Oyster Reef Surfaces in the Central Wadden Sea: Intra-Reef Classification and Comprehensive Statistical Description

Abstract

The Pacific oyster (Magallana gigas) is an invasive species in the Wadden Sea transforming parts of it permanently. M. gigas, as an ecosystem engineer, builds reef structures that are characterized by highly complex and variable surfaces consisting of densely packed, sharp-edged individuals connected with cement-like bonds. To investigate the interactions between reef structure, shape and formation and wave as well as tidal currents, an understanding of the surface roughness is essential. This work reports on observations of oyster reefs for which seven new structural classes (Central Reef, Transitional Zone, Cluster I, Cluster II, Patch I, Patch II, and Garland) are proposed. For each class, high resolution Digital Elevation Models (DEMs) have been elaborated based on Structure-from-Motion (SfM) photogrammetry and analyzed using spatial statistics. By determining probability density functions (PDFs), vertical porosity distributions, abundances, orientations and second-order structure functions (SSFs), topographical parameters that influence the hydraulic bed roughness have been determined. The results suggest, that by applying the structural classification and their distinct topographical roughness parameters, the oyster reef surfaces can be described appropriately accounting for their complexity. The roughness accounts to a total roughness height kt = 103 ± 15 mm and root-mean-square roughness height krms = 23 ± 5 mm. These values were found similar across all structural classes, yet the shape of the PDFs reveal differences. With decreasing abundance, the distributions become more positively skewed and are characterized by more extreme outliers. This is reflected in the higher statistical moments, as the skewness ranges between Sk = 0.4–2.1 and the kurtosis between Ku = 2.2–11.5. The analysis of the orientations and the SSFs confirms anisotropic behavior across all structural classes. Further, the SSFs reveal the oyster shells as significant roughness elements with exception of Cluster I and II, where the clusters are identified as significant roughness elements. The provided set of topographical roughness parameters enhances the knowledge of oyster reef surfaces and gives insights into the interactions between biogenic structure and surrounding hydrodynamics. The new intra-reef classification allows for more accurate determination of the overall roughness as well as the population dynamics of the habitat forming oyster. Combined with hydraulic measurements, the results can be used to estimate the hydraulic bed roughness induced by the oyster reef surfaces.

Understanding the Role of Sharp Edges in the Propagation of Surface Gravity Waves

“…The lamella type consists of aluminum sheets with a thickness of 1.0 mm and a height of 0.05 m. For the mix type, identical wooden palisades as for the semicircle were cut to a height of 0.04 m and sculpted with an aluminum sheet of 0.01 m height, resulting in a total height of 0.05 m. Hence, all surrogate surfaces have a total roughness height k t = 0.05 m, measured from the bottom to the maximum elevation of the roughness element (peak‐to‐trough roughness height (Chung et al., 2021)). On prototype scale, k t,prototype = 0.15 m is similar to measured heights of protruding oyster shells (∼0.10–0.15 m) (Hitzegrad, Brohmann et al., 2022; Kitsikoudis et al., 2020; Manis et al., 2015). The total length of the surrogate surfaces of 16.0 m reflects 48.0 m in prototype scale, which is in the same order of magnitude as the widths of intertidal oyster reefs in the central Wadden Sea (Folmer et al., 2017; Hitzegrad, Brohmann et al., 2022; Reise et al., 2017).…”

“…Oyster reefs surfaces are highly variable surfaces consisting of areas with densely packed individuals over clusters with lower abundances to bare sediment (Bungenstock et al, 2021;Markert, 2020). Hitzegrad, Brohmann et al (2022) introduced seven structural classes of intertidal M. gigas reefs and identified associated topographical roughness parameters. Applying the results of this work implies that the more heterogeneous areas that are less densely settled and clustered (e.g., Transitional Zone, Cluster I, and Cluster II in Hitzegrad, Brohmann et al (2022)) are most effective at attenuating waves and are subjected to the strongest forces induced by the bed shear stress.…”

“…Hitzegrad, Brohmann et al (2022) introduced seven structural classes of intertidal M. gigas reefs and identified associated topographical roughness parameters. Applying the results of this work implies that the more heterogeneous areas that are less densely settled and clustered (e.g., Transitional Zone, Cluster I, and Cluster II in Hitzegrad, Brohmann et al (2022)) are most effective at attenuating waves and are subjected to the strongest forces induced by the bed shear stress. A study by McClenachan et al (2020) confirms an increased cumulative effect of several small-scale restoration projects compared to their individual influence regarding shoreline protection.…”

“…In contrast to the two-dimensional roughness elements investigated in this work, for example, oyster reef surfaces consist of highly three-dimensional agglomerations of oyster individuals. The surfaces vary in abundance of individuals, as well as the individuals' size, the shell morphology, and their orientation (Hitzegrad, Brohmann et al, 2022;Markert, 2020). Furthermore, the shaping of the shell edge depends on oyster species contributing to the complexity of the bed roughness (Gosling, 2015).…”

“…Individuals typically protrude from the surrounding sediment and form areas with high abundance in which individuals are densely packed and oriented vertically. The resulting ultra-rough surfaces are characterized by reef-wide attributes (e.g., the habitat morphology, abundance, and cluster formation), as well as the characteristics of the individual oysters (e.g., the species, shell shape, shell size, and shell orientation; Hitzegrad, Brohmann et al (2022)). The two most common reef-building oyster species are the Pacific oyster Magallana gigas (Thunberg, 1793 formerly referred to as Crassostrea gigas) and the American oyster Crassostrea virginica (Gmelin, 1791), which inherit similar and differentiating morphological properties (Dame, 2016;Gosling, 2015;Hayward & Ryland, 2017).…”