![\"IMG_0508.JPG\"](\"http:\/\/wsa.seagrassonline.org\/blog\/wp-content\/uploads\/2015\/01\/IMG_0508.jpg\")
<\/a>\nFigure 1 \u2013 The location of the Albemarle-Pamlico Sound Estuary System relative to the State of North Carolina and the southeastern United States.<\/p>\n
The beds are dominated by a mixture of two species, Zostera marina<\/em> (eelgrass) and Halodule wrightii<\/em> (shoalgrass), with seasonally abundant Ruppia maritima<\/em> (widgeongrass) in quiescent areas. This region represents the southern geographic limit of Z. marina<\/em> and the northern geographic limit of H. wrightii<\/em> on the east coast of North America and is the only known overlapping acreage of these two species in the world. H. wrightii <\/em>has higher tolerances to fluctuations in light and water temperature than Z. marina<\/em>, and so it has been suggested that the Albemarle-Pamlico Sound Estuary System could revert to a Halodule<\/em>-dominated system over time as a result of climate change (i.e., increasing water temperature and sea level rise). Therefore, we have embarked on a field study to evaluate changes in the seagrass community (shoot density, biomass, species composition) over the last 20+ years. <\/p>\n The field study involves sampling at 10 historic study sites which were thoroughly evaluated in 1992 by staff here at NOAA\u2019s Center for Coastal Fisheries and Habitat Research (CCFHR) in Beaufort, North Carolina. The 10 sites exist along an increasing gradient of tidal current speed and wind-wave exposure which results in seagrass seascapes ranging from continuous meadows (extending tens of kilometers) to aggregations of patchy mounds (less than a meter across).<\/p>\n The project was initiated in 2013. This past spring and summer we collected our second set of field data. Because the two seagrass species achieve peak biomass at different times of the year, we must sample in late May to early June for Z. marina<\/em> and again in late August to early September for H. wrightii. The patchy sites are mapped by physically tracing the perimeters of all seagrass patches located within a 50 x 100 meter area using a handheld GPS unit. <\/p>\n We also conducted elevation surveys at patchy sites using Real Time Kinematic (RTK) GPS. The elevation surveys require that an off-site GPS base receiver station is established within 5 kilometers of the study site in conjunction with an existing local reference benchmark (a geographic point whose coordinates and\/or elevation has been measured and recorded to a high level of accuracy). The benchmark is linked to a local tidal datum using NGS VDatum utility. The local tidal datum is then used as a reference to measure local water levels. The GPS errors calculated at the reference base station are radio broadcast to the GPS rover receiver being used at the study site. The rover receiver also collects its own set of coordinates which are then corrected on the fly relative to the reference benchmark. After post-processing, this results in a submerged, topographic map of the seagrass patches with centimeter-level vertical accuracy. <\/p>\n A key limitation of conducting surveys via RTK GPS is that the antenna for the rover receiver must be in contact with the surface of the substrate at all times because coordinates are collected continuously at one-second intervals. For our surveys, we adopted an innovative technique that developed at our lab for use in salt marsh elevation surveys where the GPS antenna for the rover receiver is mounted to a bicycle that is manually maneuvered by hand across the study site following a grid pattern to ensure far-reaching coverage of data points. <\/p>\n In addition to mapping, we performed Braun-Blanquet estimates of percent cover and extract core samples for measuring shoot density, leaf lengths, and above- and belowground biomass and for determination of species composition at all sites. Water depth is variable at the sites which can create a sampling challenge. Although it is easier to extract cores in shallow water, estimating percent cover is not as easy when plants are exposed! In deeper water, we use a lot of breath holding to extract cores! <\/p>\n<\/a>
\nFigure 2 \u2013 Zostera marina<\/em> (top) and Halodule wrightii<\/em> (bottom). Visually, the two species appear very similar (strap-like blades) but are easily distinguished by their differing blades tips, blade widths, and rhizome structures.<\/p>\n<\/a>
\nFigure 3 \u2013 Seagrass seascapes in coastal North Carolina range from continuous meadows to patchy seagrass as a result of increasing hydrodynamic stress.<\/p>\n<\/a>
\nFigure 4 \u2013 Here, lead PI Amy Uhrin traces the perimeter of a seagrass patch using a handheld GPS unit.<\/p>\n<\/a>
\nFigure 5 \u2013 CCFHR staff Amit Malhotra and Don Field install the off-site base station and radio antenna (left). The base station antenna is positioned directly over the existing GPS benchmark (middle). The final antenna arrangement is shown in the photo at the right.<\/p>\n<\/a>
\nFigure 6 \u2013 CCFHR staff Amit Malhotra and Troy Rezek maneuver the bicycle with attached antenna across seagrass patches at one of our study sites.<\/p>\n