156
5 Spatial and Telllporal D"tnhulion Patterns
pub!.). Individual densities of macrobenthos varied in the three study years between 300-53500 individuals per m 2 and the lowest values were also recorded
immediately after the ice winter. The highest values occurred in summer 1996,
due to massive recruitments of several polychaete and bivalve species (Chaps. 5.3
and 7). Macrobenthos abundances varied not only between years and seasons, but
also daily. Especially abundant species with patchy distribution patterns (e.g. the
amphipod Urothoe poseidonis and the polychaete Capitella capitata) varied in
numbers between days. Yet, in most cases the variance was higher between subsamples taken on the same day than between samples from consecutive days. The
high variability of benthos abundances was not related to the occurrence of organisms of the respective species in the driftfauna.
A classification of trophic groups showed that their shares varied over the years
in relation to variations in the abundances of single species and their respective
recruitment (Chap. 5.3).
5.5.4
Lanice conchilega: Density and Distribution
Lanice occurred at different population densities at the study sites of the Groninger
Plate and Dornumer Nacken and also showed small-scale density variations at
each of the sites (Table 5.5.2). The density of Lanice also varied between years,
with a decline both inter- and subtidally (Reichert pers. comm.) after the ice winter of 1995/96.
Lanice had a small-scale distribution pattern with patches of 1-2 m in size, alternating with ambient tidal flat areas, where it was absent or occurred with lower
densities. Multicorer analyses revealed that an increase in density was combined
with a higher small-scale aggregation. In the studied backbarrier system, Lanice
occurred mainly in tidal flats of intermediate or higher elevation. Lanicedominated tidal flats often had a small-scale topography with low mounds around
the Lanice-patches and slight depressions in between (van Bernem et al. unpubl.).
Size classes of Lanice were assessed by measuring the diameter of the thoracic
segments (Reichert pers. comm.). Adult Lanice had an average thorax diameter of
0.3 cm. At the end of the year 1995, over 50 % of the Lanice-population at the
Dornumer Nacken were adults with a thorax diameter of 0.3-0.45 cm. After the
ice winter 1995/96, no living Lanice were recorded in the study area until June
1996. In July, juvenile Lanice with a thorax diameter of 0.05-0.2 cm appeared
again on the tidal tlats, but only a few individuals had grown to the size of 0.3 cm
by the end of the year.
The tubes of Lanice are a biogenic structure reaching about 20 cm into the
sediment and protruding 2-3 cm above the sediment. Dense aggregations of polychaete tubes can affect the currents at the benthic boundary layer (Friedrichs 1996)
and thus alter small-scale habitat conditions. Thus, the topography of mounds and
depressions described above develops. After the ice winter 1995/96 and the
recolonization by Lanice in the following summer, this topography was not pronounced, as the juvenile worms were obviously too small to exert the same
5 Spatial and Telllporal D"tnhulion Patterns
pub!.). Individual densities of macrobenthos varied in the three study years between 300-53500 individuals per m 2 and the lowest values were also recorded
immediately after the ice winter. The highest values occurred in summer 1996,
due to massive recruitments of several polychaete and bivalve species (Chaps. 5.3
and 7). Macrobenthos abundances varied not only between years and seasons, but
also daily. Especially abundant species with patchy distribution patterns (e.g. the
amphipod Urothoe poseidonis and the polychaete Capitella capitata) varied in
numbers between days. Yet, in most cases the variance was higher between subsamples taken on the same day than between samples from consecutive days. The
high variability of benthos abundances was not related to the occurrence of organisms of the respective species in the driftfauna.
A classification of trophic groups showed that their shares varied over the years
in relation to variations in the abundances of single species and their respective
recruitment (Chap. 5.3).
5.5.4
Lanice conchilega: Density and Distribution
Lanice occurred at different population densities at the study sites of the Groninger
Plate and Dornumer Nacken and also showed small-scale density variations at
each of the sites (Table 5.5.2). The density of Lanice also varied between years,
with a decline both inter- and subtidally (Reichert pers. comm.) after the ice winter of 1995/96.
Lanice had a small-scale distribution pattern with patches of 1-2 m in size, alternating with ambient tidal flat areas, where it was absent or occurred with lower
densities. Multicorer analyses revealed that an increase in density was combined
with a higher small-scale aggregation. In the studied backbarrier system, Lanice
occurred mainly in tidal flats of intermediate or higher elevation. Lanicedominated tidal flats often had a small-scale topography with low mounds around
the Lanice-patches and slight depressions in between (van Bernem et al. unpubl.).
Size classes of Lanice were assessed by measuring the diameter of the thoracic
segments (Reichert pers. comm.). Adult Lanice had an average thorax diameter of
0.3 cm. At the end of the year 1995, over 50 % of the Lanice-population at the
Dornumer Nacken were adults with a thorax diameter of 0.3-0.45 cm. After the
ice winter 1995/96, no living Lanice were recorded in the study area until June
1996. In July, juvenile Lanice with a thorax diameter of 0.05-0.2 cm appeared
again on the tidal tlats, but only a few individuals had grown to the size of 0.3 cm
by the end of the year.
The tubes of Lanice are a biogenic structure reaching about 20 cm into the
sediment and protruding 2-3 cm above the sediment. Dense aggregations of polychaete tubes can affect the currents at the benthic boundary layer (Friedrichs 1996)
and thus alter small-scale habitat conditions. Thus, the topography of mounds and
depressions described above develops. After the ice winter 1995/96 and the
recolonization by Lanice in the following summer, this topography was not pronounced, as the juvenile worms were obviously too small to exert the same