142
:' ) Spallal and Temporal Disirihuiloll Patterns
southern tidal channel exceeded 10 days. Though the Groninger Plate is more
exposed compared to the Oddewatt (a sheltered tidal flat at the island of Sylt) the
turnover rate of this species was lower as estimated by Armonies (\ 994) for the
latter area. The reason for this observed difference can be manifold: Armonies
obtained his value probably from smaller individuals, and the methods applied
were different. Repeated experiments at the Groninger Plate in late summer 1995,
when the mean individual size of the bivalves was higher, suggest the turnover rate
to be size dependent. Thus, turnover of an entire O-group of one species is not
continuous over time; e.g. 50 % of the M. balthica postlarvae had disappeared
from the experimental plot within 2 days, but even after 10 days stained specimen
could still be observed.
5.3.5
Small-Scale Temporal Variability in Benthic Distribution Patterns:
Patchiness vs. Mobility
Considering the high mobility of juvenile benthic macrofauna the question arose
whether adults disperse also due to active behaviour or passive resuspension. For
this purpose, local hydrodynamics, short term fluctuations in macrofaunal abundances and water column dispersal of adults were investigated simultaneously to
test for external factors driving spatial variability and variations in abundance of
macrofauna (Niemeyer, Michaelis, Steuwer, Kaiser, FleSner unpublished data). In
spring and autumn of 1994 and 1995 two campaigns of 14 days duration each were
carried out in which the above mentioned parameters were measured on a daily
basis. In addition import experiments were carried out to determine the local colonization by adult macrofauna.
The results of the benthos sampling yielded high fluctuations in abundance between days, but in most cases without significant changes (Steuwer unpublished
data). Specifically high variability was observed for Urothoe poseidonis (median
between 50 and 1500 ind m
2 ) and Capitella capitata (median between 50 and 1200
ind m'\ and a low daily variability for Arenicola marina. Applying the time series
analysing method developed by Ibanez & Dauvin (1988), the analysis of the 1994
spring data gave no indication for cyclic patterns in the changes of abundance of
selected macrofauna species related to the spring tide rhythm.
For some species (e.g. Phyllodoce macu/ata, P. elegans, S. armiger) spring and
autumn abundances were significantly different depending on the yearly recruitment success.
The general large deviation from the mean abundance indicates that in spite of
the high number of samples (9-16 parallel samples, each of 177 cm 2 area) some of
the species were not sufficiently sampled and temporal signals due to discrete
events were superimposed by the "noise" of spatial inhomogeneity. Subsequently,
for example, predators feeding on these organisms face a high variability of their
prey items in time (scale of tides) and space (scale of km2). This could affect the
food web, making estimates of energy flux which do not address the proper scales
erroneous.
During the sampling periods no extreme event such as a storm occurred, so that
the influence of hydrodynamics on variations in macrofauna abundance could not
be analysed. Only twice a deviation from normal hydrodynamic conditions was
:' ) Spallal and Temporal Disirihuiloll Patterns
southern tidal channel exceeded 10 days. Though the Groninger Plate is more
exposed compared to the Oddewatt (a sheltered tidal flat at the island of Sylt) the
turnover rate of this species was lower as estimated by Armonies (\ 994) for the
latter area. The reason for this observed difference can be manifold: Armonies
obtained his value probably from smaller individuals, and the methods applied
were different. Repeated experiments at the Groninger Plate in late summer 1995,
when the mean individual size of the bivalves was higher, suggest the turnover rate
to be size dependent. Thus, turnover of an entire O-group of one species is not
continuous over time; e.g. 50 % of the M. balthica postlarvae had disappeared
from the experimental plot within 2 days, but even after 10 days stained specimen
could still be observed.
5.3.5
Small-Scale Temporal Variability in Benthic Distribution Patterns:
Patchiness vs. Mobility
Considering the high mobility of juvenile benthic macrofauna the question arose
whether adults disperse also due to active behaviour or passive resuspension. For
this purpose, local hydrodynamics, short term fluctuations in macrofaunal abundances and water column dispersal of adults were investigated simultaneously to
test for external factors driving spatial variability and variations in abundance of
macrofauna (Niemeyer, Michaelis, Steuwer, Kaiser, FleSner unpublished data). In
spring and autumn of 1994 and 1995 two campaigns of 14 days duration each were
carried out in which the above mentioned parameters were measured on a daily
basis. In addition import experiments were carried out to determine the local colonization by adult macrofauna.
The results of the benthos sampling yielded high fluctuations in abundance between days, but in most cases without significant changes (Steuwer unpublished
data). Specifically high variability was observed for Urothoe poseidonis (median
between 50 and 1500 ind m
2 ) and Capitella capitata (median between 50 and 1200
ind m'\ and a low daily variability for Arenicola marina. Applying the time series
analysing method developed by Ibanez & Dauvin (1988), the analysis of the 1994
spring data gave no indication for cyclic patterns in the changes of abundance of
selected macrofauna species related to the spring tide rhythm.
For some species (e.g. Phyllodoce macu/ata, P. elegans, S. armiger) spring and
autumn abundances were significantly different depending on the yearly recruitment success.
The general large deviation from the mean abundance indicates that in spite of
the high number of samples (9-16 parallel samples, each of 177 cm 2 area) some of
the species were not sufficiently sampled and temporal signals due to discrete
events were superimposed by the "noise" of spatial inhomogeneity. Subsequently,
for example, predators feeding on these organisms face a high variability of their
prey items in time (scale of tides) and space (scale of km2). This could affect the
food web, making estimates of energy flux which do not address the proper scales
erroneous.
During the sampling periods no extreme event such as a storm occurred, so that
the influence of hydrodynamics on variations in macrofauna abundance could not
be analysed. Only twice a deviation from normal hydrodynamic conditions was