C),4 The Ahiotic Ingredients of the Wadden Sea
23C)
Equilibria, however, can only emerge if negative feedback between some components of the system occurs (Wissel 1989). This is indeed the case with the morphodynamics of the Wadden Sea: fast currents lead to erosion, whereas slow currents allow for the re-sedimentation of eroded material. Since the morphology
determines domains of fast and slow currents, negative feedback exists between
currents and morphology: very slow currents increase sedimentation, which in turn
changes morphology such that, roughly speaking, according to Bernoulli's law,
higher current velocities result, and vice versa.
This negative feedback and the resulting equilibrium are the decisive stability
mechanisms which have allowed the landscape "Wadden Sea" to follow the rising
sea level for thousands of years while maintaining its characteristic features (morphology with tidal channels, sand and mudflats, and salt marshes).
Strictly speaking, we should refer to "quasi-equilibrium" because "equilibrium"
means by definition that the variables of interest considered do not change at all,
whereas "quasi-equilibrium" means that these variables still change, albeit very
slowly. Under natural conditions (i.e., without dikes), however, these changes are
not relevant for the organisms living in the Wadden Sea, because they proceed
much slower than the typical life cycles of the organisms, and because they do not
alter the essential properties of the landscape.
Extreme scenarios
The most extreme scenario conceivable with respect to hydrography would be the
construction of dikes connecting the barrier islands. Although this scenario is absurd, it helps highlight the "essence" of the Wadden Sea: the tides and the key
processes driven by the tides. This essence is abiotic and will be very important for
the following considerations in this chapter.
A less unrealistic scenario, which has indeed actually emerged in the North
Frisian part of the Wadden Sea, is the construction of dams along the tidal watersheds. They would, as is known from the Sylt-R0m0-bight (Reise et al. 1998),
have enormous consequences on hydrography, morphology and sedimentation.
This is because a large proportion of the exchange of water between a tidal basin
and other regions (basically the North Sea) takes place across the tidal watersheds
(for the area behind Spiekeroog, about 30 %; Chap. 3.3). Additionally, retlection
of the tides at the darns would strongly alter the morphology of the landscape.
Damming of the Zuiderzee in the Netherlands in 1932 has caused major changes in
the hydrography and geomorphology of the western Dutch Wadden Sea (Steyaert
& Bakker 1994).
An extreme sedimentation scenario would be the complete loss of mudtlats
(which is indeed not unlikely). In this scenario, the diversity of benthic species and
birds would decrease because some species depend on mudflats as their habitat or
feeding grounds (for example, the amphipod Corophium volutator, the mudsnail
Hydrobia uLvae, the avocet Recurvirostra avosetta, the redshank Tringa totanus,
juveniles of flatfish and brown shrimps; Reise 1985, 1998).
Less clear are the consequences of a total disappearance of the mudflats for the
primary production of microphytobenthos and plankton, as well as for species
which prefer sandflats as adults but use mudflats as "breeding grounds", i.e. as a
habitat for the juveniles (Arenico/a marina). Living in breeding grounds during the
23C)
Equilibria, however, can only emerge if negative feedback between some components of the system occurs (Wissel 1989). This is indeed the case with the morphodynamics of the Wadden Sea: fast currents lead to erosion, whereas slow currents allow for the re-sedimentation of eroded material. Since the morphology
determines domains of fast and slow currents, negative feedback exists between
currents and morphology: very slow currents increase sedimentation, which in turn
changes morphology such that, roughly speaking, according to Bernoulli's law,
higher current velocities result, and vice versa.
This negative feedback and the resulting equilibrium are the decisive stability
mechanisms which have allowed the landscape "Wadden Sea" to follow the rising
sea level for thousands of years while maintaining its characteristic features (morphology with tidal channels, sand and mudflats, and salt marshes).
Strictly speaking, we should refer to "quasi-equilibrium" because "equilibrium"
means by definition that the variables of interest considered do not change at all,
whereas "quasi-equilibrium" means that these variables still change, albeit very
slowly. Under natural conditions (i.e., without dikes), however, these changes are
not relevant for the organisms living in the Wadden Sea, because they proceed
much slower than the typical life cycles of the organisms, and because they do not
alter the essential properties of the landscape.
Extreme scenarios
The most extreme scenario conceivable with respect to hydrography would be the
construction of dikes connecting the barrier islands. Although this scenario is absurd, it helps highlight the "essence" of the Wadden Sea: the tides and the key
processes driven by the tides. This essence is abiotic and will be very important for
the following considerations in this chapter.
A less unrealistic scenario, which has indeed actually emerged in the North
Frisian part of the Wadden Sea, is the construction of dams along the tidal watersheds. They would, as is known from the Sylt-R0m0-bight (Reise et al. 1998),
have enormous consequences on hydrography, morphology and sedimentation.
This is because a large proportion of the exchange of water between a tidal basin
and other regions (basically the North Sea) takes place across the tidal watersheds
(for the area behind Spiekeroog, about 30 %; Chap. 3.3). Additionally, retlection
of the tides at the darns would strongly alter the morphology of the landscape.
Damming of the Zuiderzee in the Netherlands in 1932 has caused major changes in
the hydrography and geomorphology of the western Dutch Wadden Sea (Steyaert
& Bakker 1994).
An extreme sedimentation scenario would be the complete loss of mudtlats
(which is indeed not unlikely). In this scenario, the diversity of benthic species and
birds would decrease because some species depend on mudflats as their habitat or
feeding grounds (for example, the amphipod Corophium volutator, the mudsnail
Hydrobia uLvae, the avocet Recurvirostra avosetta, the redshank Tringa totanus,
juveniles of flatfish and brown shrimps; Reise 1985, 1998).
Less clear are the consequences of a total disappearance of the mudflats for the
primary production of microphytobenthos and plankton, as well as for species
which prefer sandflats as adults but use mudflats as "breeding grounds", i.e. as a
habitat for the juveniles (Arenico/a marina). Living in breeding grounds during the