Assessing the spatial distribution of coral bleaching using small unmanned aerial systems

Abstract

Small unmanned aerial systems (sUAS) are an affordable, effective complement to existing coral reef monitoring and assessment tools. sUAS provide repeatable low-altitude, high-resolution photogrammetry to address fundamental questions of spatial ecology and community dynamics for shallow coral reef ecosystems. Here, we qualitatively describe the use of sUAS to survey the spatial characteristics of coral cover and the distribution of coral bleaching across patch reefs in Kāne'ohe Bay, Hawaii, and address limitations and anticipated technology advancements within the field of UAS. Overlapping subdecimeter low-altitude aerial reef imagery collected during the 2015 coral bleaching event was used to construct highresolution reef image mosaics of coral bleaching responses on four Kāne'ohe Bay patch reefs, totaling 60,000 m 2 . Using sUAS imagery, we determined that paled, bleached and healthy corals on all four reefs were spatially clustered. Comparative analyses of data from sUAS imagery and in situ diver surveys found as much as 14% difference in coral cover values between survey methods, depending on the size of the reef and area surveyed. When comparing the abundance of unhealthy coral (paled and bleached) between sUAS and in situ diver surveys, we found differences in cover from 1 to 49%, depending on the depth of in situ surveys, the percent of reef area covered with sUAS surveys and patchiness of the bleaching response. This study demonstrates the effective use of sUAS surveys for assessing the spatial dynamics of coral bleaching at colonyscale resolutions across entire patch reefs and evaluates the complementarity of data from both sUAS and in situ diver surveys to more accurately characterize the spatial ecology of coral communities on reef flats and slopes.

Rapid assessment of coral cover from environmental DNA in Hawai'i

“…Coral reefs are among the most threatened ecosystems on Earth (Baker, Glynn, & Riegl, ; Brown, ; Burke, Reytar, Spalding, & Perry, ; De'ath, Fabricius, Sweatman, & Puotinen, ; Edmunds et al, ; Erftemeijer, Riegl, Hoeksema, & Todd, ; Hughes et al, ; Pandolfi et al, ) and there is an urgent need to increase the frequency and scale of monitoring (Parrish, Braun, & Unnasch, ; Roberts et al, ) in the face of global change. Currently, biodiversity and ecosystem functioning on reefs are surveyed by numerous complementary methods (Brown et al, ; Levy, Hunter, Lukacazyk, & Franklin, ; Mumby, Green, Edwards, & Clark, ; Rodgers, Jokiel, Bird, & Brown, ; Williams et al, ). Underwater visual surveys are most widely conducted using SCUBA, which can be both time consuming and logistically challenging.…”

Coral Reef Monitoring, Reef Assessment Technologies, and Ecosystem-Based Management

“…Small Unmanned Aerial Systems (sUAS) have been used to effectively assess spatial distributions of coral bleaching on shallow-water patch reefs in Kaneohe Bay, Oahu. sUAS were able to collect imagery and create orthomosaics with ∼1 cm resolution at a rate of 2000 m 2 /min vs. 10 m 2 /min using in situ video surveys; the most efficient survey methods as detailed in Jokiel et al (2015) and Levy et al (2018). With the ability Use NOAA Coral Reef Watch forecasts, and regional derivatives, and communicate with authorities and stakeholders on likelihood of bleaching.…”

“…Current work attempts to integrate multi-domain unmanned systems with a variety of environmental and physical sensors to collect fine-scale data, enhancing our understanding of environmental stressor impacts on coastal coral reef health. Unmanned systems can be tasked with collecting physical data such as imagery, and bathymetry data, in addition to environmental data such as oxygen, temperature, chlorophyll, turbidity, and salinity (Grasmueck et al, 2006;Chirayath and Earle, 2016;Koparan et al, 2018;Levy et al, 2018;Monk et al, 2018).…”

“…This offers the ability to acquire statistically-robust observations about 3-D reef structure, overall coral biomass, and, importantly, key coral vital rates such as growth (Edwards et al, 2017). This technique has been used with both aerial and in situ imagery (Burns et al, 2015;Casella et al, 2016;Levy et al, 2018). Over the coming decade, these technologies will enable tremendous advances in spatial and temporal coverage of coral reef monitoring observations that will be easily transferred to the use of volunteers and citizen-scientists for shallow waters and the use of autonomous vehicles for deeper depths.…”

UAVs in Support of Algal Bloom Research: A Review of Current Applications and Future Opportunities

“…When no ground control is possible, imagery accuracy will depend on the on-board capabilities of the UAV: the Global Positioning System (GPS) and Inertial Measurement Unit (IMU), the link between the camera and the GPS and IMU, and the stored telemetry data that records the UAV position and relevant information for each camera image in a data log which can be used to correct the location of the imagery post-flight [24,55,56]. Additionally, well-anchored floating buoys that have precise GPS measurements are a potential option for open water imaging [57]. In regards to spectral value normalization of UAV-based algal blooms, indices are difficult to standardize without radiometric calibration and potentially atmospheric correction.…”