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Shifts in Seagrass Species Composition: A Sentinel Indicator for Sea Level Rise and Climate Change

This project began in May 2013 and was completed in December 2015

We are using aerial and satellite imagery, field surveys, and tank experiments to answer questions regarding climate change impacts to the coastal seagrass beds of North Carolina, particularly seagrass susceptibility to hurricanes. Our findings will identify areas vulnerable to change and enable managers to prioritize areas for regular mapping and monitoring efforts.

Why We Care
Seagrasses (submerged, rooted, marine flowering plants) are integral components of the North Carolina coastal environment. The sediment binding capabilities of the seagrass root system in conjunction with the wave-attenuating properties of the canopy act to physically stabilize the seafloor, reducing erosion and buffering the coast against storms. Seagrass beds also function as nurseries, feeding grounds, and refuge for several fishery species during multiple stages of their lifecycle, including bay scallops and black sea bass, which are currently listed as overfished in North Carolina. Threatened in the short-term by reduced water quality and coastal development, seagrass is also susceptible to impacts from increased hurricane activity and sea level rise as a result of climate change.

Little is known about the susceptibility of patchy versus continuous seagrass beds to extreme wind events such as hurricanes, and whether hurricane impacts are enhanced or impeded across a patchy versus continuous landscape. It is possible that patchy areas are less resistant to such disturbances and more vulnerable to shifting to an unvegetated state. Direct loss of seagrass habitat would have profound effects on shoreline stabilization as well as fisheries.

North Carolina is located at the intersection of northern species and southern species for many marine organisms. There is a unique co-occurrence of two seagrass species, the temperate seagrass Zostera marinaand the subtropical seagrass Halodule wrightii. As climate and sea level change, it has been hypothesized that conditions will favor Halodule with potentially profound cascade effects on the local seagrass ecosystem, given the significantly different communities that each seagrass species supports. A seagrass species composition shift in North Carolina from co-dominants to a Halodule-dominated system may cause dramatic shifts in local faunal populations that in turn could influence fisheries production through the altered functional role of the grass beds.

What We Are Doing
We are using remote sensing tools, a field study, and a mesocosm experiment to answer questions regarding the response of seagrass ecosystems to climate change and climate variability.

  • Remote Sensing: We are developing and evaluating a novel technique for classifying seagrass habitat from aerial and satellite imagery for use in characterizing the spatial distribution and pattern of seagrass landscapes. Pilot trials indicate that the new classification technique improves seagrass change detection analysis over methods where seagrass habitat is visually interpreted. We will use this new technique to classify historic aerial imagery and conduct change analysis.
  • Field Study: In summer 2013 we surveyed eighteen historically studied seagrass sites where Zostera and Halodule coexist for seagrass distribution, percent cover, biomass/density, persistence, and species composition. We are also conducting fine-scale mapping and elevation surveys of the sites. The sites represent an increasing gradient of tidal currents and wave exposure.
  • Tank Experiment: We have successfully initiated an outdoor tank experiment to document competitive/facilitative interactions between Zostera and Halodule.

Expected Outcomes and Benefits of Our Work

  • The new seagrass classification technique and subsequent change analysis will provide fisheries managers with more accurate assessments of the acreage and extent of seagrass and its availability for use by commercial species.
  • Determining changes in seagrass distribution and landscape pattern as a function of local tidal current speed, wave exposure, and past hurricane activity will allow for identification of vulnerable areas.
  • Observations of whether these coastal seagrass ecosystems are prone to dramatic shifts from vegetated to unvegetated states as a result of hurricane activity will allow for prioritization of areas for mapping, monitoring, and potential restoration efforts.
  • Documentation of potential seagrass species composition shifts and competitive interactions between seagrass species will allow managers to mitigate the loss of Essential Fish Habitat and the subsequent cascading impacts to fishery species.

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