Influence of river sediment management on the sedimentology and ecology of shellfish beds within the World Heritage site ‘Wadden Sea’

Abstract

Soft sediment coasts are shaped by interactions between sedimentological and biological processes. Intertidal shellfish beds of the World Heritage site ‘Wadden Sea’ are an important habitat with a variety of sedimentological and ecological functions. They are dominated by two ecosystem engineers, the native Blue mussel (Mytilus edulis) and the invasive Pacific oyster (Magallana gigas). Their extents and biomasses are highly variable and are influenced by many environmental factors as well as human interventions. Among the latter, continuous dredging and dumping of sediments for the maintenance of shipping channels and harbours in the large German estuaries and rivers have led to a change in natural sediment dynamics and are suspected to influence the occurrence of mussel beds and seagrass areas in the Wadden Sea National Parks. The outer Ems estuary is a prime example in this context. River deepening and maintenance dredging have been taking place for decades and have led to heavy siltation and lack of oxygen in the lower Ems and an increased water turbidity can also be observed in the outer Ems. In addition, coastal construction measures have caused morphological changes in ​​the Ems estuary. Within an interdisciplinary project, changes in the morphodynamics, sediment transport and currents are being investigated and methods are developed that allow an assessment of resulting ecological effects on mussel beds, as well as their sedimentological feedbacks in the outer Ems estuary. Investigations on the extent to which sediment shift and deposition affect the occurrence of mussel beds are lacking in the Lower Saxon Wadden Sea. For this purpose, detailed measurements of sedimentation and erosion on selected mussel beds and analyses of the sediment composition were conducted with high spatial and temporal resolution between 2019 and 2023 using sedimentation-erosion-bars and analysis of grain size composition. Mussel beds with different distance to the sediment deposition sites and areas with dense mussel and oyster colonization, gaps in the mussel bank and the tidal flats around the banks were compared. In addition, the extent and biomass of the mussel beds and the condition of Blue mussels were examined and related to sedimentation rates. The results will be presented and discussed against the background of sea level rise in the Wadden Sea and will be provided as a basis for the development of an ecological sediment management for the outer Ems estuary. Within the interdisciplinary project it will be considered how sediments can be sensibly directed to prevent siltation of mussel and seagrass areas on the one hand and to support the natural growth of the seabed where sediments are needed on the other hand, safeguarding the outstanding universal value of the World Heritage site.