Low-cost UAV monitoring

insights into seasonal volumetric changes of an oyster reef in the German Wadden Sea

authored by

Tom K. Hoffmann, Kai Pfennings, Jan Hitzegrad, Leon Brohmann, Mario Welzel, Maike Paul, Nils Goseberg, Achim Wehrmann, Torsten Schlurmann

Abstract

This study aims to quantify the dimensions of an oyster reef over two years via low-cost unoccupied aerial vehicle (UAV) monitoring and to examine the seasonal volumetric changes. No current study investigated via UAV monitoring the seasonal changes of the reef-building Pacific oyster (Magallana gigas) in the German Wadden Sea, considering the uncertainty of measurements and processing. Previous studies have concentrated on classifying and mapping smaller oyster reefs using terrestrial laser scanning (TLS) or hyperspectral remote sensing data recorded by UAVs or satellites. This study employed a consumer-grade UAV with a low spectral resolution to semi-annually record the reef dimensions for generating digital elevation models (DEM) and orthomosaics via structure from motion (SfM), enabling identifying oysters. The machine learning algorithm Random Forest (RF) proved to be an accurate classifier to identify oysters in low-spectral UAV data. Based on the classified data, the reef was spatially analysed, and digital elevation models of difference (DoDs) were used to estimate the volumetric changes. The introduction of propagation errors supported determining the uncertainty of the vertical and volumetric changes with a confidence level of 68% and 95%, highlighting the significant change detection. The results indicate a volume increase of 22 m³ and a loss of 2 m³ in the study period, considering a confidence level of 95%. In particular, the reef lost an area between September 2020 and March 2021, when the reef was exposed to air for more than ten hours. The reef top elevation increased from -15.5 ± 3.6 cm NHN in March 2020 to -14.8 ± 3.9 cm NHN in March 2022, but the study could not determine a consistent annual growth rate. As long as the environmental and hydrodynamic conditions are given, the reef is expected to continue growing on higher elevations of tidal flats, only limited by air exposure. The growth rates suggest a further reef expansion, resulting in an increased roughness surface area that contributes to flow damping and altering sedimentation processes. Further studies are proposed to investigate the volumetric changes and limiting stressors, providing robust evidence regarding the influence of air exposure on reef loss.

Organisation(s)

Ludwig-Franzius-Institute of Hydraulics, Estuarine and Coastal Engineering
Coastal Research Centre

External Organisation(s)

Senckenberg Naturhistorische Sammlungen Dresden
Technische Universität Braunschweig

Type

Article

Journal

Frontiers in Marine Science

Volume

10

ISSN

2296-7745

Publication date

09.10.2023

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Oceanography, Global and Planetary Change, Aquatic Science, Water Science and Technology, Environmental Science (miscellaneous), Ocean Engineering

Sustainable Development Goals

SDG 15 - Life on Land

Electronic version(s)

https://doi.org/10.3389/fmars.2023.1245926 (Access: Open)