6.5 DiscussioIl
187
could have affected the recolonization process. A lateral input of sediment can
occur into depressions. Furthermore, the water-filled depressions could have contained a concentration of pelagic or other mobile stages of benthic organisms.
Juvenile shrimps (Crangon crangon) were also recorded frequently in the experimental plots, where they could have preyed upon the developing benthic assemblages. Thus in 1995 the plots were raked after the removal of the tarpaulins to
reduce sediment compaction. In a further experiment the time of cover had been
shortened to simulate a disturbance of reduced intensity.
The variety of treatments allowed comparisons of the recolonization process in
dependence of the timing and intensity of disturbance. It was also possible to
compare recolonization in the two consecutive years 1994 and 1995, as in both
years plots which had been covered for 4 weeks were uncovered in May. The year
1995 differed from the preceding year by low benthic abundances in the studied
tidal flats and an overall low recruitment after a mild winter 1994/95 (Chaps. 5.3;
5.5). These low abundances made it difficult to analyse the data from the experiments in 1995. The lower individual numbers recorded in the experimental plots in
1995 illustrated that recolonization also depends on year to year climatic variations
(Chaps. 3.2; 5.1).
The recolonization experiments were of a central interest for ELA W AT
(Chap. 2) and thus several sub-projects collaborated for this experiment to sample
macro- and meiofauna, microphytobenthos and sediment chemistry. Still, experimental designs to jointly assess recolonization, dispersal and exchange rates of
benthic organisms are recommended for the future.
6.5.1
Recolonization in Dependence of the Sediment Chemistry
In the sediments that were covered with a tarpaulin for several weeks, sulphide
concentrations increased due to the remineralization activity of the microbes.
Although the overall concentrations measured were not especially high, they
reached toxic levels for benthic organisms temporarily and locally. Macrobenthic
organisms vary in their oxygen demand and sulphide tolerance (Thierman et al.
1996). In similar defaunation experiments on the Baltic coast, sulphide-tolerant
ostracods were the first colonizers. They were also among the first colonizers in
the experiments in the backbarrier tidal flats of Spiekeroog. However, it remains
unclear here, whether they had survived in the disturbed plots because of their
sulphide tolerance or whether they had immigrated into these plots actively or by
passive sediment transport. A sulphide tolerance is also discussed for opportunistic
Capitella spp. (Grassle & Grassle 1974). Capitellids occurred in the experimental
plots, but did not dominate the benthic assemblage at any stage.
Organic enrichment can also affect the benthic species composition (Pearson &
Rosenberg 1978; Flothmann & Werner 1992). The decay of the benthic organisms
who died underneath the tarpaulins has probably increased the content of organic
matter in these plots. Unfortunately this was not measured and it remains unclear
whether there was a related effect on the colonization.
187
could have affected the recolonization process. A lateral input of sediment can
occur into depressions. Furthermore, the water-filled depressions could have contained a concentration of pelagic or other mobile stages of benthic organisms.
Juvenile shrimps (Crangon crangon) were also recorded frequently in the experimental plots, where they could have preyed upon the developing benthic assemblages. Thus in 1995 the plots were raked after the removal of the tarpaulins to
reduce sediment compaction. In a further experiment the time of cover had been
shortened to simulate a disturbance of reduced intensity.
The variety of treatments allowed comparisons of the recolonization process in
dependence of the timing and intensity of disturbance. It was also possible to
compare recolonization in the two consecutive years 1994 and 1995, as in both
years plots which had been covered for 4 weeks were uncovered in May. The year
1995 differed from the preceding year by low benthic abundances in the studied
tidal flats and an overall low recruitment after a mild winter 1994/95 (Chaps. 5.3;
5.5). These low abundances made it difficult to analyse the data from the experiments in 1995. The lower individual numbers recorded in the experimental plots in
1995 illustrated that recolonization also depends on year to year climatic variations
(Chaps. 3.2; 5.1).
The recolonization experiments were of a central interest for ELA W AT
(Chap. 2) and thus several sub-projects collaborated for this experiment to sample
macro- and meiofauna, microphytobenthos and sediment chemistry. Still, experimental designs to jointly assess recolonization, dispersal and exchange rates of
benthic organisms are recommended for the future.
6.5.1
Recolonization in Dependence of the Sediment Chemistry
In the sediments that were covered with a tarpaulin for several weeks, sulphide
concentrations increased due to the remineralization activity of the microbes.
Although the overall concentrations measured were not especially high, they
reached toxic levels for benthic organisms temporarily and locally. Macrobenthic
organisms vary in their oxygen demand and sulphide tolerance (Thierman et al.
1996). In similar defaunation experiments on the Baltic coast, sulphide-tolerant
ostracods were the first colonizers. They were also among the first colonizers in
the experiments in the backbarrier tidal flats of Spiekeroog. However, it remains
unclear here, whether they had survived in the disturbed plots because of their
sulphide tolerance or whether they had immigrated into these plots actively or by
passive sediment transport. A sulphide tolerance is also discussed for opportunistic
Capitella spp. (Grassle & Grassle 1974). Capitellids occurred in the experimental
plots, but did not dominate the benthic assemblage at any stage.
Organic enrichment can also affect the benthic species composition (Pearson &
Rosenberg 1978; Flothmann & Werner 1992). The decay of the benthic organisms
who died underneath the tarpaulins has probably increased the content of organic
matter in these plots. Unfortunately this was not measured and it remains unclear
whether there was a related effect on the colonization.