PREP Reports & Publications

Eelgrass Distribution in the Great Bay Estuary for 2014

Frederick T. Short, University of New Hampshire, Jackson Estuarine Laboratory


Headlines for 2014:

• The Great Bay Estuary gained 12% in eelgrass distribution from 2013 to 2014.

• Great Bay itself experienced a 16% increase in eelgrass distribution from 2013 to 2014.

• Eelgrass biomass in Great Bay increased 8% between 2013 and 2014.

• But in Portsmouth and Little Harbors, eelgrass distribution decreased 14% from 2013 to 2014.

Long-term story, the eelgrass decline since 1996:

• The Great Bay Estuary has lost 44% of its eelgrass distribution.

• The Great Bay itself has lost 41% of its eelgrass distribution and 79% of its eelgrass biomass.

• Little Bay and the Piscataqua River have lost 94% of their eelgrass distribution.

• Portsmouth and Little Harbors have lost 54% of their eelgrass distribution.

Eelgrass in the Great Bay Estuary gained in areal distribution between 2013 and 2014. In Great Bay, where the majority of 2014 eelgrass gains were seen, shallow new beds of eelgrass seedlings were largely responsible for the increased eelgrass area and biomass. Total eelgrass area in Great Bay was 1466 acres (16% increase) with an 8% gain in biomass. Areal gains outpaced biomass gains because eelgrass seedlings and young plants are typically low in biomass. Nuisance macroalgae and epiphytes continued to proliferate in 2014, especially in Great Bay. As in 2013, there was no eelgrass in Little Bay in 2014. The patchy 4-acre eelgrass area in the Piscataqua River grew more dense but was still less than 30% cover overall. In Portsmouth and Little Harbors, including the Back Channel, eelgrass continued to decline with eelgrass distribution down 14% from 2013, to 150 acres.

Despite the gains seen in 2014, the long-term trend of eelgrass loss continues (Figure 2) and it will take much larger and longer-term increases in eelgrass area and biomass to represent true recovery of the habitat. To put this in perspective, the gains of 2014 do not even make up for the losses in eelgrass seen between 2012 and 2013. In the Great Bay Estuary, eelgrass distribution has declined 44% since 1996. Great Bay itself has lost 79% of eelgrass biomass in that same time, down from 1630 tons in 1996 to 348 tons in 2014.

Worldwide, the main causes of seagrass decline are increased nitrogen and sediment loading. Also, loss of eelgrass leads to increases in the nitrogen available in the water column. Furthermore, eelgrass loss leads to greater resuspension of sediments, which in turn leads to decreasing water clarity.