The late Holocene demise of a sublittoral oyster bed in the North Sea

Abstract

A fossil oyster bed (Ostrea edulis) was recently encountered offshore Helgoland (German Bight). Oysters are important filter feeders in marine environments and their habitat structure supports a large associated biodiversity. The European flat oyster Ostrea edulis has historically occurred in vast populations in the North Sea, but declined massively in the early 20th century. The ecological restoration of Ostrea habitats is a current focal point in the North Sea. To better understand the mechanisms that caused the local collapse of the oyster population, this study investigated the size structure, weight, and age of the shells, along with the spatial dimensions, seafloor properties, and environmental context of the oyster bed. The results show that the demise of the population occurred around 700 CE, ruling out excessive harvest as a driver of decline. Synchronicity of increased geomorphological activity of rivers and concurrent major land use changes in early medieval Europe suggest that increased sedimentation was a viable stressor that reduced the performance of the oysters. The shells provided no indication of a demographically poor state of the oyster bed prior to its demise, but manifested evidence of the wide-spread occurrence of the boring sponge Cliona sp. Our study challenges the assumption of a stable preindustrial state of the European flat oyster in the North Sea, and we conclude that the long-term variability of environmental conditions needs to be addressed to benchmark success criteria for the restoration of O. edulis.

Fig 1

(A) Overview map of the North Sea basin with the historical distribution of Ostrea edulis [20], with background data from [42, 43], (B) Bathymetry and location of the studied oyster bed offshore Helgoland, with background data from [44], (C) Drift-video transects and the surface composition around the oyster bed, (D) Still-image example of oyster shells under a thin veneer of sediment (NB: laser for scale = 10 cm). See S1 Fig in S1 File for examples of all seafloor classes.

Fig 2. Overview of the length, width and weight of all measured shells (n = 532) from the 16 grab sampling locations.

Fig 3

Ostrea edulis shells with boring holes of Cliona sp. showing different degrees of infestation: (A) adult shell, size class ≤9 cm with heavy infestation covering the complete shell, (B) and (C) outer and inner side of a smaller size class with infestation around the umbo, a typical sign for ante-mortem infection, (D) and (E) outer and inner side of an un-infested smaller size class. Scale = 1 cm.

Fig 4. Size classes of sampled Ostrea edulis shells and infestation with Cliona sp. distributed over different size classes (length) indicated as number of holes per area and individual.

Inner and outer shell infestation was assessed separately (n = 1064).

Fig 5

An overview of important environmental dynamics in the southern North Sea over the late Holocene (A-C) and the ¹⁴C ages on shells from the studied oyster bed (D-E). (A) Changes in sedimentation rates in the Helgoland Mud Area (HMA) [65], (B) Periods of increased storminess from records in Atlantic Europe [ and references therein], (C) A record of Holocene river activity (-) and stability (-) in Germany [69]. The green area to the right marks the time window from the first documentation of offshore oysters in the German Bight to their functional extinction [15].