Eelgrass Distribution in the Great Bay Estuary 2005

Eelgrass (Zostera marina) is an essential habitat for the Great Bay Estuary (GBE) because it provides food for wintering waterfowl and habitat for juvenile fish and shellfish. Eelgrass is the basis of an estuarine food chain that supports many of the recreationally, commercially and ecologically important species in the estuary. Additionally, eelgrass filters estuarine waters, removing both nutrients and suspended sediments from the water column. Eelgrass in the Great Bay Estuary is the largest monoculture in the State of New Hampshire and is considered a vital resource to the State’s marine environment. The present report describes and interprets the eelgrass distribution data collected in 2005 for the Great Bay Estuary.The Great Bay Estuary is experiencing an alarming decline in both eelgrass biomass and distribution that appears to be related to the declining water clarity of the estuary. Eelgrass biomass in Great Bay itself (grams of eelgrass per meter square) has declined steadily (Trowbridge 2006) over the past decade, although the distribution has been relatively constant in Great Bay for the past 10 years at approximately 2,000 acres. In the Piscataqua River, recent declines in both natural and transplanted eelgrass beds are now evident (Short and Beem, in prep) and are a combination of both loss of biomass and loss of distribution. In Portsmouth Harbor in the past 3 years, eelgrass has receded at the deep edge of the meadows, creating an overall loss of distribution which has been accompanied by losses in biomass (Rivers 2007).In this study, we refer to eelgrass biomass as measured by percent cover, i.e., the percent of the bottom which is vegetated with eelgrass. Biomass is determined through a regression of field measured biomass and field-measured percent cover. The percent cover map from the aerial distribution can then be converted to biomassFor the first time, Ruppia maritima (called here by its common name, ruppia) was observed in large beds in several of the tributaries of GBE, both in aerial photographs and while ground truthing. Therefore, ruppia has been added as an element of the seagrass distribution maps. Ruppia has always been found in the GBE at low levels, particularly in association with salt marsh pannes and in the upper reaches of the estuary. It is mapped in 2005 for the first time because it appeared in large beds in parts of the tributaries where eelgrass could be expected. Ruppia occurs as both an annual and perennial plant, and the persistence of these beds is impossible to predict. Although ruppia is a seagrass and provides some of the functions of an eelgrass meadow, its low canopy height (less than 10 cm in these beds) creates different habitat conditions. Almost two decades ago, in 1989, there was a dramatic decline in eelgrass area in Great Bay itself to only 300 acres (15% of normal levels). The cause of this crash was an outbreak of a slime mold, Labryrinthula zosterae, commonly called “wasting disease”. More recently, the greatest extent of eelgrass in the GBE was observed in 1996 after the beds had recovered from the wasting disease episode. The decline in eelgrass biomass seen over the past decade (1996 – 2006) is not a result of wasting disease, and shows all the signs of being caused by anthropogenic impacts, namely nutrient loading and sedimentation. The University of New Hampshire provided digitized eelgrass distribution information in Great Bay Estuary for the years 1999-2001 to the NHEP database. Additionally, the 2002, 2003 and 2004 eelgrass coverages are now in the NHEP database. In 2006, the NHEP funded annual monitoring for eelgrass in GBE. We collected aerial photography of eelgrass coverage for 2006 and mapped eelgrass distribution for 2005 from the information gathered in the summer of 2005 (aerial photography and ground truthing). Here, I report on the eelgrass distribution and cover class information for 2005 in the Great Bay Estuary