Loss of shallow water physiotope areas in tidal estuaries of the North Sea since the nineteenth century

Purpose The main aim of this study was to quantify changes in shallow water physiotopes in the North Sea estuaries of Elbe, Humber, Scheldt, and Weser and to collect information on engineering activities which influenced the estuarine hydrology and potentially the spatial range of physiotopes, both between the nineteenth century and present day. A second aim was to provide a database on the recent physiotope distribution for the assessment of future interventions on these estuaries. Methodically Physiotopes were defined by water depth and salinity. The area of the sub-, supra-, and intertidal physiotopes was calculated from digitised maps from different times since the nineteenth century. The spatial changes were opposed to the dates of implementation of the hydraulic engineering activities. Results From the nineteenth century onwards, the shallow water physiotopes of Elbe, Sea Scheldt, and Weser decreased spatially between 4 and 30%, whereas for the Humber estuary the areas have remained stable. The loss of areas was particularly located in the inner estuaries and concerned mostly freshwater physiotopes. The oligohaline and mesohaline zones exhibited less of a reduction in size, although, in the same order of magnitude, while the area of polyhaline physiotopes remained nearly unchanged. Conclusion The loss of physiotopes which occurred mainly in the freshwater and oligohaline zones is important because physiotopes host different salinity-dependent biological communities. This loss of physiotopes could be observed after the implementation of hydraulic engineering projects, which could have led to higher current velocities, further upstream tidal penetration and very likely a corresponding shift of the salt-/freshwater interface upstream. All these effects altered the physiotope areas of the estuaries, except of the Humber estuary. The Humber had been modified to its today’s shape essentially before the nineteenth century. Nevertheless, the data describing the recent size of physiotopes provide a reference base for the assessment of future interventions in the estuaries of Elbe, Humber, Sea Scheldt, and Weser.