Low-cost UAV monitoring: insights into seasonal volumetric changes of an oyster reef in the German Wadden Sea

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.

Surfaces of coastal biogenic structures: exploiting advanced digital design and fabrication strategies for the manufacturing of oyster reef and mussel bed surrogates

“…When individuals die, their shells persist, serving as a favorable settlement substrate for succeeding generations (Quayle, 1988;Arakawa, 1990;Diederich, 2005). By establishing new generations on the bound shells of deceased individuals, permanent reef structures are formed, which can continue to expand horizontally, vertically, and volumetrically, depending on environmental influences (Folmer et al, 2017;Bungenstock et al, 2021;Hoffmann et al, 2023).…”

“…Consequently, at the macroscale, similar spatial extents and patterns to the formerly predominant M. edulis beds exist in the newly-formed M. gigas reefs (Figure 1A). At the microscale, M. gigas reefs exhibit highly heterogeneous surfaces ranging from isolated clusters over coherent patches to spatially extensive, uniformly populated surfaces (Markert, 2020;Hoffmann et al, 2023) (Figure 1B). Hitzegrad et al (2022) proposed a structural classification in seven classes based on spatial patterns and statistical surface parameters (Cluster I, Cluster II, Patch I, Patch II, Garland, Transitional Zone, and Central Reef).…”

“…With the advent of advanced digital manufacturing, new methods are now available to streamline and optimize experimental modeling workflows with the benefit of greater costeffectiveness and more accurate surface surrogate models, yet without being applied to complex coastal surfaces (Aberle et al, 2021). High-resolution, three-dimensional (3D) topographic data in the form of digital elevation models (DEMs) are however a requirement for advanced digital manufacturing and have only recently become widely available due to the development of novel acquisition techniques, e.g., terrestrial laser scanning, sonar survey, and structure-from-motion photogrammetry (SfM) (Faulhaber, 2007;Westoby et al, 2012;Leon et al, 2015;Hodge and Hoey, 2016a;James et al, 2017;Mazzoleni et al, 2020;Hoffmann et al, 2023). When combined with digital fabrication methods, this data enables the generation of 3D, high-resolution surrogate models (Henry et al, 2018).…”

“…Kaiserbalje study are presented inHitzegrad et al (2022) andHoffmann et al (2023). This study chooses the class Central Reef of the structural classification introduced byHitzegrad et al (2022) as the focal structure, as it represents the structural class oyster reefs are predominantly formed of.…”