2015 SAV Report
Multispectral aerial imagery with a ground sample distance of 24 cm 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 2015. There were 186 flight lines that yielded aerial imagery frames that were 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 2015 was tabulated and entered into the VIMS SAV GIS digital database.The aerial imagery is carefully examined to identify all visible SAV beds. Photographs covering SAV beds are scanned and orthorectified 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 2015 is tabulated and entered into the SAV geographic information system (GIS). SAV distribution data are presented and discussed based on the 2003 revised Chesapeake Bay Program (CBP) segmentation and zonation scheme (DAWG, 1997). This segmentation scheme is mapped and listed by salinity regime.
The CBP Segmentation scheme defines 93 segments that are grouped into 4 salinity zones to reflect the communities of SAV species found in Chesapeake Bay:
SAV distribution in the Delmarva Peninsula coastal bays is presented and discussed separately from Chesapeake Bay. A fourth zone, the Delmarva Peninsula Coastal Bays Zone, is defined to include the region from Assawoman Bay to Magothy Bay and is subdivided into 10 segments: Assawoman, Isle of Wight, Sinepuxent, Chincoteague, Burtons, Hog Island, Spidercrab,Cobb, South, and Magothy bays.
The term "submerged aquatic vegetation" (SAV) for the purpose of this report encompasses 23 taxa from 12 vascular macrophyte families and 3 taxa from 1 freshwater macrophytic algal family, the Characeae. The term "SAV" in this report excludes all other algae, both benthic and planktonic that 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 9 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.
Z. 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 2015 aerial photography was obtained primarily by Air Photographics (Martinsburg, West Virginia) from multispectral digital imagery. The multispectral imagery was acquired using a ZI DMC-II 230 multispectral (RGB,NIR) digital mapping camera and IMU 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 13,200 feet, yielding a ground sample distance (GSD) of approximately 24 cm. This imagery was supplemented for a small set of areas with black and white aerial photographs 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. The black and white 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 RC-30 camera to acquire IMU data.
186 flight lines, which cover 3,937 flight line kilometers, were flown. These lines were designed to include land features necessary to establish control points for accurate mapping, augementing and checking the IMU data. 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 imagery address tidal stage, plant growth, sun angle, atmospheric transparency, turbidity, wind, sensor operation, and land features. Adherence to the guidelines assured acquisition of imagery 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. To minimize image degradation due to sun glint, lines and frames were designed 60% line overlap and 20% sidelap. The scale, altitude, camera settings, 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 maximun cloud cover level was based on pilot experience. Records of this parameter were kept in a flight notebook. Every attempt was made to acquire imagery when there was no cloud cover below 13,000 feet. Cloud cover did not exceed 5% of the area covered by the camera frame. A thin haze layer above 13,000 feet was generally acceptable. Experience with 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 imagery. Digital imagery was delevered on a portable hard drive. Black and white film was scanned photogramettrically and also delivered on a hard drive. Each black and whte 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.
Digital multispectral imagery with a ground sample distance of 24 cm and black and white aerial photography are carefully examined to identify all visible SAV beds. Aerial imagery covering SAV beds are orthorectified. Digital imagery is orthorectified and combined 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 2015 is tabulated and entered into the SAV geographic information system (GIS).
USGS 7.5 minute quadrangle maps are used to organize the mapping process, including interpretation of SAV beds from aerial photography, mapping ground survey data, and compiling SAV bed area measurements. The SAV quadrangle index page gives locations of the 258 quadrangles in the study area that includes all regions with potential for SAV growth. Most quadrangles are sequentially numbered north to south for efficient access to data.
Orthorectification and Mosaic Production
The orthophotographs are mosaicked use a set of ArcGIS mosaic datasets for each flight line that are mosacked into a single Baywide mosiac dataset that is shared as an ArcGIS image service.
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.
Standard operating procedures (SOPs) were developed to facilitate orderly and efficient processing of 2015 SAV maps and SAV computer files produced from them, and to comply with the need for consistency, quality assurance, and quality control. SOPs included: a detailed procedure for orthorectification, mosaicking, and photo-interpretation; tracking sheets to record the processing of flight lines and quadrangles; and weekly summary progress reports of all operations.
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 2015 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.
Species and Ground Surveyors Key
Lists and Figures
SAV species found in Chesapeake Bay
Location of 2015 SAV beds in Chesapeake Bay