LJ..I The Abiotic Ingredients of thc Wadden Sca
235
Wind direction may affect the Wadden Sea if winds from the East predominate
over longer periods of time, because both the area which falls dry at low tide and
the time span of emergence may increase considerably.
Stability property: constancy
The data presented in Chap. 3.2 document the enormous variation of the meteorological conditions. Each study year had its own peculiarities, with the ice winter
1995/1996 being most prominent.
The ice winter frequency in the Wadden Sea is estimated as one every eight to
ten years (Beukema 1979; Strubing 1996), probably related to the North Atlantic
Oscillation (NAO) (Kroncke et al. 1998). This mean frequency seems to be constant. With respect to storms, the situation is less clear. The interpretation of data
of the last decades is debatable. A time series of strong wind events (wind force
> 6) during the last thirty years seems to indicate that within the study area of
ELA W AT strong wind events have occurred with an unusually high frequency
since the end of the 1980s. The slight decrease in the elevation of the sandflat
"Groninger Plate" could be an indicator for this change in the wind regime. However, the qualifier "unusual" depends on the temporal and spatial scale of observation considered for this statement.
The meteorological data set of ELA W AT suggests that there are no "normal" or
"usual" years in the Wadden Sea. As in all systems with high variability, the average over longer periods of time only has statistical significance and actually occurs
only by chance and momentarily. The mean thus does not represent "normal"
years, but can only be used to assess constancy or trends in the abiotic regime.
Moreover, even if the mean is constant, for example the frequency of ice winters,
the time interval between ice winters may vary considerably.
Stability mechanisms
In the Wadden Sea, stability properties of the meteorological boundary conditions
and of the disturbance regime are determined by climatic processes of the entire
Northern Atlantic. They are therefore "boundary" conditions imposed from outside
which define the abiotic boundaries for the organisms and the processes in the
Wadden Sea. Long-term predictions about the Wadden Sea are thus necessarily
limited by possible climate changes.
Extreme scenarios
A drastic increase or decrease in the frequency of storms or ice winters would have
different consequences for the various species of the Wadden Sea (Beukema et al.
1978; Beukema 1979, 1985). It seems unlikely that benthic species of the Wadden
Sea would be driven to extinction by sea level variations, since most of them have
been around during the sea level rise of the past 10000 years (Wolff 1987). The
chance for invading species to become established in the Wadden Sea in the course
of climate change cannot be predicted in general nor can the effects be foreseen, as
these are species-specific.
235
Wind direction may affect the Wadden Sea if winds from the East predominate
over longer periods of time, because both the area which falls dry at low tide and
the time span of emergence may increase considerably.
Stability property: constancy
The data presented in Chap. 3.2 document the enormous variation of the meteorological conditions. Each study year had its own peculiarities, with the ice winter
1995/1996 being most prominent.
The ice winter frequency in the Wadden Sea is estimated as one every eight to
ten years (Beukema 1979; Strubing 1996), probably related to the North Atlantic
Oscillation (NAO) (Kroncke et al. 1998). This mean frequency seems to be constant. With respect to storms, the situation is less clear. The interpretation of data
of the last decades is debatable. A time series of strong wind events (wind force
> 6) during the last thirty years seems to indicate that within the study area of
ELA W AT strong wind events have occurred with an unusually high frequency
since the end of the 1980s. The slight decrease in the elevation of the sandflat
"Groninger Plate" could be an indicator for this change in the wind regime. However, the qualifier "unusual" depends on the temporal and spatial scale of observation considered for this statement.
The meteorological data set of ELA W AT suggests that there are no "normal" or
"usual" years in the Wadden Sea. As in all systems with high variability, the average over longer periods of time only has statistical significance and actually occurs
only by chance and momentarily. The mean thus does not represent "normal"
years, but can only be used to assess constancy or trends in the abiotic regime.
Moreover, even if the mean is constant, for example the frequency of ice winters,
the time interval between ice winters may vary considerably.
Stability mechanisms
In the Wadden Sea, stability properties of the meteorological boundary conditions
and of the disturbance regime are determined by climatic processes of the entire
Northern Atlantic. They are therefore "boundary" conditions imposed from outside
which define the abiotic boundaries for the organisms and the processes in the
Wadden Sea. Long-term predictions about the Wadden Sea are thus necessarily
limited by possible climate changes.
Extreme scenarios
A drastic increase or decrease in the frequency of storms or ice winters would have
different consequences for the various species of the Wadden Sea (Beukema et al.
1978; Beukema 1979, 1985). It seems unlikely that benthic species of the Wadden
Sea would be driven to extinction by sea level variations, since most of them have
been around during the sea level rise of the past 10000 years (Wolff 1987). The
chance for invading species to become established in the Wadden Sea in the course
of climate change cannot be predicted in general nor can the effects be foreseen, as
these are species-specific.