136
5 Spatial and Temporal DistributIon Patterns
Thus, investigations in ELA W AT focused on the polychaete species Lanice
conchilega, Scoloplos armiger and Heteromastus filiformis, with S. armiger as a
representative for polychaetes with an exclusively benthic mode of reproduction.
Furthermore, each of these species is of major importance or a representative for
one of the different macrofauna associations of the Groninger Plate: L. conchilega
for the so-called Lanice association, S. armiger is common in Arenicola marina
sandflats and H. filiformis dominates in muddy sands. For the Wadden Sea, no
information was available on the long-term population trend of L. conchilega, but
increasing abundances were recorded for S. armiger and H. filiformis, with the
latter species reaching untypically high abundances in sandy sediments (Obert
1982; Beukema 1989).
In the course of ELA W AT, investigations were carried out on the Groninger
Plate in the backbarrier system of Spiekeroog island. In the years 1994, 1995 and
1996 sampling was carried out from spring to autumn with different objectives
(Table 5.3.1).
Sampling was carried out with a corer of 33 cm 2 area (penetration depth 20 cm,
5-7 replicates) in time intervals of 1-2 weeks during the main sampling phase.
Samples were fixed intact with 5 % buffered formalin. In the laboratory the samples were sieved using a sieve column of 500, 250 and 125 flm mesh size. Each of
the fractions was analysed separately. If necessary, specimens of the 500 flm fraction were separated into age groups.
The method of drift fauna sampling was developed by Armonies (1992). First
results from the Groninger Plate were published by Jaklin & Gunther (1996).
5.3.3.1
Spatial Variability of Initial Settlement of Selected Species
The tube building polychaete L. conchilega was found to be a hard substrate settler, selecting preferentially tubes of conspecies adults (Heuers, 1998). Single
juveniles were found to settle on eroded shells of cockles (Cerastoderma edule)
and soft-shell clams (Mya arenaria). Beside the blue mussel (Mytilus edulis)
L. conchilega is one of the rare hard substrate settlers of importance in the soft
bottom environment of the Wadden Sea. In contrast to Bayne (1964) who described that M. edulis settles exclusively attached to hard substrate, early postlarval
stages of this species were observed in the soft sediments on the southern edge of
the study area (GP6). But early L. conchilega were never observed in the sediment.
A metamorphosis of the aulophora larva without available hard substrate was only
described to occur under experimental conditions and was apparently due to low
current velocities (Bhaud & Cazaux 1990). More detailed results on the population
dynamics of L. cOl7chilega will be presented in Chap. 5.3.6.
M. balthica, Ensis americanus, C. edule, M. arenaria and H. filiformis settle on
soft substrates (Jaklin, Heuers unpublished data; Heuers 1998). In 1995 these species settled mainly in the area of the tidal channel at the southern border of the
Gr(ininger Plate (GP6, Fig. 5.3.1). As bivalve larvae are known to settle passively
(Table 5.3.2; Gunther 1990; Armonies 1998), this pattern indicates that the hydrodynamics on this site facilitated settlement. Interannual variations in spatial
5 Spatial and Temporal DistributIon Patterns
Thus, investigations in ELA W AT focused on the polychaete species Lanice
conchilega, Scoloplos armiger and Heteromastus filiformis, with S. armiger as a
representative for polychaetes with an exclusively benthic mode of reproduction.
Furthermore, each of these species is of major importance or a representative for
one of the different macrofauna associations of the Groninger Plate: L. conchilega
for the so-called Lanice association, S. armiger is common in Arenicola marina
sandflats and H. filiformis dominates in muddy sands. For the Wadden Sea, no
information was available on the long-term population trend of L. conchilega, but
increasing abundances were recorded for S. armiger and H. filiformis, with the
latter species reaching untypically high abundances in sandy sediments (Obert
1982; Beukema 1989).
In the course of ELA W AT, investigations were carried out on the Groninger
Plate in the backbarrier system of Spiekeroog island. In the years 1994, 1995 and
1996 sampling was carried out from spring to autumn with different objectives
(Table 5.3.1).
Sampling was carried out with a corer of 33 cm 2 area (penetration depth 20 cm,
5-7 replicates) in time intervals of 1-2 weeks during the main sampling phase.
Samples were fixed intact with 5 % buffered formalin. In the laboratory the samples were sieved using a sieve column of 500, 250 and 125 flm mesh size. Each of
the fractions was analysed separately. If necessary, specimens of the 500 flm fraction were separated into age groups.
The method of drift fauna sampling was developed by Armonies (1992). First
results from the Groninger Plate were published by Jaklin & Gunther (1996).
5.3.3.1
Spatial Variability of Initial Settlement of Selected Species
The tube building polychaete L. conchilega was found to be a hard substrate settler, selecting preferentially tubes of conspecies adults (Heuers, 1998). Single
juveniles were found to settle on eroded shells of cockles (Cerastoderma edule)
and soft-shell clams (Mya arenaria). Beside the blue mussel (Mytilus edulis)
L. conchilega is one of the rare hard substrate settlers of importance in the soft
bottom environment of the Wadden Sea. In contrast to Bayne (1964) who described that M. edulis settles exclusively attached to hard substrate, early postlarval
stages of this species were observed in the soft sediments on the southern edge of
the study area (GP6). But early L. conchilega were never observed in the sediment.
A metamorphosis of the aulophora larva without available hard substrate was only
described to occur under experimental conditions and was apparently due to low
current velocities (Bhaud & Cazaux 1990). More detailed results on the population
dynamics of L. cOl7chilega will be presented in Chap. 5.3.6.
M. balthica, Ensis americanus, C. edule, M. arenaria and H. filiformis settle on
soft substrates (Jaklin, Heuers unpublished data; Heuers 1998). In 1995 these species settled mainly in the area of the tidal channel at the southern border of the
Gr(ininger Plate (GP6, Fig. 5.3.1). As bivalve larvae are known to settle passively
(Table 5.3.2; Gunther 1990; Armonies 1998), this pattern indicates that the hydrodynamics on this site facilitated settlement. Interannual variations in spatial