Satellites, Seagrass, and Blue Crabs: Understanding Inter-Annual Fluctuations and Linkages in the York River
Master of Science (M.Sc.)
Virginia Institute of Marine Science
Robert J Orth
Rochelle D Seitz
Jeffrey D Shields
To protect and manage ecosystems over large spatial scales, repeated mapping with remote sensing, such as aerial photography, is valuable, but several potential problems need to be overcome to generate accurate maps. For instance, to monitor submerged aquatic vegetation (SAV), such as seagrass, satellite imagery must often capture seasonal and interannual variation as well as disturbances. We used a model system, SAV and the blue crab Callinectes sapidus in the lower Chesapeake Bay, to examine (i) if Planet Lab (PL) satellite imagery can be used to accurately estimate SAV coverage by comparing PL images coincident with those of the VIMS SAV survey; (ii) if PL imagery can capture seasonal and episodic changes in SAV accurately; and (iii) if PL and VIMS SAV survey imagery can be integrated to assess the relationship between SAV nursery habitat and recruitment of young juvenile blue crabs in mid-summer through early fall. To do so, we analyzed data from six selected sites with high salinity in lower Chesapeake Bay. Our findings were (i) PL satellite imagery was a suitable surrogate for VIMS aerial surveys of SAV conducted annually at the selected sites, with the caveat that PL imagery is at a lower resolution (3 m) than the VIMS SAV survey (24 cm), which could affect the utility of PL imagery for some goals; (ii) PL imagery was able to capture seasonal and episodic changes in SAV cover in the Bay; and (iii) remote sensing imagery taken in late spring and early summer was not representative of SAV cover available to the blue crab during the recruitment period in mid-summer through fall. Consequently, PL imagery can be used to estimate SAV bed area over time scales that are relevant to recruitment of the blue crab in lower Chesapeake Bay. Understanding SAV dynamics and future effects of climate change on SAV can be improved with broad-scale data from remote sensing techniques, such as aerial photography and satellite imagery. However, new platforms such as Planet Lab can provide accurate spatial and temporal distribution patterns for SAV beds relative to abundance of the blue crab during critical phases in its life history. At two locations in the York River, lower Chesapeake Bay, we conducted a mensurative field experiment by sampling percent cover of SAV (eelgrass Zostera marina, widgeon grass Ruppia maritima) and algae (mostly Gracilaria vermiculophylla), density of blue crab juveniles, bed area by Planet Lab, and select independent variables bimonthly over two years. The main findings were: (i) juvenile blue crab density was inversely related to SAV bed area, but reductions in crab density as bed area increased were more than offset by higher total abundance of crabs as bed area enlarged; (ii) crab density was positively related to percent cover of algae (Gracilaria), Ruppia and Zostera; (iii) location, year, season and water depth were not significant predictors of crab density in SAV beds after accounting for the effects of bed area and SAV percent cover; and (iv) potential loss of Zostera in the lower Chesapeake Bay due to global warming was projected to cause either only a modest reduction in crab density if other SAV species do not compensate and bed area remains constant, or crab density could even increase if algae and Ruppia were to compensate for the loss of Zostera.
© The Author
Bachand, Kristen, "Satellites, Seagrass, and Blue Crabs: Understanding Inter-Annual Fluctuations and Linkages in the York River" (2019). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1681950285.
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