l)4 The AbiotIc Ingredients of the Wadden Sca
241
re-form within a few tides because anoxic sediment on the surface would be oxygenized by oxygen from the air and water very quickly (Chap. 6).
The thickness of the oxic layer depends on the physical and chemical properties
of the sediment. season and biotic activities. The thickness decreases as the amount
of organic matter in the sediment increases, because bacteria use oxygen when
decomposing organic matter. This is the case when the organic matter of plankton
blooms is decomposed and was particularly pronounced after the bloom of Coscinodiscus concinnus in June 1996 (Chap. 7).
The anoxic sediment horizon reaches the surface of the sediment ("black spots")
when locally very high amounts of dead organic matter are processed by the bacteria, for example dead Mya arenaria or macroalgal mats, which are buried in the
sediment (Hopner & Michaelis 1994). With layers of macroalgal mats, in addition
to the organic matter decomposing, the diffusion of oxygen from water and air into
the sediment is hampered.
The local input of large amounts of organic matter can be considered as a disturbance of the oxic layer. This disturbance is caused not by the organic matter
itself, but by the decomposition of the material by the bacteria, which reduces the
oxygen concentration. If the oxic layer re-forms after such a disturbance, this can
be interpreted as resilience. This kind of resilience has so far been observed for all
disturbances caused by organic input into the sediment. However, elasticity, i.e.
the return time until the oxic layer rebuilds and reaches its original thickness, depends on the strength of the disturbance. It can take weeks or even months before
the dead organic matter is decomposed in black spots and the activity of heterotrophic bacteria is in turn decreased to a level where the oxic layer may build up
again.
Although black spots are detrimental for the higher benthic fauna, they are
probably not "warning signals" (Lozan et a!. 1994) per se. Instead, the increase in
the frequency and size of black spots (which then become "black areas") may be a
warning signal, i.e. an indicator of the consequences of the increased anthropogenic pollution of the Wadden Sea with nutrients (Hopner & Michaelis 1994).
After the ice winter in 1995/1 996, in some backbarrier areas of the East Frisian
Wadden Sea, large black areas emerged the size of soccer fields. They looked
dramatic, because the sediment was covered with dead benthic fauna. It turned out,
however, that these black areas could be explained as the result of a chain of rare
but natural events, including an "oily" layer which was presumably caused by a
bloom of the diatom C. concinnus (Chap. 7). Even for these large black areas, the
oxic layer was re-established after the organic pollution had decomposed. It could,
however, have taken much longer before the normal physical exchange processes
between the sediment, air and water re-formed the oxic layer, because these processes were hampered by the lipid film on the sediment surface (Chap. 7). However, a storm which occurred a few weeks after the emergence of the black areas
seems have to destroyed this lipid film and thus re-established the normal exchange processes between sediment, air and water.
Stability mechanisms
The decisive stability mechanisms for the persistence of the oxic layer are physical
and chemical processes which are assisted by biotic processes (bioturbation, pri-
241
re-form within a few tides because anoxic sediment on the surface would be oxygenized by oxygen from the air and water very quickly (Chap. 6).
The thickness of the oxic layer depends on the physical and chemical properties
of the sediment. season and biotic activities. The thickness decreases as the amount
of organic matter in the sediment increases, because bacteria use oxygen when
decomposing organic matter. This is the case when the organic matter of plankton
blooms is decomposed and was particularly pronounced after the bloom of Coscinodiscus concinnus in June 1996 (Chap. 7).
The anoxic sediment horizon reaches the surface of the sediment ("black spots")
when locally very high amounts of dead organic matter are processed by the bacteria, for example dead Mya arenaria or macroalgal mats, which are buried in the
sediment (Hopner & Michaelis 1994). With layers of macroalgal mats, in addition
to the organic matter decomposing, the diffusion of oxygen from water and air into
the sediment is hampered.
The local input of large amounts of organic matter can be considered as a disturbance of the oxic layer. This disturbance is caused not by the organic matter
itself, but by the decomposition of the material by the bacteria, which reduces the
oxygen concentration. If the oxic layer re-forms after such a disturbance, this can
be interpreted as resilience. This kind of resilience has so far been observed for all
disturbances caused by organic input into the sediment. However, elasticity, i.e.
the return time until the oxic layer rebuilds and reaches its original thickness, depends on the strength of the disturbance. It can take weeks or even months before
the dead organic matter is decomposed in black spots and the activity of heterotrophic bacteria is in turn decreased to a level where the oxic layer may build up
again.
Although black spots are detrimental for the higher benthic fauna, they are
probably not "warning signals" (Lozan et a!. 1994) per se. Instead, the increase in
the frequency and size of black spots (which then become "black areas") may be a
warning signal, i.e. an indicator of the consequences of the increased anthropogenic pollution of the Wadden Sea with nutrients (Hopner & Michaelis 1994).
After the ice winter in 1995/1 996, in some backbarrier areas of the East Frisian
Wadden Sea, large black areas emerged the size of soccer fields. They looked
dramatic, because the sediment was covered with dead benthic fauna. It turned out,
however, that these black areas could be explained as the result of a chain of rare
but natural events, including an "oily" layer which was presumably caused by a
bloom of the diatom C. concinnus (Chap. 7). Even for these large black areas, the
oxic layer was re-established after the organic pollution had decomposed. It could,
however, have taken much longer before the normal physical exchange processes
between the sediment, air and water re-formed the oxic layer, because these processes were hampered by the lipid film on the sediment surface (Chap. 7). However, a storm which occurred a few weeks after the emergence of the black areas
seems have to destroyed this lipid film and thus re-established the normal exchange processes between sediment, air and water.
Stability mechanisms
The decisive stability mechanisms for the persistence of the oxic layer are physical
and chemical processes which are assisted by biotic processes (bioturbation, pri-