February 2006
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Overview
Seagrass beds are critical wetlands components of shallow coastal ecosystems throughout the state. Seagrass beds provide food and cover for a great variety of commercially and recreationally important fauna and their prey. The leaf canopy of the seagrass bed calms the water, filters suspended matter and together with extensive roots and rhizomes, stabilizes sediment.
Eelgrass and other seagrasses are often referred to as "Submerged Aquatic Vegetation" or SAV. This distinguishes them from algae, which are not classified as "plants" by biologists (rather they are often placed in the kingdom protista), and distinguishes them from the "emergent" saltwater plants found in salt marshes. In addition to the term SAV, some coastal managers use the term SRV or submerged rooted vegetation. This term probably arose to avoid confusion because non-scientists considered both seagrasses and algae as "plants" or "vegetation," and did not realize the term SAV excluded algae. 1
The DEP Eelgrass layer, produced from data collected in 2001, is the second statewide mapping of the eelgrass resources along the coast. The data were compiled from similar methodologies as the earlier 1995 dataset. A similar third iteration of this statewide mapping is planned for the 2006-07 seasons.
Eelgrass, Zostera marina, is the most common seagrass present on the Massachusetts coastline. The other species found in embayments of the Massachusetts coast is Ruppia maritima, widgeon grass, is present in areas of less salinity along the Cape Cod and Buzzards Bay coast.
The purpose of the second statewide mapping was to determine areas where the Zostera marina resource was decreasing in area and cover. The methods used for the 2001 were similar to those used in the earlier 1995. One exception to this specification was the change in the delineatation of apparent areas of no Zostera within polygons of the resource (i.e. holes). This change in mapping convention was derived from a better understanding of the scientific literature of the dynamics of Zostera beds which describes subtle “infra” movement within beds in response to day-to-day wave action, currents and sand migration, and storm events.
Another difference between the earlier 1995 mapping and the 2001 is that two areas of the coastline, the Elizabeth Islands and Mount Hope Bay were not re-mapped in the 2001 project. The reason for this change is that the Elizabeth Islands are relatively pristine and without environmental stressors to the resource and Mount Hope Bay had no Zostera found in the 1995 mapping project.
The third mapping project scheduled for 2006-07 will include the Elizabeth Islands.
The layer is named EELGRASS_POLY. With the February 2006 update, which added the 2001 data, the EGRASVPT_PT layer of field verified points was discontinued.
Methodology
The mapping process involved the following steps:
- Acquisition of Aerial Imagery
- Photointerpretation of SAV resources
- Fieldwork to confirm Photo-interpreted features
- Compilation to Digital Base Map
Acquisition of Aerial Photography: Aerial imagery (Aerocolor 2448 color positive film) at a scale of 1:20,000 was acquired over the period of 1999-2001. Collecting aerial imagery under the proper environmental conditions is critical to successful benthic mapping. The image acquisition mission required collecting data at low tide, during the period of high seagrass biomass from May 15 to August 31, sun angle between 20 and 35 degrees, low water turbidity, and weather conditions of no clouds and winds less than 10 mph. The imagery was ortho-rectified at a resolution of 1 meter with 90% of the pixels accurate to within 3 meters.
Photointerpretation: The accurate identification of SAV in aerial photographs requires visual evaluation of the fundamental elements of image interpretation (tone, color, contrast, texture and shadow). It also requires extensive experience at ground level in the particular study area. The photographic images of SAV and other benthic images vary in ways that cannot readily be modeled, described or communicated. Training for SAV photointerpretation includes: literature research, discussions with local ecologists and biologists, site visits, overflights in a small plane, and examinations of historical aerial photographs of the area.
SAV are observed best using stereo pairs of photographs and high quality stereoscopic instruments (DEP WCP uses a Cartographic Engineering Ltd. Model SB 190). SRV polygons are drafted onto overlays fixed to each photograph. Minimum mapping unit is 20 meters. SRV (and other benthic features) in a given area will present a variety of signatures depending upon the bottom sediment, depth, season and haze. Shadows from clouds or trees, turbid water, white caps, or sun glint may obscure SRV signatures in the photograph.
Fieldwork: Extensive fieldwork was required to verify that the photosignature was SAV or some other type of benthic feature (macroalgae, mussel bed, dark sand, shells, rock, or other). SAV can also be combined with any of these other benthic features. Fieldwork was conducted in a small boat using surface observation and underwater observation for which an underwater video camera system was utilized. Field notes were compiled digitally in an onboard computer equipped with real-time differentially-corrected GPS positions.
Attributes
The features from the 1995 dataset and the 2001 dataset were combined into one layer, the SAV type and whether SAV existed in that year were used as attributes. To make drawing the polygons easier, a code item was added to specify whether SAV existed for either year or both. The items are:
ATTRIBUTE TYPE VALUE(S) HABITAT95 Char eelgrass HABITAT01 Char eelgrass, ruppia, Freshwater sp (for freshwater species) CODE Char 95 ONLY, 01 ONLY, 95 AND 0
Maintenance/Availability
The next round of eelgrass mapping is scheduled for 2006-07, data availability will follow. To download information and maps of eelgrass extents, go to the MASS DEP Wetlands Conservancy Program Eelgrass Mapping website at http://www.mass.gov/dep/water/resources/eelgrass.htm.
Also see DEP Eelgrass for Selected Embayments .
1 http://www.buzzardsbay.org/eelgrass.htm
Last Updated 3/30/2010
