13 Summary
291
• The distribution patterns of the polychaetes Lanice conchilega, Heteromastus
filiformis and Scoloplos armiger were mainly determined by their initial settlement while those of the molluscs Macoma balthica, Ensis americanus, Cerastoderma edule and Mya arenaria followed from postlarval dispersion. However,
postlarval dispersion can also be decisive for polychaetes, e.g. for the recolonization after ice winters.
• The polychaete Lanice conchilega requires hard substrate for settlement. No
correlation was found between the larval density and number of settling larvae.
Results of a model indicated that the density of a developing Lanice-patch is
determined by the near-bottom current velocity.
• The occurrence of L. conchilega on the tidal tlats was subject to high spatial and
temporal variations. Abundances and species numbers of benthic organisms
were higher within natural Lanice-patches and artificial tube fields than in ambient sediments without L. conchilega. Juvenile mussels attached to the protruding tube structures. Among the infauna, predatory polychaetes and spionid
polychaetes were more abundant in the Lanice-patches. Small-scale associations
around the Lanice tubes were detected for nematode- and copepod species.
• The mobility of the benthic organisms in the Wadden Sea contributes to the
high variability of densities. A staining technique revealed a high turn-over. For
example, all stained Pygospio elegans were exchanged on a test field within 6
days, although the low numbers of worms caught in drift nets and sediment
traps had indicated a low capability for dispersal.
• A survey of the shore crab Carcinus maenas showed that crabs occurring in the
intertidal are not a representative part of the entire population. The intertidal is
especially important for early life stages which occur in mussel beds. Furthermore, medium-sized crabs migrate into the tidal tlats, while large crabs remain
in the subtidal. Shore crabs can occur in different colour forms which go along
with physiological and behavioural differences (phenotypic plasticity). Green
crabs were more abundant intertidally. The colour forms are discussed as life
stage strategies beneficial for living in a constantly changing environment like
the Wadden Sea.
• Experimentally disturbed sandtlat sites were recolonized within a few days or
weeks by diatoms and nematodes and within several months by smaller macrofauna. Recolonization was faster in spring than in autumn. Varying the intensity
of disturbance gave faster recolonization rates the smaller the disturbance. The
course of the recolonization depended mainly on the availability of larvae and
postlarvae. Juvenile polychaetes were more abundant on disturbed than control
sites. Among the first species to colonize the experimentally disturbed sites
were the polychaete Pvgospio elegans, ostracods and amphipods. They were
also frequent after the ice winter. The results gave indications for both resilience
in-situ and resilience by migration.
• The ice winter 1995/1996 had different effects on the organisms in the Wadden
Sea. Benthic diatoms were only slightly affected. Three cold-adapted species
appeared which had not been recorded in preceding years. The bloom of Coscinodiscus concillllUS in the East Frisian Wadden Sea and nearshore North Sea is
seen as the main course for the development of large anoxic sediment areas in
several backbarrier systems. Abundances of nematodes and many macrobenthic
species were reduced right after the ice winter, but recovered within a year. Re-
291
• The distribution patterns of the polychaetes Lanice conchilega, Heteromastus
filiformis and Scoloplos armiger were mainly determined by their initial settlement while those of the molluscs Macoma balthica, Ensis americanus, Cerastoderma edule and Mya arenaria followed from postlarval dispersion. However,
postlarval dispersion can also be decisive for polychaetes, e.g. for the recolonization after ice winters.
• The polychaete Lanice conchilega requires hard substrate for settlement. No
correlation was found between the larval density and number of settling larvae.
Results of a model indicated that the density of a developing Lanice-patch is
determined by the near-bottom current velocity.
• The occurrence of L. conchilega on the tidal tlats was subject to high spatial and
temporal variations. Abundances and species numbers of benthic organisms
were higher within natural Lanice-patches and artificial tube fields than in ambient sediments without L. conchilega. Juvenile mussels attached to the protruding tube structures. Among the infauna, predatory polychaetes and spionid
polychaetes were more abundant in the Lanice-patches. Small-scale associations
around the Lanice tubes were detected for nematode- and copepod species.
• The mobility of the benthic organisms in the Wadden Sea contributes to the
high variability of densities. A staining technique revealed a high turn-over. For
example, all stained Pygospio elegans were exchanged on a test field within 6
days, although the low numbers of worms caught in drift nets and sediment
traps had indicated a low capability for dispersal.
• A survey of the shore crab Carcinus maenas showed that crabs occurring in the
intertidal are not a representative part of the entire population. The intertidal is
especially important for early life stages which occur in mussel beds. Furthermore, medium-sized crabs migrate into the tidal tlats, while large crabs remain
in the subtidal. Shore crabs can occur in different colour forms which go along
with physiological and behavioural differences (phenotypic plasticity). Green
crabs were more abundant intertidally. The colour forms are discussed as life
stage strategies beneficial for living in a constantly changing environment like
the Wadden Sea.
• Experimentally disturbed sandtlat sites were recolonized within a few days or
weeks by diatoms and nematodes and within several months by smaller macrofauna. Recolonization was faster in spring than in autumn. Varying the intensity
of disturbance gave faster recolonization rates the smaller the disturbance. The
course of the recolonization depended mainly on the availability of larvae and
postlarvae. Juvenile polychaetes were more abundant on disturbed than control
sites. Among the first species to colonize the experimentally disturbed sites
were the polychaete Pvgospio elegans, ostracods and amphipods. They were
also frequent after the ice winter. The results gave indications for both resilience
in-situ and resilience by migration.
• The ice winter 1995/1996 had different effects on the organisms in the Wadden
Sea. Benthic diatoms were only slightly affected. Three cold-adapted species
appeared which had not been recorded in preceding years. The bloom of Coscinodiscus concillllUS in the East Frisian Wadden Sea and nearshore North Sea is
seen as the main course for the development of large anoxic sediment areas in
several backbarrier systems. Abundances of nematodes and many macrobenthic
species were reduced right after the ice winter, but recovered within a year. Re-