[M-AG32] Satellite Land Physical Processes Monitoring at Medium/High/Very High Resolution

[MAG32-01] Assessing the spatial distribution and temporal dynamics of a cryptogenic macroalgal invader in the Northwestern Hawaiian Islands with high-resolution satellite imagery

★Invited Papers

*Tomoaki Miura¹, Keolohilani Lopes¹, Leiana Beyer¹, Kauaoa Fraiola², Heather L. Spalding³ (1.University of Hawaii at Manoa, 2.United States Fish and Wildlife Services, 3.College of Charleston)

Keywords:commercial remote sensing, high-resolution remote sensing, Chondria tumulosa, invasive species

An unknown red macroalga was observed growing on the forereef of Manawai Atoll within the Northwestern Hawaiian Islands, or Papahānaumokuākea Marine National Monument (PMNM) in 2015. Subsequent expeditions revealed an increased abundance of this alga and the formation of dense mats overgrowing native corals and macroalgae. The alga is of unknown origin and displayed invasive traits, which alarmed resource managers tasked with conserving the near-pristine, yet remote ecosystems in PMNM. This new alga species was named Chondria tumulosa in 2020; understanding the spatial extent and temporal variability of the algal cover were urgent tasks for management. In this study, we employed Maxar WorldView (WV)-2 and -3 commercial high-resolution satellite image data, assessed their utility, and used their temporal dataset to map the areal extent of C. tumulosa cover for Manawai Atoll. First, we discovered and verified that algal cover was observed as distinctive dark patches in WV-2 and -3 images. Analyzing a temporal sequence of the images allowed the identification of dark patches suspected to be C. tumulosa based on its visible spread starting in 2015 or 2016. Second, we mapped and analyzed those dark patches found along the interface regions of the backreef and lagoon which corresponded to the accumulation of unattached C. tumulosa. The image analysis revealed that these meandering accumulations began to form in 2015, continued to expand at the rate of ~16.02 km /year (length), and reached the maximum possible extent in 2021. Lastly, we employed a machine-learning algorithm and mapped the current spatial extent of the alga cover over the entire forereef and backreef region of Manawai Atoll with an overall accuracy of 76%. The estimated distribution indicated C. tumulosa could be found throughout the forereefs and backreefs with the highest cover in the western and northern regions. The study has demonstrated the utility of high resolution satellite imagery for monitoring and mapping the distribution of a cryptogenic invasive macroalga at Manawai Atoll.