Measurement and statistical Analysis of Oyster Reef Surfaces in the German Wadden Sea

The pacific oyster (Crassostrea gigas) is an introduced species to the German Wadden Sea, predominantly settling on the formerly native, loosely-connected blue mussel beds (Mytilus edulis); C. gigas are now typically found on the intertidal flats where they build rigid reef structures. These oyster reefs are characterized by highly complex surfaces consisting of densely-packed, sharp-edged individuals connected with cement-like bonds. In order to investigate the interactions between reef structure, shape, formation on the one and wave as well as tidal currents on the other hand, a currently unavailable understanding of the rough surface is crucial, as it cannot be expressed sufficiently well by conventional roughness estimates.

Here we report observations of oyster reefs for which we first suggest seven new structural classifications. For each structural class, precise digital terrain models (DTMs) are elaborated based on 3D laser scanning and structure-from-motion data. These DTMs are decomposed, deriving spatial properties over some orders of magnitude. They are then evaluated by determination of statistical moments and structure functions. This spatial 3D analysis aims at deriving insight into the impact oyster individuals have on the overarching rough structure they build. The results are validated by comparison to population dynamics for each structure type, e.g. density, length-frequency distribution or coverage ratio.

Our results suggest, that by defining structural classes of oyster reefs and conducting statistical analysis, the physical roughness and overall orientation of oyster reef patches allows for a more precise evaluation of its response to external forcing, such as mean tidal flow and mean wave directions. The results of this investigation will contribute to the development of digitally fabricated surrogate models for experimental and numerical investigation, aimed at deepening the understanding of the interactions between reef formation and hydrodynamics.