshore marine environment. Gulfwatch fact sheet.

Salmon Habitat Monitoring Program

The salmon program monitors water quality and biological communities in Maine's Atlantic Salmon rivers and streams in close collaboration with state and federal agencies and non-profit groups, with the goal to restore and enhance the endangered populations of this species.


Marine water quality standards assessments are documented biennially in Integrated Water Quality Monitoring and Assessment Reports, also known as the 305(b) reports, which include lists of attaining and impaired waterbodies. Assessments are based on data collected by the Marine Unit and external organizations with approved Quality Assurance Project Plans.

The Marine Unit coordinates Maine's portion of the U.S. Environmental Protection Agency's National Coastal Condition Assessment (NCCA) every five years. The NCCA consists of coast-wide sampling of the water column and sediment to characterize the health of the physical, chemical and biological environment. Reports of the status of Maine waters within regional and national contexts are produced by EPA.

Emerging Issues

Coastal Acidification - primarily driven by global carbon dioxide emissions caused by human activity, the Gulf of Maine is especially vulnerable to acidification due to its cold temperatures and the large amount of freshwater flowing into the system. Local factors including river runoff that is lower in pH and higher in nitrogen may be exacerbating coastal acidification. Coastal acidification is a long term and pervasive threat to the marine environment, impacting marine biota including commercially important species like oysters, clams, and lobsters. The Marine Unit is partnering to monitor the impacts of coastal acidification.

Maine Ocean and Coastal Acidification (MOCA) Partnership

Northeast Coastal Acidification Network

Data and Maps

Marine data are housed in DEP's central database, the Environmental and Geographic Analysis Database (EGAD) and in downloadable map projects.

Partnering Organizations

Maine Healthy Beaches

Casco Bay Estuary Partnership

Friends of Casco Bay

Kennebec Estuary Land Trust

Boothbay Region Land Trust

Maine Coastal Observing Alliance