2014 SAV Report
Black-and-white aerial photography at a scale of 1:24,000 and digital imagery was the principal source of information used to assess distribution and abundance of SAV in Chesapeake Bay, its tributaries, and the Delmarva Peninsula coastal bays from Assawoman Bay to Magothy Bay in 2014. There were 168 flight lines that yielded aerial photography negatives that were scanned and orthorectified to create orthophoto mosaics. These mosaics were carefully examined on-screen and outlines were drawn to identify all SAV beds visible on the photography, providing a geographic information system (GIS) digital database for analysis of bed areas and locations.Ground survey information collected in 2014 was tabulated and entered into the VIMS SAV GIS digital database.
The CBP Segmentation scheme defines 93 segments that are grouped into four salinity zones to reflect the communities of SAV species found in the Chesapeake Bay:
The term "submerged aquatic vegetation" (SAV) for the purpose of this report encompasses twenty-three taxa from twelve vascular macrophyte families and three taxa from one freshwater macrophytic algal family, the Characeae. The term "SAV" in this report excludes all other algae, both benthic and planktonic, which occur in Chesapeake Bay, its tributaries, and the Delmarva Peninsula coastal bays. Although these other algae species constitute a portion of the SAV biomass in this region (Humm, 1979), this survey did not attempt to identify, delineate, or discuss the algal component of the vegetation nor its relative importance in the flora. The aerial survey cannot differentiate epiphytic algae on submersed vascular plants or differentiate many benthic marine algae species, including many macrophytes, which can co-occur in the same SAV beds.
Seventeen species of submerged aquatic vegetation are commonly found in Chesapeake Bay and its tributaries. Zostera marina (eelgrass), the only "true" seagrass species, can tolerate salinities as low as 10 ppt and is dominant in the lower reaches of the bay. Myriophyllum spicatum (Eurasian watermilfoil), Stuckenia pectinata (sago pondweed), Potamogeton perfoliatus (redhead grass), Potamogeton crispus (Curly pondweed), Potamogeton pusillus (Slender pondweed), Zannichellia palustris (horned pondweed), Vallisneria americana (wild celery), Elodea canadensis (common elodea), Ceratophyllum demersum (coontail), Hydrilla verticillata (hydrilla), Heteranthera dubia (water stargrass), Najas guadalupensis (southern naiad), Najas minor, Najas gracillima, and Najas sp. are freshwater species, some of which have the capacity to tolerate some level of salt, and are found in the middle and upper reaches of the bay (Stevenson and Confer, 1978; Orth et al., 1979; Orth and Moore, 1981, 1983; Moore et al., 2000). Ruppia maritima (widgeon grass) is tolerant of a wide range of salinities and is found from the bay mouth to the Susquehanna Flats. Approximately nine other species are only occasionally found. When present, these less common species occur primarily in the middle and upper reaches of the bay and the tidal rivers. Of all species of SAV, the most abundant are Z. marina, R. maritima, V. americana, H. verticillata, P. perfoliatus, Stuckenia pectinata (P. pectinatus), and M. spicatum.
Zostera marina and R. maritima are the dominant SAV species found in the Delmarva Peninsula coastal bays.
An online key to Chesapeake Bay SAV is available from the Maryland Department of Natural Resources web page.
The 2014 aerial multispectral digital imagery was obtained by Air Photographics (Martinsburg, West Virginia) using a Wild RC-30 camera, with a 153 mm (6 inch) focal length Aviogon lens and Agfa Pan 80 film, mounted in the bottom fuselage of a Piper Aztec, a twin engine reconnaissance aircraft. Photography was acquired from an altitude of approximately 12,000 feet, yielding 1:24,000 scale photographs. A Novatel DL dual frequency GPS and an Applanix Phalanx IMU was attached to the camera to acquire IMU data. Multispectral digital imagery was acquired on June 15, 2014 over the mid-bay islands and the Eastern Shore using a ZI DMC-II 230 multispectral (RGB,NIR) digital mapping camera with a 92 mm focal length, a 5.6 μm pixel size, and a 15552 x 14144 image size. That imagery was acquired at an approximate altitude of 16,300 feet, yielding a ground sample distance (GSD) of approximately 30 cm. Additional imagery for the Potomac River, which happened to be acquired under conditions that were suitable for delineating SAV, was obtained from the USDA Aerial Photography Field Office NAIP Program.
The 168 flight lines, which cover 3,693 flight line kilometers, were numbered and included land features necessary to establish control points for accurate mapping if IMU data was not available. The flight lines used to obtain the photography were positioned to include all areas known to have SAV, as well as most areas that could potentially have SAV (i.e., all areas where water depths were less than two meters at mean low water).
Flight lines were prioritized by sections and flights were timed during the peak growing season of species known to inhabit each area. In addition, specific areas with significant SAV coverage were given priority.
Guidelines for acquisition of aerial photography address tidal stage, plant growth, sun angle, atmospheric transparency, turbidity, wind, sensor operation, and land features. Adherence to the guidelines assured acquisition of photography under nearly optimal conditions for detection of SAV, thus ensuring accurate photo interpretation. Deviation from any of these guidelines required prior approval by VIMS staff. Quality assurance and calibration procedures were consistently followed. The altimeter was calibrated annually by the Federal Aviation Administration and the aerial camera was calibrated by USGS.
Camera settings were selected by automatic exposure control. Sun angle was measured with a sensor on the plane. Flight lines were plotted on 1:250,000 scale maps to allow for overlap of photography. To minimize image degradation due to sun glint, the camera was equipped with a computer controlled intervalometer which established 60% line overlap and 20% sidelap. An automatic bubble level held the camera to within one-degree tilt. The scale, altitude, film, and focal length combination was coordinated so that SAV patches of one square meter could be resolved. Ground-level wind speed was monitored hourly. Under normal operating conditions, flights were usually conducted under wind speeds less than 10 mph. Above this speed, wind-generated waves stir bottom sediments, which can easily obscure SAV beds in less than one hour. The pilot used experiential knowledge to determine the acceptable level of turbidity that would allow complete delineation of SAV beds. During optimum flight conditions the pilot was able to distinguish bottom features such as SAV or algae at low tide. Excessively turbid conditions precluded photography. Determination of optimum cloud cover level was based on pilot experience. Records of this parameter were kept in a flight notebook. Every attempt was made to acquire photographs when there was no cloud cover below 12,000 feet. Cloud cover did not exceed 5% of the area covered by the camera frame. A thin haze layer above 12,000 feet was generally acceptable. Experience with the Chesapeake Bay has shown that optimal atmospheric conditions generally occur two to three days following passage of a cold front, when winds have shifted from north-northwest to south and have moderated to less than 10 mph. Within the guidelines for prioritizing and executing the photography, the flights were planned to coincide with these atmospheric conditions when possible. Air Photographics coordinated the processing of all film. A 9-inch by 9-inch, black-and-white contact print was produced for each exposed frame. Each photograph was labeled with the date of acquisition as well as the flight line number. Film and photographs are stored under appropriate environmental conditions to prevent degradation.
Black-and-white aerial photography at a scale of 1:24,000 and digital imagery are carefully examined to identify all visible SAV beds. Aerial photography negatives covering SAV beds are scanned and orthorectified. Digital imagery is orthorectified and combined with the scanned images to create orthophoto mosaics. Outlines of SAV beds are then interpreted on-screen, providing a digital database for analysis of bed areas and locations. Ground survey information collected in 2014 is tabulated and entered into the SAV geographic information system (GIS).
Orthorectification and Mosaic Production
Photo Interpretation and Bed Delineation
In addition to delineating SAV bed boundaries, an estimate of SAV density within each bed was made by visually comparing each bed to an enlarged crown density scale similar to those developed for estimating crown cover of forest trees from aerial photography (Paine, 1981). Bed density was categorized into one of four classes based on a subjective comparison with the density scale. These were: 1, very sparse (<10% coverage); 2, sparse (10-40%); 3, moderate (40-70%); or 4, dense (70-100%). Either the entire bed or subsections within the bed were assigned a bed density number (1 to 4) corresponding to the above density classes. Some beds were subsectioned to delineate variations of SAV density. Additionally, each distinct SAV bed or bed subsection was assigned an identifying one or two letter designation unique to its map. Coupled with the appropriate SAV quadrangle number and year of photography, these letter designations uniquely identify each SAV bed in the database.
Calculation of Area
An ArcGIS geodatabase in a Universal Transverse Mercator (UTM) Zone 18 projection was used to calculate area in square meters for all SAV beds. These areas are summarized in tables by USGS 7.5 minute quadrangle, Chesapeake Bay Program and Delmarva Peninsula coastal bay segments, zone, and by state. Segment and zone totals were calculated using an overlay operation of segment and zone regions on the SAV beds.
Ground surveys were accomplished by cooperative efforts from a number of agencies and individuals. Although not all areas of Chesapeake Bay were ground surveyed, the data did provide valuable supplemental information. The ground surveys confirmed the existence of some SAV beds mapped from the 2014 aerial imagery, as well as SAV beds that were too small to be visible on the imagery. The surveys also provided species data for many of the SAV beds. Ground survey information supplied to VIMS researchers is included on the SAV distribution and abundance digital maps and included in the VIMS SAV GIS Database. The group that performed each survey is designated by a unique symbol to identify the different methods of sampling. In many cases the symbols on the SAV maps have been offset from the actual sampling point to avoid confusion with the mapped SAV bed. Where species information was available, it is included on the map. Because of space limitations on the maps, occasionally one or more survey points are combined where the information was duplicated. All ground survey data supplied to VIMS are tabulated in the ground survey table.
Lists and Figures
Location of 2014 SAV beds in Chesapeake Bay