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.

The world was our oyster: Records reveal the vast historical extent of European oyster reef ecosystems

“…As oyster reefs typically occurred in otherwise soft-bottom habitats, their presence also supported the persistence of a community whose composition differed from surrounding habitats, likely contributing to a higher beta-diversity across the wider system (e.g. Henry et al 2010;Sea et al 2022). The complex, three-dimensional structure of reefs may also have impacted local hydrodynamicregime and sedimentation processes (Lee et al 2021).…”

“…Andrews 1948), changing hydrographic flows or sedimentation rates (e.g. Sander et al 2021), or different environmental conditions for growth meaning reef habitats are less likely to form towards the edge of their range. The situation is made more complex still by the historically uncertain nomenclature of the Ostrea species complex in the Mediterranean (González-Wangüemert et al 2004).…”

“…Glimpses of wider environmental changes and their impacts upon oyster reef persistence are also observed. These include reports of oyster mortalities under very cold winter conditions, which frequently affected shallow-water oyster layings (Select Committee 1876), and the influence of changing hydrographic regimes, such as the 19th century expansion of flat oysters into the western Limfjord after storm-induced hydrodynamic and salinity changes (Huxley 1883;Gaardner and Bjerkan 1934;Berghahn and Ruth 2005;Sander et al 2021). While disease was reported as having significant impacts upon the persistence of oyster populations from the 20th century onwards (e.g.…”

Site selection for European native oyster (Ostrea edulis) habitat restoration projects: An expert‐derived consensus

“…Even inshore, determining the exact age of extinction of historical native oyster beds can present a significant challenge, as shell material can persist for millennia, and written records from locations distant to key trading ports is often scant (Fariñas‐Franco et al, 2018; Bergström, Thorngren & Lindegarth, 2022). Nevertheless, there is significant evidence that oyster beds were extremely widely distributed throughout the North Sea (Gercken & Schmidt, 2014; Bennema, Engelhard & Lindeboom, 2020; Sander et al, 2021), and in many coastal estuaries where they are no longer in evidence (Beck et al, 2011 ) . Given the strong evidence for the widespread distribution of the habitat historically, and the connectivity at the seascape scale that this implies, the expert participants articulated that it was appropriate not to insist on physical evidence of the historical presence of oyster beds in all cases.…”

Sea bed disturbance increases flat oyster recruitment for low to moderate stock densities

“…These depositions can occur on areas known for shellfish settlement, and were up to 8 cm deep over oyster beds off West Mersea, Essex, UK after the 2018 storm ‘Beast from the East’ (Lown, 2019). Sediment deposition can reduce the survival of shellfish and bury the habitats that they create on different timescales (Berghahn & Ruth, 2005; Bromley et al, 2016; Allison et al, 2020; Sander et al, 2021). It is widely recognized that harrowing of the seabed to disturb these sediments is a globally common practice despite a lack of strong peer‐reviewed evidence of its effectiveness (Waugh, 1972; Bromley et al, 2016).…”