144
5 Spallal and Temporal Dlslrihulion Palterns
occurs facilitating settlement. In case the current velocity is lower than this value
no turbulence will develop and the probability of larval settlement is reduced.
At which current speed turbulence develops depends on the density of tubes,
their diameter and the surface roughness in combination with currents. In the natural habitat as well as under experimental conditions higher abundances of tubes
had positive effects on settlement of larvae. Most likely the probability for smallscale areas with low currents rises with increasing abundances of tubes (Carey
1983, 1987). The intensity with which dense tube lawns may affect the small scale
hydrodynamics was demonstrated in experiments with densely packed artificial
tubes leading to a sediment rise of several cm above the surrounding sediment
surface (Zuhlke et al. 1998). A similar accumulation of sediment was observed in
dense patches of Polydora sp. (Daro & Polk 1973).
From the investigations carried out at the Groninger Plate it appeared that recruitment including initial settlement into habitats with an adult population and
subsequent detachment of juvenile worms from the adult tube to build their own
solitaire tube maintains the spatial distribution of this species, provided the juveniles do not disperse far from their place of initial settlement. The low numbers of
juvenile L. como'hi/ega in the driftfauna nets as well as in the epibenthos net suggest water column transport to be limited, though bedload transport cannot be
excluded.
In the summer 1996 after the ice winter patches of 20 to 70 L. conchilega developed on the Gr(ininger Plate without hard substrate locally available. The spatial
distribution of the aggregates was similar to the zonation patterns of this species in
the previous years. But how they developed could not be completely explained.
The temporal gap between larval supply and the appearance of juveniles in the
benthos presumably indicates that recolonization of the Groninger Plate was
achieved by secondary dispersal of juveniles. Their initial settlement may have
taken place on shells of bivalves, filamentous algae or in dense Pygospio lawns,
where they have been observed to settle in 1996.
In 1996 recruitment of L. conchilega must have originated from regions relatively far from the Spiekeroog backbarrier system. In the ice winter 1995/96 not
only the intertidal part of the population disappeared completely but also subtidal
ones down to 10m water depth in the tidal inlets and seaward off the East Frisian
islands (Reichert pers. comm.).
Strong differences in growth of juvenile L. conchilega at the different sites on
the Gr(minger Plate in 1996 point to the importance of local differences in food
supply, which may affect the large-scale spatial distribution of this species on the
long run. High growth rates of juveniles in the central part of the Groninger Plate
imply a good food availability at this site of a former adult habitat. Food supply
will be the main advantage for the aulophora larvae to settle on adults tubes, accordingly competition between adults and newly metamorphosed juveniles is supposed to be low. This is a contradictory life history pattern to that of Arenicola
rnarilla. Habitat changes of juvenile lugworms are interpreted as avoidance of
intraspecific competition between adults and juveniles (Beukema & De VIas 1979;
Reise 1985).
5 Spallal and Temporal Dlslrihulion Palterns
occurs facilitating settlement. In case the current velocity is lower than this value
no turbulence will develop and the probability of larval settlement is reduced.
At which current speed turbulence develops depends on the density of tubes,
their diameter and the surface roughness in combination with currents. In the natural habitat as well as under experimental conditions higher abundances of tubes
had positive effects on settlement of larvae. Most likely the probability for smallscale areas with low currents rises with increasing abundances of tubes (Carey
1983, 1987). The intensity with which dense tube lawns may affect the small scale
hydrodynamics was demonstrated in experiments with densely packed artificial
tubes leading to a sediment rise of several cm above the surrounding sediment
surface (Zuhlke et al. 1998). A similar accumulation of sediment was observed in
dense patches of Polydora sp. (Daro & Polk 1973).
From the investigations carried out at the Groninger Plate it appeared that recruitment including initial settlement into habitats with an adult population and
subsequent detachment of juvenile worms from the adult tube to build their own
solitaire tube maintains the spatial distribution of this species, provided the juveniles do not disperse far from their place of initial settlement. The low numbers of
juvenile L. como'hi/ega in the driftfauna nets as well as in the epibenthos net suggest water column transport to be limited, though bedload transport cannot be
excluded.
In the summer 1996 after the ice winter patches of 20 to 70 L. conchilega developed on the Gr(ininger Plate without hard substrate locally available. The spatial
distribution of the aggregates was similar to the zonation patterns of this species in
the previous years. But how they developed could not be completely explained.
The temporal gap between larval supply and the appearance of juveniles in the
benthos presumably indicates that recolonization of the Groninger Plate was
achieved by secondary dispersal of juveniles. Their initial settlement may have
taken place on shells of bivalves, filamentous algae or in dense Pygospio lawns,
where they have been observed to settle in 1996.
In 1996 recruitment of L. conchilega must have originated from regions relatively far from the Spiekeroog backbarrier system. In the ice winter 1995/96 not
only the intertidal part of the population disappeared completely but also subtidal
ones down to 10m water depth in the tidal inlets and seaward off the East Frisian
islands (Reichert pers. comm.).
Strong differences in growth of juvenile L. conchilega at the different sites on
the Gr(minger Plate in 1996 point to the importance of local differences in food
supply, which may affect the large-scale spatial distribution of this species on the
long run. High growth rates of juveniles in the central part of the Groninger Plate
imply a good food availability at this site of a former adult habitat. Food supply
will be the main advantage for the aulophora larvae to settle on adults tubes, accordingly competition between adults and newly metamorphosed juveniles is supposed to be low. This is a contradictory life history pattern to that of Arenicola
rnarilla. Habitat changes of juvenile lugworms are interpreted as avoidance of
intraspecific competition between adults and juveniles (Beukema & De VIas 1979;
Reise 1985).