240
') Stahiltty Properties in the Wadden Sea
juvenile stages avoids competition between juveniles and adults (Reise 1985). This
mechanism, which seems to be part of the "success story" of Arenicola marina in
the Wadden Sea, would work much less effectively without breeding grounds in
the upper intertidal.
Biogenic mudbanks cannot substitute mudflats, because they differ with respect
to position and other properties from mudflats, and because they are strongly affected by the disturbance regime imposed by storms and ice winters (Chaps. 3.6,
5.2 and 8).
9.4.3
Chemistry of the Sediments
Variables of interest
Numerous chemical and physical variables can be used to characterize the chemical properties of the sediment. Concerning the entire ecosystem of the Wadden
Sea, however, one aspect is of crucial importance: the existence of a layer on the
surface of the sediment which contains oxygen. The thickness of this oxic layer is
the variable of interest which is considered with respect to resilience, whereas as
far as persistence is concerned, merely the existence of this layer forms the (binary) variable of interest.
Significance of the ingredient for the Wadden Sea
Apart from bacteria with an obligate or optional anaerobic metabolism, all organisms depend on oxygen and cannot therefore survive in an anoxic medium.
Moreover, anoxic sediment is often enriched with sulphide, which is toxic for
higher organisms, although tolerances to different concentrations and exposure
time vary among species (Chaps. 5.2.2 and 6).
A completely and permanently anoxic sediment would be almost exclusively
populated by bacteria. For this reason, and because sulphide concentrations in the
water column might become toxic, a Wadden Sea without an oxic layer at the
sediment surface could no longer be a habitat for benthic organisms, juvenile fish,
crabs or birds. Thus, the existence and resilience of the oxic layer is a decisive
precondition for "higher" life in the Wadden Sea, i.e. for organisms beyond bacteria. "Higher" life includes the usage of the Wadden Sea by man.
Stability mechanisms: persistence and resilience
The oxic layer exists because of the diffusion of oxygen from the air and water into
the sediment. This basic mechanism is supported by the turbation of the sediment
by the tidal currents and swell, drainage of oxygen rich water during ebb through
the sediment, the bioturbation by the infauna, and the oxygen produced by the
microphytobenthos.
Since the physical mechanisms which contribute to the oxic layer are related to
the tides, which are the most predictable aspect of the Wadden Sea, there can be no
doubt about the persistence of the oxic layer under natural conditions. For the same
reason, the oxic layer is also resilient. If the layer were to be destroyed, it would
') Stahiltty Properties in the Wadden Sea
juvenile stages avoids competition between juveniles and adults (Reise 1985). This
mechanism, which seems to be part of the "success story" of Arenicola marina in
the Wadden Sea, would work much less effectively without breeding grounds in
the upper intertidal.
Biogenic mudbanks cannot substitute mudflats, because they differ with respect
to position and other properties from mudflats, and because they are strongly affected by the disturbance regime imposed by storms and ice winters (Chaps. 3.6,
5.2 and 8).
9.4.3
Chemistry of the Sediments
Variables of interest
Numerous chemical and physical variables can be used to characterize the chemical properties of the sediment. Concerning the entire ecosystem of the Wadden
Sea, however, one aspect is of crucial importance: the existence of a layer on the
surface of the sediment which contains oxygen. The thickness of this oxic layer is
the variable of interest which is considered with respect to resilience, whereas as
far as persistence is concerned, merely the existence of this layer forms the (binary) variable of interest.
Significance of the ingredient for the Wadden Sea
Apart from bacteria with an obligate or optional anaerobic metabolism, all organisms depend on oxygen and cannot therefore survive in an anoxic medium.
Moreover, anoxic sediment is often enriched with sulphide, which is toxic for
higher organisms, although tolerances to different concentrations and exposure
time vary among species (Chaps. 5.2.2 and 6).
A completely and permanently anoxic sediment would be almost exclusively
populated by bacteria. For this reason, and because sulphide concentrations in the
water column might become toxic, a Wadden Sea without an oxic layer at the
sediment surface could no longer be a habitat for benthic organisms, juvenile fish,
crabs or birds. Thus, the existence and resilience of the oxic layer is a decisive
precondition for "higher" life in the Wadden Sea, i.e. for organisms beyond bacteria. "Higher" life includes the usage of the Wadden Sea by man.
Stability mechanisms: persistence and resilience
The oxic layer exists because of the diffusion of oxygen from the air and water into
the sediment. This basic mechanism is supported by the turbation of the sediment
by the tidal currents and swell, drainage of oxygen rich water during ebb through
the sediment, the bioturbation by the infauna, and the oxygen produced by the
microphytobenthos.
Since the physical mechanisms which contribute to the oxic layer are related to
the tides, which are the most predictable aspect of the Wadden Sea, there can be no
doubt about the persistence of the oxic layer under natural conditions. For the same
reason, the oxic layer is also resilient. If the layer were to be destroyed, it would