Buzzards Bay Eelgrass GIS data
Considerations for mapping eelgrass beds with aerial photographs
On outer coasts, where eelgrass typically grows on sand, eelgrass appears as a dark region on a light background of sand. In protected embayments with muddy bottoms, contrast between eelgrass and its background is reduced, but eelgrass can still be discerned as a dark patch on a slightly lighter bottom. On the outer coast, only algae covered rock and cobble can be mistaken for eelgrass, but the different “texture” of these beds on photographs makes their identification possible. In protected embayments, eelgrass is more difficult to identify because it typically grows on mud bottoms, which offers reduced contrast between the beds especially on black and white photographs. Furthermore, there is a large accumulation of attached or drift algae in these areas which can be mistaken for eelgrass. When eelgrass grows on a bottom covered with drift algae, however, the beds appear as a slightly lighter patch on a dark background.
The time of year that aerial photographs are taken is an important consideration in their interpretation. Eelgrass is a perennial, but where it does not over winter because of icing or does not survive the late summer heat and desiccation when tidally exposed, it is a functional annual. In general, eelgrass growing in deeper waters on the outer coast can be seen on aerial photographs year-round. In winter and spring photographs, eelgrass beds appear less dense, and estimates of percent habitat cover may be less than photographs taken later in the growing season. In many embayments, particularly in shallows, eelgrass beds that are present in summer may be absent or have very reduced density between December and early June. Thus, photographs covering shallow embayments taken this time of year must be interpreted with caution. Most aerial surveys of this region have been taken between December and April during leaf-off periods. Those photographs taken during September and October that also coincide with good water transparency tend to be the best for mapping eelgrass cover because the beds during this time of year tend to show the peak summertime extent of eelgrass cover and still have very high leaf cover and biomass.
The depth that eelgrass grows depends principally on water transparency. In poorly flushed embayments with poor water transparency, eelgrass may grow only to depths of 4 to 6 feet mean low water (MLW), and often less. Outside of embayments in Buzzards Bay, where water transparency is better, eelgrass often grows to depths between 10 and 15 feet MLW, and sometime down to 20 feet MLW or more in clear water areas like around the Elizabeth Islands. A good approximation of maximum eelgrass depth of growth is the average secchi disk depth during the growing season. From a mapping perspective, the significance of this information is that the lower depth of eelgrass visible in aerial photographs is often near the maximum depth of visible detail of aerial photographs during average conditions. Thus, during periods of poor water transparency, the lower limit of eelgrass may not be easily discerned for either offshore areas or protected embayments.
Eelgrass can grow in subtidal areas abutting the intertidal zone, in intertidal pools, and can even grow in the lowest areas of the exposed intertidal (although they may be exposed less than one week each month). However, the existence of eelgrass beds in the intertidal is serendipitous, and depends on the right circumstances. Eelgrass beds growing in these areas are functional annuals, growing from seed, because they generally do not survive winter icing of late summer heat exposure during spring low tides. In the intertidal, for the beds to survive into the summer, you must have both good seed set early on and the right environmental conditions as they grow. Generally, these beds become established in late May or June. The early survival of intertidal beds might require overcast or rainy days coincide with spring monthly low tides while the seedlings are young and small. With sufficient density, intertidal beds can survive the increasing summer heat because large number of leaves and shoots overlay each other, trapping some water, and generally only the top leaves die. Large intertidal eelgrass beds like these appeared in 1984 on the tidal flats in the West Branch of the Westport River. These beds grew to flowering shoots and were developing mature seeds by early July.
Intertidal and immediate subtidal eelgrass beds are probably less common today than in years past. This is because protected, low-energy-environment estuaries and coastal lagoons are the areas that have tidal flats that are stable enough for intertidal eelgrass beds to be established. However, for intertidal beds to become established and grow, they need a large stock of seeds to colonize the area. Unfortunately, many of these shallow protected embayment ecosystems are now impacted by coastal eutrophication and have lost most of their deeper eelgrass beds that would have provided sufficient seed stock to help establish these functional annual intertidal and near subtidal beds. Outer coast areas with cleaner water do not have eelgrass in near subtidal because wave energy is too high to create a stable substrate for the growth of new seedlings.
Costa 1988 Thesis Data
With funding from the Buzzards Bay Program in 1985, Costa mapped current and historical distribution of eelgrass in Buzzards Bay based on field surveys, historical aerial photographs, and sediment cores (Costa 1988a, 1988b). To map the beds, vertical aerial photographs were overlain with Mylar and eelgrass bed outlines were traced and characterized for percent cover. Images of these tracings were then projected onto USGS 1:24000 topographic maps, traced, then digitized using proprietary software of the period.
Because the base maps used were sometimes photocopies of topographic maps rather than originals, this introduced positional errors in these maps and data files that were apparent on the original ArcView version of this database distributed in a limited fashion during the 1990s. In this version of the dataset, these errors were corrected by shifting or rotating slightly individual polygons using ESRI ArcMap software in order to have the beds coincide with the existing 1980s era topographic base maps as originally intended (these base maps are available from MassGIS and have a positional accuracy of less than 2 meters). To make the adjustments original field notes and maps used by Costa surveys, and in in some cases, the original aerial images were consulted.
A challenge when mapping eelgrass is the fact that eelgrass ‘beds’ are seldom continuous monocultures. Either bare patches or other benthic habitat may be interspersed. Depending upon the scale of any aerial survey, different levels of patchiness are apparent. In attempting to delineate eelgrass bed boundaries, a subjective decision must often be made to decide where the boundary is, and there are multiple ways in which a polygon coverage may be created depending upon the level of detail desired. In this study, each bed was assigned a percent cover estimate based on the cover of eelgrass beds within each polygon. This interpretation of percent cover was based on what is evident at a roughly 1:25,000 projection on the aerial photographs.
Besides patchiness between eelgrass beds and bare areas, eelgrass beds may exist within sandy patches in rock and cobble areas in offshore high wave energy areas, or mixed with drift and attached algae in sheltered embayments. Both these areas pose special problems in characterization because eelgrass may be present, but only in small percentage of cover. In the Costa 1980s surveys, such a mixed area were delineated as an eelgrass ‘bed’, even if eelgrass cover may have been characterized as 20% to 50%, because each polygon was assigned an attribute based on percent cover.
Most of the aerial surveys used in the Costa study were not taken with mapping coastal vegetation in mind. In many cases, glare or poor water clarity from strong winds or recent storms resulted in the loss of information. If several recent surveys were taken of the same area, the best information from each was used to make the maps. Sometimes the lower boundaries of eelgrass beds could not be identified on any photographs and in these cases, bed boundaries were approximated based on local bathymetry and the typical depth of eelgrass growth for that area.
Field verification was accomplished either by diving excursions from shore, surface verification with boats, or boat diving with skin or SCUBA. Within the limited scope of this study, it was impossible to field-verify every bed. This was unnecessary, since with experience, most features on photographs could be distinguished. Areas where no field verification was made by DEP or in previous studies, and where doubt remains on the identity of the mapped vegetation are detailed in the results section. In some embayments, interpretation of photographs was aided by information supplied by shellfish wardens, other researchers, local residents, or reliable anecdotal data. Consult the EPA report to determine the actual date of photographs used for each part of Buzzards Bay. In general the data the aerial images used were principally taken between 1978 and 1983, with most based on the October 1981 NOAA coastal survey images.
2011 UPDATE: Costa Thesis shapefile updated; previous version had polygons with degraded reduced resolution.
DOWNLOAD THIS DATA: eelgrass_costa1988thesis_2011update.zip
(384 kb zipped Shapefile Coverage).
Do you use Google Earth (a free program)? Try opening this kmz file: eelgrass_costa1988thesis_2011update.kmz
Key for percent cover in Google Earth coverage.
Citation for this Data
Additional information in: Costa, J. E. 1988b. Distribution, production, and historical changes in abundance of eelgrass (Zostera marina L. ) in Southeastern MA. Ph. D. Thesis, Boston University, 395 pp. (7.5 MB pdf)
DEP 1996 and 2001 GIS Data
In 2006, DEP and MassGIS released updated maps of eelgrass cover in Buzzards Bay based on a 2001 aerial survey and field work in 2001 and 2002. Information on this program and updated maps are available at:
New Historical Assessments by the Buzzards Bay NEP
[Under construction. We are digitizing eelgrass beds from each photograph series we have on hand to creating historical change GIS data sets that will eventually supersede the Costa 1988 GIS data set.]