Mapping Eelgrass with Aerial Photographs

Interpretation of Aerial Photographs

Since the 1980s, scientists have routinely used vertical aerial photographs to map current and historical distributions of eelgrass. These photographs may have been taken for documenting land-based features, but some older surveys, and some newer ones were taken to maximize visibility of subtidal features. In Buzzards Bay, aerial photographs were used to map eelgrass by Costa in the 1980s, and by DEP initially in 1995 and 2001 (Fig.1). Since those initial surveys, DEP has interpreted from additional years, aerial surveys for selected areas of Buzzards Bay.

Both Costa and DEP in their first surveys extensively used field verification with boats, and Costa used diving (skin or SCUBA) as well. Because it is impossible to verify every eelgrass bed mapped, most field surveys might validate 5 to 10% of the beds, and often focus on features that may be difficult to interpret. The ability to map eelgrass habitat in an area for a particular period depends on the availability of aerial photographs and their quality, available field information, and the experience of the analyst. If previous surveys confirmed the presence of eelgrass with field observations, this information is of value when interpreting historical photographs of lesser quality. The most important foundation for the interpretation of aerial photographs for eelgrass habitat is to study how the texture, color, and general appearance of different subtidal habitat features appear in color, infra red, and black and white imagery of different scale, quality, tidal elevations, weather conditions, and times of year.

Eelgrass surveys near the mouth of West Falmouth Harbors

Fig. 1. Comparison of eelgrass distribution near the mouth of West Falmouth Harbor based on three different dates of aerial photographs (1981, 1995, 2001).

With respect to eelgrass mapping, the time of year that aerial photographs are taken is one of the most important considerations. Eelgrass is a perennial, but where it does not over winter or does not survive late summer heat, 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 late summer may be absent or have very reduced density between December and early May. Occasionally, some lower subtidal flats do not have eelgrass cover (grown from seed) until late June. Thus, photographs covering shallow embayments, particularly between January and June, must be interpreted with caution. Most aerial surveys of this region have been taken between March and December, but photographs taken during September and October that coincide with good water transparency are often the best surveys for interpreting aerial photographs. The series of photos in Fig. 2 illustrate these points.

Comparison of April and June aerial photographs

The top photograph was taken on April 1, 2001 and the middle one on June 7, 2001. The latter was the basis of the DEP’s eelgrass 2001 eelgrass survey. This photograph was photo enhanced to improve brightness and contrast, which may not have been done for the final eelgrass bed boundary interpretation (bottom photo).

Fig. 2 top was taken on April 1, 2001 and the middle one on June 7, 2001. The latter was the basis of the DEP’s eelgrass 2001 eelgrass survey. This photograph was photo enhanced to improve brightness and contrast, which may not have been done for the final eelgrass bed boundary interpretation (bottom photo, green boundary lines). As shown in the top photograph, many eelgrass beds were present in April, although they did not have as dense an appearance as the June photograph when the leaf biomass (especially density and length) was much greater. The expansion of some eelgrass beds by June is apparent by comparing the two photographs (e.g. at “b”). Notice that the shallow area “c” in the top photograph was unvegetated in April, but eelgrass beds had grown in by June. Also note that the deep bed at “a” in the top photo was not included in the final DEP map (bottom), because this area was quite dark in the original photograph and difficult to discern, although deep area “d” was discerned. Area “b” was an eelgrass bed (perhaps also with rocks) that was not included in the DEP survey because it may have been too small of a bed for the mapping criteria. Area “f” was quite sparsely vegetated in both photographs but still included in the survey coverage. The increased darkening of the area shoreward of this bed in the June photograph suggests that there was probably considerable eelgrass growth within the boulder field closer to shore. Area “g” certainly had considerable eelgrass, but the photo interpreter may have assumed that this area was predominantly algal covered boulders. Area “e” had the largest bare patch included in the DEP survey, but other smaller bare areas were not added as “holes” several sandbars. These decisions were compromises for simplicity.

The photographs in Fig. 2 also illustrate a second important point about interpreting aerial photographs: it is beneficial to have several different dates of photographs to interpret for an area, even if the photographs were not taken under ideal conditions. The above photograph pairs are ideal because they were taken only months apart. If an area is sand, but later vegetated, it can only be eelgrass or drift algae. If a dynamic offshore area is identical from year in photographs, it is likely an algal covered rock field. Algal covered rock fields, drift algae, and eelgrass beds can be further distinguished by other clues of texture and color. For example in fall color aerial photographs, eelgrass beds often have a distinct brownish hue from the growth of algal epiphytes on the leaves, whereas deeper drift algal accumulations have a more uniform darker color.

One of the biggest challenges to characterizing eelgrass bed distribution and creating a GIS polygon coverage is the fact that eelgrass “beds” are seldom continuous monocultures, and either bare patches or other benthic habitat may be interspersed. Depending upon the scale of any aerial survey, different levels of patchiness are apparent. An aerial survey taken from 10,000 feet may show bare patches within an eelgrass bed, and other areas may seem continuous. These continuous areas may in fact have smaller bare patches evident in low altitude aerial surveys. Even in lower altitude photographs, when diving in what appears to be a continuous bed, small bare areas from less than a meter to several meters may be evident. Sometimes eelgrass beds do not have distinct or discernable boundaries, and show a gradual tapering off. 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 the Costa 1988 study, most of the interpretation was based on what is evident at a roughly 1:25,000 projection.

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 a small percentage of bottom cover. In the Costa 1980s surveys, such mixed areas 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. In the DEP survey, such areas may have been characterized principally as algae, and not included in the 1996 maps.

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 that 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.

Benthic Video and Photo Surveys

There have been a number of recent benthic video and photographic surveys in Buzzards Bay, that when studied, provide great insight into the features and textures visible in certain aerial surveys where there has been great water transparency.  Fig. 3 shows an area near the entrance of the Cape Cod Canal, near Mashnee Island.  The aerial photograph shows previously mapped eelgrass beds (red outlines) on a 2014 aerial image. In the center of the photograph are dark, fine striations.  These are eelgrass beds, and their appearance is caused by vegetative growth of eelgrass shoots in a high current sandy bottom environment.  The striation pattern in these environments is parallel to the ebb and flood currents. Benthic video surveys conducted by Hubbard and Massachusetts Maritime Academy Students (insets in Fig. 3), affirm this interpretation.

Aerial photograph of eelgrass beds near the entrance of the canal with benthic imagery superimposed.

Fig. 3. Aerial photograph of eelgrass beds near the entrance of the canal (major boundaries defined in red) with benthic images shown for selected sites (green dots). Benthic imagery from Hubbard (2017). Final Report MMA/CAF Benthic Studies in Buzzards Bay, MA. See explanation in text for eelgrass beds with thin striated appearance in the center of the image. Image contrast and brightness were adjusted. Click the image to enlarge.

Not all dark features on aerial imagery is eelgrass.  Such features may be cobble or boulder fields, drift algae, attached algae, silty sediments (generally only in protected embayments), dredged channels, mollusk reefs, or any other potential features. Interpretation of these features require several other data sources be available, preferably in a GIS software environment.  The most important of these ancillary data sets are bathymetric charts or bathymetric digital elevation models in a GIS environment.  Past eelgrass surveys should be reviewed to recognize the maximum depth eelgrass was previously mapped. Deeper eelgrass beds may be overlooked in past surveys because of poor water transparency, but areas well beyond previous maximum depth of eelgrass are generally other types of habitat.  In upper Buzzards Bay, dark features are evident in many areas below the 12 ft bathymetric contour. These features appear to be reefs of the slipper shell, Crepidula fornicata, based on the benthic surveys by Hubbard (2015, 2017); see Fig. 4).

Mollusk reefs, principally the sessile snail Crepidula fornicata, in deeper waters near the Pocasset River, in Bourne.

Fig. 4. Mollusk reefs, principally the sessile snail Crepidula fornicata, in deeper waters near the Pocasset River, in Bourne. Note in this site, eelgrass is in shallow waters, attached algae on rock and cobble begins at about the 6 foot depth (see bathymetry inset), and in deeper water, below 6 feet MLLW (or deeper), are extensive areas dominated by Crepidula. Click the image to enlarge.  Photograph inserts taken from Hubbard 2017 study.

Large subtidal boulder fields can be found in many areas of Buzzards Bay. These are glacial deposits, and are particularly prominent along the glacial moraines that stretch through Bourne, Falmouth, and the Elizabeth Islands.  In the Elizabeth Islands, near shore, generally in less than 15 feet of water, eelgrass can be found in abundance in sandy patches within these boulder fields. In deeper waters within the photic zone for macroalgae ( to 15 feet at the north end of the bay, to 30 feet at the south end), red and brown algae can be the dominant cover on these rocks, making them distinct from surrounding sandy areas in aerial photographs where water transparency is good.  An example of a feature like this is shown in Fig. 5.

 

Boulders in the photic zone of Buzzards Bay (generally 15 to 30 feet depending on location in the bay), are typically covered by Red and brown algae and encrusting animals.

This aerial photograph near Woods Hole shows large boulder fields (red arrows), which at this site are covered by red algae (inset photo, probably Chondrus), and a colonial bryozoan (tan). Benthic image from Ackerman et al., (2014), Geological Sampling Data and Benthic Biota Classification: Buzzards Bay and Vineyard Sound, Massachusetts. Area delineated with a green line along the coast to the right (Penzance Point), shows previously mapped eelgrass habitat in sandy areas within a boulder field.  The delineated area includes both algal covered boulders and eelgrass patches. Click to enlarge.