9.4 The AhlOtic Ingredients of the Wadden Sea
237
tain types of sediment (mudflats, muddy sand, sandflats; Dbrjes 1978). Thus the
spatial distribution of sediment types determines the distribution of benthic species, especially macrobenthic species, to a large degree.
Stability properties: constancy and resilience
Within a tidal cycle, living conditions change so sharply that, according to
Costanza et a!. (1993), tidal flats are mainly characterized by their unpredictability,
i.e. the high variation in abiotic conditions (see definition of "unpredictability"
above).
However, this notion is incomplete because of the apparent paradox that the unpredictability driven by the tides is by far the most predictable aspect of the Wadden Sea. After all, whatever happens, the next tide is bound to come in! Thus the
process that moulds the Wadden Sea, i.e. the tides, is extremely constant with
respect to the occurrence of low and high tides.
The seeming contradiction between the two different notions of tidal f1ats as
being characterized by unpredictability (Costanza et a!. 1993) or by constancy does
not really exist, because Costanza et a!. refer to the unpredictability of states, for
example abiotic conditions, whereas we refer to the constancy (or predictability) of
the dynamics of the living conditions. "Constancy" in this context refers to the
overall characteristics of the dynamics, not to particular values of the variables of
interest. There is, of course, variation in all variables of interest which are determined by the tides, e.g., the size of the areas falling dry (Chap. 3.3) and the time
span over which the tidal tlats fall dry, which is also inf1uenced by the lunar cycles
and the current wind direction.
Likewise, the currents driven by the tides are highly variable if considered on
small spatial and temporal scales (Chap. 3.3), but considered over longer periods
of time (decades) tidal currents presumably show high constancy. This has two
main reasons: the predictability of the tides and the tidal range, and the fact that the
tidal currents are mainly determined by the geomorpology of the tidal f1ats which,
in turn, are constant and resilient (see below). In fact, the stability properties of the
tidal currents and the geomorphology are inseparably linked.
It is not known whether the hydrographic regime of the Wadden Sea follows a
long-term trend. Global warming would certainly change the parameters of the
tides in the Wadden Sea. In the case of the sea level rising, a natural Wadden Sea,
i.e. without dikes, would - as happened several thousand years ago - shift landwards without changing its main characteristics (unless the topography of the
flooded land shows major irregularities; Wolff 1993). This natural adaptation to
changing hydrographical conditions is no longer possible today because the landward boundaries of the Wadden Sea are fixed by dikes ( cf. Wolff 1987; Reise et al.
1998).
The morphology of the tidal tlats was extremely variable on small spatial and
temporal scales. Certain parts of the "Swinnplate", for example, changed due to
major transport processes in winter, biodeposition of the mussel beds in summer,
and seasonally occurring, ephemeral small tidal channels (Chap. 3.4). All in all,
however, it seems as if the position, size, and morphology of the sandf1ats in the
Spiekeroog backbarrier area only change extremely slowly. For example, the relief
237
tain types of sediment (mudflats, muddy sand, sandflats; Dbrjes 1978). Thus the
spatial distribution of sediment types determines the distribution of benthic species, especially macrobenthic species, to a large degree.
Stability properties: constancy and resilience
Within a tidal cycle, living conditions change so sharply that, according to
Costanza et a!. (1993), tidal flats are mainly characterized by their unpredictability,
i.e. the high variation in abiotic conditions (see definition of "unpredictability"
above).
However, this notion is incomplete because of the apparent paradox that the unpredictability driven by the tides is by far the most predictable aspect of the Wadden Sea. After all, whatever happens, the next tide is bound to come in! Thus the
process that moulds the Wadden Sea, i.e. the tides, is extremely constant with
respect to the occurrence of low and high tides.
The seeming contradiction between the two different notions of tidal f1ats as
being characterized by unpredictability (Costanza et a!. 1993) or by constancy does
not really exist, because Costanza et a!. refer to the unpredictability of states, for
example abiotic conditions, whereas we refer to the constancy (or predictability) of
the dynamics of the living conditions. "Constancy" in this context refers to the
overall characteristics of the dynamics, not to particular values of the variables of
interest. There is, of course, variation in all variables of interest which are determined by the tides, e.g., the size of the areas falling dry (Chap. 3.3) and the time
span over which the tidal tlats fall dry, which is also inf1uenced by the lunar cycles
and the current wind direction.
Likewise, the currents driven by the tides are highly variable if considered on
small spatial and temporal scales (Chap. 3.3), but considered over longer periods
of time (decades) tidal currents presumably show high constancy. This has two
main reasons: the predictability of the tides and the tidal range, and the fact that the
tidal currents are mainly determined by the geomorpology of the tidal f1ats which,
in turn, are constant and resilient (see below). In fact, the stability properties of the
tidal currents and the geomorphology are inseparably linked.
It is not known whether the hydrographic regime of the Wadden Sea follows a
long-term trend. Global warming would certainly change the parameters of the
tides in the Wadden Sea. In the case of the sea level rising, a natural Wadden Sea,
i.e. without dikes, would - as happened several thousand years ago - shift landwards without changing its main characteristics (unless the topography of the
flooded land shows major irregularities; Wolff 1993). This natural adaptation to
changing hydrographical conditions is no longer possible today because the landward boundaries of the Wadden Sea are fixed by dikes ( cf. Wolff 1987; Reise et al.
1998).
The morphology of the tidal tlats was extremely variable on small spatial and
temporal scales. Certain parts of the "Swinnplate", for example, changed due to
major transport processes in winter, biodeposition of the mussel beds in summer,
and seasonally occurring, ephemeral small tidal channels (Chap. 3.4). All in all,
however, it seems as if the position, size, and morphology of the sandf1ats in the
Spiekeroog backbarrier area only change extremely slowly. For example, the relief