Project part-financed by the European Union (European Regional Development Fund)

The Interreg IVB North Sea Region Programme

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Shallow water areas in North Sea estuaries. Changing patterns and sizes of habitats influenced by human activities in the Elbe, Humber, Scheldt and Weser

1. Summary

Diverse uses in the estuaries form a challenge for balancing economic as well as environmental interests in a sustainable way. The project TIDE aimed to consider the ecosystem service approach in estuarine management. Since the services of an ecosystem are dependent on the area of habitats hosting the respective organisms of the ecosystem, it was the scope of this study to compare the change of habitat areas and patterns in the North Sea estuaries Elbe, Humber, Scheldt and Weser.
The evolution of the habitats of the four estuaries was analysed in three time steps: (1) at the end of 19th century/early 20th century, (2) at the middle of the 20th century and (3) in recent times. This approach has allowed to compare the development of the overall sizes of the estuaries, the areas related to a certain salinity zone and different habitats within these zones.
Sea charts, different types of maps and areal photos were digitized and the extensions of the respective habitat types within different salinity zones along the river were calculated applying GIS tools for the three time steps. Additionally, river specific inventories of human activities, potentially influencing the hydromorphology of the rivers were assembled.
In geological time scales, North Sea estuaries have changed as the result of climate changes after the Ice age. The melting of glaciers and inland ice shields led to a flooding of the North Sea basin due to a sea level rise of approximately 120 m. In addition, the North Sea estuaries are influenced by isostatic movement of the Earth´s surface. The surface is still recovering from the load of the ice masses, which covered the North Sea area. Areas covered by the ice shield such as Scandinavia rose and are still rising while areas which were not covered by ice show a subsidence. As an effect of subsidence, the sea-level is apparently rising because parts of eastern England, Belgium, The Netherlands and the German Wadden Coast are sinking.
Considering historic times, the estuaries have been modified by mankind with respect to protect the landscape from flooding and erosion, to claim wetlands for agriculture and settlement and to allow and maintain navigation along the fairways. In particular, navigation became more important because the settlements developed into prosperous trading places being reliant on these shipping routes.
It is notable that the estuaries generally show the highest loss of area in the inner freshwater and oligohaline zones. In the mesohaline zone less space has been lost and within polyhaline areas there was more or less a balance in the losses and gains. Across all salinity zones, the Elbe, Weser and Sea Scheldt show decreases of their overall area between 5 to more than 30 percentages. While the overall size of the Humber reduced greatly in the 18th and 19th century through land-claim for agriculture and port developments, more recently it has remained relatively stable.
In the Sea Scheldt appr. 1,850 ha respectively even more than half of the freshwater area has disappeared. In the mesohaline zone 500 ha got lost which is less than 20%.
In the Elbe 17% of the area (17,000 ha) is lost in the last century, most of it in the freshwater (-3,600 ha) and oligohaline zones (-5,700 ha) whereas the polyhaline zone remains stable.
The Weser shows a total loss of 4.5% of its area (-4,600 ha) with one third in the freshwater area (-2,000 ha) and a fourth of the oligohaline area (-1,700 ha). As well, here the area of the polyhaline zone did not change a lot. The loss of estuarine areas as well as different types of habitats can be related to several anthropogenic impacts such as building dykes and land claim, straightening and deepening the rivers, protecting shorelines and cutting-off tributaries or side channels.
These measures have partly led to higher current velocities, a further upstream penetration of the tides and tidal amplification. These changes in hydromorphological features changed the habitat patterns, decreased the overall area of the estuaries and in turn affected the ecological functioning of the estuarine environment.

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