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Wrack, comprised of dead marsh grass, occurs naturally in salt marshes. Wrack can reduce biomass in underlying vegetation and affect salt marsh function. Unmanned aerial vehicles (UAV) provide a more efficient and cost-effective method than traditional field sampling for characterizing the distribution of wrack at a fine spatial scale. We used a DJI Matrice 210 UAV with a MicaSense Altum to collect a total of 20 images from January 2020-December 2021 in a salt marsh on Sapelo Island, GA. Wrack was classified using principal component analysis. Classified images were then used to characterize the size-frequency distribution, landscape position, and potential environmental drivers of wrack. We observed ~ 2100 wrack patches over the course of the study, most of which were present for only a single month. Wrack was found most frequently at the mean higher high water line (~ 1 m), although the areas with the highest frequency of wrack as a proportion of available marsh area were at a higher elevation (> 1.3 m) and closer to creeks or shorelines (~ 40-50 m). High tide events were found to decrease the distance to water of wrack and increase the standard deviation of wrack elevation. This study provides a methodology for understanding wrack dynamics at a landscape scale using frequent, high-resolution UAV data.

Lynn, T., Alber, M., Shalack, J. and Mishra, D. 2023. Utilizing Repeat UAV Imagery to Evaluate the Spatiotemporal Patterns and Environmental Drivers of Wrack in a Coastal Georgia Salt Marsh. Estuaries and Coasts. (DOI: https://doi.org/10.1007/s12237-023-01265-z)