Great Bay Non-Point Nitrogen Sources Project Background


Based on previous experimental studies, dissolved oxygen violations and eelgrass loss are evident in most of the Great Bay estuary and this impairment is strongly controlled by high levels of nitrogen (N) (NHDES 2010). In areas with low levels of dissolved oxygen, fish and other aquatic life become stressed and die or are forced to flee their habitat. For example, algal blooms fueled by excess nutrients such as nitrogen limit available light for eelgrass and trigger die offs and associated fish kills.

It is also understood that anthropogenic activities have significantly altered the global nitrogen (N) cycle in the past half century, with N pollution in rivers, streams and estuaries becoming a significant problem for coastal ecosystems worldwide (Vitousek et al. 1997, Galloway et al. 2002). Increased deliveries of N from non-point sources (NPS) as well as point sources have contributed to symptoms of eutrophication and loss of species habitat throughout the northeastern US (Nixon 1995, Caraco and Cole 1999, Valiela and Bowen 2002). For managers, problems associated with excess nitrogen inputs to the estuary are being increasingly recognized as a high priority issue. Therefore, a better understanding of the types of NPS sources and spatial extent that contribute to the Great Bay is needed. There are several non-point sources to consider: residential septic systems, fertilizers used by residents, golf courses and farms, urban runoff from parking lots and roadways and aging sewer lines as well as atmospheric deposition from fossil fuel emissions.


  • Great Bay, New Hampshire’s most significant estuary
  • High economic and recreational value
  • Watershed is home to almost 25% of NH’s population
  • Watershed intersects 39 NH and 9 ME communities (48 total)

Previous Studies Have Found

  • Deterioration of water quality and aquatic life  (i.e., increased suspended sediment, chlorophyll-a and nitrogen levels)
  • Dissolved inorganic nitrogen up 42% over the last 5 years (New Hampshire, Piscataqua River Estuarine Partnership).
  • Eelgrass biomass in the estuary declined by 64% between 1990 and 2008
  • Adult oyster populations have decreased from 125,000 (1997) to 10,044 (2009).
  • NHDES has listed the Bellamy River, Cocheco River, Lamprey River, Salmon Falls River, Oyster River, Piscataqua River and Great Bay as impaired or threatened by water pollution


Great Bay Estuarine Reserve: A rare inland estuary, where salt water meets fresh, spans more than 13,000 acres. Nearly a quarter of the state’s population lives within its watershed.

Geology – The region surrounding the Great Bay Reserve was formed by the most recent glaciation in the Wisconsin stage of the Pleistocene epoch (10,000 to 20,000 yrs. B.P.) Substantial amounts of glacial till were deposited as the glacier receded. Crustal depression in New Hampshire from glacial weight was on the order of 12.2 m (40 ft.) However, the uplift was not uniform throughout the region and Great and Little bays represent a sag along the surface. The low-lying area was filled by rising sea level from glacial melting. Thus, the Great Bay estuary is representative of a drowned-river valley. Present sea level was reached approximately 3,000 to 5,000 years ago.

Tidal Range - Great Bay is a mesotidal estuary with the average tidal range varying from 2.5 meters (8.2 feet) at the mouth of the estuary to 2.0 meters (6.6 feet) at Dover Point, increasing slightly to 2.1 meters (6.9 feet) at the mouth of the Squamscott River.
The flushing time for water entering the head of the estuary is 58 tidal cycles (26.0 days) during high river flow and 48.5 cycles (25.1 days) during low river flow. On average, it can take up to 39 tidal cycles, or 20 days, for a parcel water in Great Bay to completely move to the ocean.

What is in a watershed? The dictionary defines a watershed as the area of land that drains rainwater into a common body of water such as a creek or bay. The Great Bay Watershed is made up of many small watersheds, each collecting rainfall and waste water that ultimately flows to the Bay. All of the water we use in this area flows directly to the Bay or the Atlantic Ocean.


A watershed is more. It is the interdependent web of living organisms that inhabit a geographic area and depend on it for clean soil, air and water. Everything we do, from mountaintop to marsh, affects the health of local water, soil, air, wildlife, our families and communities.