Multispectral aerial imagery with a ground sample distance of 24cm was the principal source of information used to assess distribution and abundance of SAV in Chesapeake Bay and its tributaries in 2017. There were 189 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.
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 2017 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:
- Tidal Fresh (less than 0.5 ppt)
- Oligohaline (0.5-5 ppt)
- Mesohaline (5-18 ppt)
- Polyhaline (18-25 ppt)
The term "submerged aquatic vegetation" (SAV) for the purpose of this report encompasses 23 taxa from 12 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, 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 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 2017 aerial multispectral digital imagery was obtained by Air Photographics (Martinsburg, West Virginia) 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. The imagery was acquired at an approximate altitude of 13,200 feet, yielding a ground sample distance (GSD) of approximately 24 cm.
A total of 189 flight lines, which cover 3,879 flight line kilometers, were flown. These lines were designed to include land features necessary to establish control points for accurate mapping, augmenting 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.
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 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 imagery. Digital imagery was delivered on a portable hard drive.
Digital multispectral imagery with a ground sample distance of 24cm 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 2017 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
Digital multispectral imagery is georectified and orthographically corrected to produce a seamless series of aerial mosaics following the standard operating procedures (SOP). ERDAS IMAGINE Photogrammetry image processing software is used to orthographically correct the individual flight lines using a bundle block solution. Camera lens calibration data is matched to the image location of fiducial points to define the interior camera model. Control points from USGS DOQQ, National Agriculture Imagery Program (NAIP), MD Dept. of IT, Virginia Base Mapping Program (VBMP), and ESRI World imagery provide the exterior control, which is enhanced by a large number of image-matching tie points produced automatically by the software when IMU data were not available. The exterior and interior models are combined with a 10-meter resolution digital elevation model (DEM) from the USGS National Elevation Dataset (NED) to produce an orthophoto for each aerial photograph.
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
The SAV beds are interpreted on-screen from the orthophoto mosaics using ESRI ArcInfo GIS software. The identification and delineation of SAV beds by photo interpretation utilizes all available information including: knowledge of aquatic grass signatures on film, distribution of SAV in 2017 from aerial imagery, 2017 ground survey information, and aerial site surveys.
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 2016 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 2017 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. All ground survey data supplied to VIMS are tabulated in the ground survey table.
Ground survey data were obtained in 2017 by:
- Rebecca Golden, Roman Jesien, Brooke Landry, and Angel Willey of the Maryland Department of Natural Resources (MD-DNR) for Susquehanna Flats; the Chester, Gunpower, Magothy, and Sassafras rivers; and Chincoteague, Sinepuxent, Isle of Wight, and Assawoman bays
- Sarah Shpak of the Havre de Grace Maritime Museum for Susquehanna Flats
- David Riter of Baltimore County for Baltimore County
- Eugene McCord for the Elk and Bohemia rivers
- Carl Callahan, Chris Guy, and Peter McGowan of the US Fish and Wildlife Service for Eastern Bay and Harris Creek
- Lindsey Hughes and Matt Pluta of ShoreRivers for the Chester, Choptank, Miles, Tred Avon, and Wye rivers, and Eastern Bay
- Terry Willis for the Chester river
- Matthew Robinson of DC Department of Energy and Environment for the Potomac river
- Chris Jones of George Mason University for the Potomac river
- Tom Harten for the Patuxent river
- Jeremy Testa of the University of Maryland for the Patuxent river
- Bo Lusk of The Nature Conservancy for Hog Island Bay
- James Reidy for Linkhorn Bay
- Corey Holbert, Lisa Kellogg, Bob Orth, Paul Richardson, Erica Smith, Melissa Southworth, and David Wilcox of the Virginia Institute of Marine Science (VIMS) for the Back, Chickahominy, James, Mattaponi, Piankatank, Poquoson, Potomac, Rappahannock, and York rivers; Harris and Hungars creeks, Mobjack and South bays; and Dameron Marsh
- DAWG. 1997. Chesapeake Bay Program Analytical Segmentation Scheme for the 1997 Re-evaluation and Beyond. Chesapeake Bay Program (CBP) Monitoring Subcommittee (MSC) Data Analysis Work Group (DAWG). Draft December 15, 1997 (amended and approved January 29, 1998).
- Godfrey, R. K. and J. W. Wooten. 1979. Aquatic and Wetland Plants of Southeastern United States: Monocotyledons. The University of Georgia Press, Athens, GA. 712 pp.
- Godfrey, R. K. and J. W. Wooten. 1981. Aquatic and Wetland Plants of Southeastern United States: Dicotyledons. The University of Georgia Press, Athens, GA. 933 pp.
- Harvill, A. M., C. E. Stevens, and D. M. E. Ware. 1977. Atlas of the Virginia Flora: Part I, Pteridophytes through Monocotyledons. Virginia Botanical Associates, Farmville, VA. 59 pp.
- Harvill, A. M., T. R. Bradley, and C. E. Stevens. 1981. Atlas of the Virginia Flora: Part II, Dicotyledons. Virginia Botanical Associates, Farmville, VA. 148 pp.
- Humm, Harold J. 1979. The Marine Algae of Virginia. Special Papers in Marine Science, Number 3, Virginia Institute of Marine Science. The University Press of Virginia, Charlottesville, VA. 263 pp.
- Kartesz, J. T. and R. Kartesz. 1980. A Synonymized Checklist of the Vascular Flora of the United States, Canada and Greenland: Volume II, The Biota of North America. The University of North Carolina Press, Chapel Hill, NC. 498 pp.
- Moore, K. A., D. J. Wilcox, and R. J. Orth. 2000. Analysis of the Abundance of Submersed Aquatic Vegetation Communities in the Chesapeake Bay. Estuaries. 23:115-127.
- Orth, R. J., K. A. Moore, and H. H. Gordon. 1979. Distribution and Abundance of Submerged Aquatic Vegetation in the Lower Chesapeake Bay, Virginia. Final Report to U.S. EPA, Chesapeake Bay Program, Annapolis, MD. EPA-600/8-79-029/SAV1. 38 pp.
- Orth, R. J. and K. A. Moore. 1981. Submerged Aquatic Vegetation in the Chesapeake Bay: Past, Present and Future. pp. 271-283. In: Proc. 46th North American Wildlife and Natural Resources Conf., Wildlife Management Institute, Washington, D.C.
- Orth, R. J. and K. A. Moore. 1983. Chesapeake Bay: An Unprecedented Decline in Submerged Aquatic Vegetation. Science. 222:51-53.
- Paine, David P. 1981. Aerial Photography and Image Interpretation for Resource Management. John Wiley & Sons, Inc., New York City, NY. 571 pp.
- Radford, A. E., H. E. Ahles, and C. R. Bell. 1968. Manual of the Vascular Flora of the Carolinas. The University of North Carolina Press, Chapel Hill, North Carolina, NC. 1183 pp.
- Stevenson, J. C. and N. Confer. 1978. Summary of Available Information on Chesapeake Bay Submerged Vegetation. U.S. Dept. of Interior, Fish and Wildlife Service. FWS/0BS-78/66. 335 pp.
- U.S. Environmental Protection Agency, Region III, Chesapeake Bay Program Office, Annapolis, Maryland and Region III, Water Protection Division, Philadelphia, Pennsylvania, in coordination with Office of Water, Office of Science and Technology, Washington, D.C. Technical Support Document for Identification of Chesapeake Bay Designated Uses and Attainability. 2004 Addendum. October 2004.
- Wood, R. D. and K. Imarhori. 1964. A Revision of the Characeae: Volume II, Iconograph of the Characeae. Verlag Von J. Cramer, Weinheim, Germany. 395 icones with index.
- Wood, R. D. and K. Imahori. 1965. A Revision of the Characeae: Volume I, Monograph of the Characeae. Verlag Von J. Cramer, Weinheim, Germany. 904 pp.
Lists and Figures
SAV species found in Chesapeake Bay
Guidelines followed during acquisition of aerial photographs
Chesapeake Bay Program and Delmarva Peninsula coastal bay segments with salinity regime
Location of 2017 SAV beds in Chesapeake Bay
Index of flight lines for 2017 SAV photography
Index of quadrangles for Chesapeake Bay and the Delmarva Peninsula coastal bays
Index of Chesapeake Bay Program Segments
Segment Comparision Map of Chesapeake Bay and its tributaries
Crown density scale used for estimating density of SAV beds from aerial photography