Interpretation of Eelgrass in Aerial Photographs
Mapping Eelgrass with 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 in Buzzards Bay in the 1980s, and by DEP in 1995 and 2001. Both Costa and DEP used field verification with boats, and Costa used diving (skin or SCUBA) for his eelgrass maps as well. Both DEP surveys had field validation (the 1995 aerial survey was validated in 1995 and 1996, the 2001 aerial survey was validated with field work in 2001 and 2002). 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.
Mapping eelgrass is based on experience, field information when available, and often best professional judgment. There are a variety of techniques, tricks, and useful strategies that are used to map subtidal features, like eelgrass beds.
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 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) unlit 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 below illustrate these points.
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 amongst 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 above also illustrate a second important 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 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. In the DEP survey, such areas may have been characterized as 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.