Quantifying seaweed and seagrass beach deposits using high-resolution UAV imagery

Yalei Li; Hege Gundersen; Robert Nøddebo Poulsen; Lina Xie; Zhenming Ge; Kasper Hancke

doi:10.1016/j.jenvman.2022.117171

Quantifying seaweed and seagrass beach deposits using high-resolution UAV imagery

J Environ Manage. 2023 Apr 1:331:117171. doi: 10.1016/j.jenvman.2022.117171. Epub 2023 Jan 7.

Authors

Yalei Li¹, Hege Gundersen², Robert Nøddebo Poulsen³, Lina Xie⁴, Zhenming Ge⁵, Kasper Hancke⁶

Affiliations

¹ Section for Marine Biology, Norwegian Institute for Water Research (NIVA), Oslo, Norway; State Key Laboratory of Estuarine and Coastal Research, Institute of Eco-Chongming, East China Normal University, Shanghai, China. Electronic address: 52173904027@stu.ecnu.edu.cn.
² Section for Marine Biology, Norwegian Institute for Water Research (NIVA), Oslo, Norway. Electronic address: Hege.Gundersen@niva.no.
³ SpectroFly ApS, Markstien 2, 4640, Faxe, Denmark. Electronic address: rnp@spectrofly.dk.
⁴ Section for Marine Biology, Norwegian Institute for Water Research (NIVA), Oslo, Norway; State Key Laboratory of Estuarine and Coastal Research, Institute of Eco-Chongming, East China Normal University, Shanghai, China. Electronic address: 52163904018@stu.ecnu.edu.cn.
⁵ State Key Laboratory of Estuarine and Coastal Research, Institute of Eco-Chongming, East China Normal University, Shanghai, China. Electronic address: zmge@sklec.ecnu.edu.cn.
⁶ Section for Marine Biology, Norwegian Institute for Water Research (NIVA), Oslo, Norway. Electronic address: kasper.hancke@niva.no.

PMID: 36623360
DOI: 10.1016/j.jenvman.2022.117171

Abstract

Macroalgae and seagrass wash ashore by tidal waters and episodic events and create an ocean-to-land transport of carbon and nutrients. On land, these deposits (beach wrack) are consumed by macrofauna, remineralized by microorganisms, or washed back to the sea, during which recycling of carbon and nitrogen affect the biochemical cycles in coastal zones. Manual quantification of beach wracks is time-consuming and often difficult due to complex topography and remote locations. Here, we present a novel method using Unoccupied Aerial Vehicle (UAV) photogrammetry combined with in situ measurements of carbon and nitrogen contents of wrack to quantify marine carbon and nutrient deposits in beach zones. The UAV method was tested against placed cubes ranging from 125 to 88,218 cm³ and demonstrated a high accuracy (R² > 0.99) for volume acquisition when compared to manual measurements. Also, the UAV-based assessments of the cross-sectional area of beach deposits demonstrated a high accuracy when compared to manual and high-precision GNSS (Global Navigation Satellite System) measurements without significant differences between the methods. This demonstrated that UAVs can provide detailed spatial maps, three-dimensional (3D) surface models, and accurate volumetric assessments of beach wrack deposits. In three case studies, combined with carbon and nitrogen measures, total organic carbon and nitrogen deposits in beach wracks were quantified ranging from 4.3 to 9.7 and from 0.3 to 0.5 kg per meter coastline, respectively. In conclusion, this UAV method demonstrated an effective tool to quantify ecosystem carbon and nitrogen deposits relevant to ecosystem assessments and quantification of blue carbon stocks. The method is optimal when the terrain below beach wrack deposits is known, as in the case with before-and-after or repeated surveys. Further, UAVs display strong time- and cost-effective advantages over manual methods which is amplified with increasing project scale. We propose it as a valuable method for multiple scientific and commercial applications related to environmental monitoring and management, including marine resource exploration and exploitation.

Keywords: Beach deposits; Carbon; Drones; Macroalgae; Seagrass; UAV photogrammetry.

MeSH terms

Carbon
Ecosystem*
Environmental Monitoring / methods
Nitrogen
Seaweed*

Substances

Carbon
Nitrogen