246
l) StahJilly Properties In the Wadden Sea
silience by migratHm", i.e. at the disturbed site all organisms are killed so that the
re-establishment of the original species composition depends on colonization ("migration") from other, undisturbed areas. In this context it does not matter whether
"migration" IS active or passive ("migration" usually refers to active movements).
The second mechanism of resilience is called "in-situ" resilience (Grubb &
Hopkins 1986). In this case not all organisms are killed at the disturbed site. When
after the disturbance the abiotic conditions become similar to the original, predisturbance conditions, the regeneration of populations may occur "in-situ" and
does not necessarily require the input of individuals from other sites. Examples of
this kind of resilience include seed banks of plants, serotinous seeds (which only
germinate after fires) or the resprouting of burned plants (Enright et al. 1996).
Some species of benthic algae are tolerant of burial and many live heterotrophic
(Admiraal et al. 1979). As was shown by the recolonization experiments during
ELA W AT (Chap. 6), some diatoms exhibited "in-situ" resilience because they
were even able to survive several weeks of anoxic conditions.
Extreme scenarios
A Wadden Sea without microphytobenthos would have lost an essential ingredient
and would therefore no longer be the Wadden Sea we know. Primary production
would be much lower. Many species which feed upon microphytobenthos would
disappear, with far-reaching consequences for the food web of the Wadden Sea. In
addition, sediment would be less "stable", which would increase the turbidity of
the water and would also presumably affect larval settlement.
Such a scenario, however, is hard to envisage because the decisive abiotic ingredients which enable the high primary production of the microphytobenthos (i.e.,
sufficiently high input of light, carbon dioxide and nutrients) are very persistent
and resilient. Whether, however, the actual rate of primary production remains
more or less constant, or follows an increasing or decreasing trend, mainly depends
on the anthropogenic input of nutrients.
With respect to phytoplankton, one extreme scenario would be such a high turbidity of the water body that photosynthesis is strongly reduced. Such strong turbidity could be caused by a much more dynamic water body containing much more
suspended matter, as it occurs, for example, in the turbidity maximum of estuaries.
Indeed, the kinetic energy of the water body is already higher than it would be
under natural conditions because of the dikes.
9.5.3
Macrozoobenthos
Variables of interest
The assessment of stability properties is much more complex for higher, heterotrophic species than for autotrophs because there is no highly aggregating variable
which would have the same significance as, for example, primary production or
the rate of remineralization. There is no single variable that would allow to characterize the "performance" of the higher heterotrophic benthic animals (worms,
mussels, crabs, etc.) in the Wadden Sea. Thus, the following considerations will be
l) StahJilly Properties In the Wadden Sea
silience by migratHm", i.e. at the disturbed site all organisms are killed so that the
re-establishment of the original species composition depends on colonization ("migration") from other, undisturbed areas. In this context it does not matter whether
"migration" IS active or passive ("migration" usually refers to active movements).
The second mechanism of resilience is called "in-situ" resilience (Grubb &
Hopkins 1986). In this case not all organisms are killed at the disturbed site. When
after the disturbance the abiotic conditions become similar to the original, predisturbance conditions, the regeneration of populations may occur "in-situ" and
does not necessarily require the input of individuals from other sites. Examples of
this kind of resilience include seed banks of plants, serotinous seeds (which only
germinate after fires) or the resprouting of burned plants (Enright et al. 1996).
Some species of benthic algae are tolerant of burial and many live heterotrophic
(Admiraal et al. 1979). As was shown by the recolonization experiments during
ELA W AT (Chap. 6), some diatoms exhibited "in-situ" resilience because they
were even able to survive several weeks of anoxic conditions.
Extreme scenarios
A Wadden Sea without microphytobenthos would have lost an essential ingredient
and would therefore no longer be the Wadden Sea we know. Primary production
would be much lower. Many species which feed upon microphytobenthos would
disappear, with far-reaching consequences for the food web of the Wadden Sea. In
addition, sediment would be less "stable", which would increase the turbidity of
the water and would also presumably affect larval settlement.
Such a scenario, however, is hard to envisage because the decisive abiotic ingredients which enable the high primary production of the microphytobenthos (i.e.,
sufficiently high input of light, carbon dioxide and nutrients) are very persistent
and resilient. Whether, however, the actual rate of primary production remains
more or less constant, or follows an increasing or decreasing trend, mainly depends
on the anthropogenic input of nutrients.
With respect to phytoplankton, one extreme scenario would be such a high turbidity of the water body that photosynthesis is strongly reduced. Such strong turbidity could be caused by a much more dynamic water body containing much more
suspended matter, as it occurs, for example, in the turbidity maximum of estuaries.
Indeed, the kinetic energy of the water body is already higher than it would be
under natural conditions because of the dikes.
9.5.3
Macrozoobenthos
Variables of interest
The assessment of stability properties is much more complex for higher, heterotrophic species than for autotrophs because there is no highly aggregating variable
which would have the same significance as, for example, primary production or
the rate of remineralization. There is no single variable that would allow to characterize the "performance" of the higher heterotrophic benthic animals (worms,
mussels, crabs, etc.) in the Wadden Sea. Thus, the following considerations will be