200
7 Effects of the Ice Wmter 1995/%
Parallel to the appearance of the black areas an extended lipid film was observed on the water surface north of the East Frisian Islands. Investigations carried
out by the BSH (Bundesamt fur Seeschiffahrt und Hydrographie) showed that this
film was of natural origin produced by lipids derived from the diatom Coscinodiscus concinnus (compare Delafontaine & Flemming 1997). Cells of this species
were also found in the sediments of the East Frisian Wadden Sea. This indicates
that algae and parts of the lipid film were transported by tidal currents into the
Wadden Sea where sedimentation took place followed by the development of anoxic sediment areas. At the same time, an increase in the density of bacteria, a
white surface film covering the sediment and an increase of lipid concentrations in
the sediment were observed. The additional input of organic matter derived from
the algal bloom as well as the reduced gas exchange between the sediment and the
water column due to a sealing effect of the lipid film may have caused the change
in sediment chemistry from aerobic to anaerobic conditions (Hapner 1996).
Aerial surveys from the 28/30th of May and 12th of June 1996 over the entire
East Frisian Wadden Sea showed that anoxic sediment surfaces occurred in the
Spiekeroog backbarrier tidal flats as well. The oxidized sediment layers of the
Groninger Plate and the Swinnplate were extremely thin, indicating that the main
study area of ELA W A T was also affected by this event. Increases of bacterial
counts and TOC concentrations were measured in the sediments. At the same time,
the density of diatoms dropped in the microphytobenthos. According to experimental results (Villbrandt, unpubl. data) the regeneration of the oxic layer depends
on temperature, the concentration of organic matter. current speed and bioturbation. Under stagnant conditions reoxidation and regeneration of anoxic sediment
surfaces was either strongly slowed down or stopped completely.
Neither at the Groninger Plate nor at the Swinnplate an increased macrofauna
mortality could be observed. As larval settlement in most of the species took place
after the "black area event", effects on recruitment were not detectable. Possibly
recruitment of the polychaete H. filiformis, feeding on bacteria, was positively
affected by the increase in bacterial density (compare Chap. 5.3).
In 1996 the C. cOflcinnus bloom and its sedimentation in the Wadden Sea were
an indirect effect of the ice winter and its hydrographic peculiarities. But
C. cOflcinnus blooms may occasionally also develop after mild winters and their
sedimentation may cause - as sedimentation of blooms of other algal species - an
overload of organic matter in the sediment and an increased mortality of macrofauna (Cadee 1996; Michaelis, pers. comm.).
Combined effects of the ice winter and the "black area event" could not be detected at the study sites of ELA W A T. They were more likely to have happened in
areas such as the Baltrum tidal flats. But also in these areas good recruitment of
macrofauna took place (Gunther, pers. obs.) and in 1997, after a period of one
month of high summer temperatures, no return to anoxic conditions was observed
(Hapner 1997).
7 Effects of the Ice Wmter 1995/%
Parallel to the appearance of the black areas an extended lipid film was observed on the water surface north of the East Frisian Islands. Investigations carried
out by the BSH (Bundesamt fur Seeschiffahrt und Hydrographie) showed that this
film was of natural origin produced by lipids derived from the diatom Coscinodiscus concinnus (compare Delafontaine & Flemming 1997). Cells of this species
were also found in the sediments of the East Frisian Wadden Sea. This indicates
that algae and parts of the lipid film were transported by tidal currents into the
Wadden Sea where sedimentation took place followed by the development of anoxic sediment areas. At the same time, an increase in the density of bacteria, a
white surface film covering the sediment and an increase of lipid concentrations in
the sediment were observed. The additional input of organic matter derived from
the algal bloom as well as the reduced gas exchange between the sediment and the
water column due to a sealing effect of the lipid film may have caused the change
in sediment chemistry from aerobic to anaerobic conditions (Hapner 1996).
Aerial surveys from the 28/30th of May and 12th of June 1996 over the entire
East Frisian Wadden Sea showed that anoxic sediment surfaces occurred in the
Spiekeroog backbarrier tidal flats as well. The oxidized sediment layers of the
Groninger Plate and the Swinnplate were extremely thin, indicating that the main
study area of ELA W A T was also affected by this event. Increases of bacterial
counts and TOC concentrations were measured in the sediments. At the same time,
the density of diatoms dropped in the microphytobenthos. According to experimental results (Villbrandt, unpubl. data) the regeneration of the oxic layer depends
on temperature, the concentration of organic matter. current speed and bioturbation. Under stagnant conditions reoxidation and regeneration of anoxic sediment
surfaces was either strongly slowed down or stopped completely.
Neither at the Groninger Plate nor at the Swinnplate an increased macrofauna
mortality could be observed. As larval settlement in most of the species took place
after the "black area event", effects on recruitment were not detectable. Possibly
recruitment of the polychaete H. filiformis, feeding on bacteria, was positively
affected by the increase in bacterial density (compare Chap. 5.3).
In 1996 the C. cOflcinnus bloom and its sedimentation in the Wadden Sea were
an indirect effect of the ice winter and its hydrographic peculiarities. But
C. cOflcinnus blooms may occasionally also develop after mild winters and their
sedimentation may cause - as sedimentation of blooms of other algal species - an
overload of organic matter in the sediment and an increased mortality of macrofauna (Cadee 1996; Michaelis, pers. comm.).
Combined effects of the ice winter and the "black area event" could not be detected at the study sites of ELA W A T. They were more likely to have happened in
areas such as the Baltrum tidal flats. But also in these areas good recruitment of
macrofauna took place (Gunther, pers. obs.) and in 1997, after a period of one
month of high summer temperatures, no return to anoxic conditions was observed
(Hapner 1997).