122
5 Spallal and Temporal DistrIhutlon Pallerns
During the study period, the benthic assemblages along the transect changed as
a result of several events like dislodgement of the mussel beds, variations in mussel densities as well as a new settlement of M. edulis at site SP5 in 1994 (Table 5.2.1). During the same year, a strong spatfall of larvae of Macoma balthica
occurred also. According to the results of the geochemical investigations, site SP5
was characterized by organic matter of high quality. During 1994 and 1995, biomass, average species number and abundances of the macrofauna were clearly
higher at this site than at the other sites of the transect. However, the new settlement of M. edulis, which caused changes in the macrofauna, did not affect other
parameters such as mud content, bacterial cell numbers or POC-concentrations in
the sediment (Sect. 5.2.1). After the ice winter, the mussel beds had disappeared
and no mussels were present until the summer 1996, when a new settlement of
Mya arenaria, Cerastoderma edulis, M. halthica and M. edulis took place in the
transect area.
Spatial and temporal variations of median species numbers, abundances and
biomass of the macrofauna were high (Table 5.2.4). Median values for species
numbers ranged from 2-11 species 20 cm' and varied at each site with season and
between years. In 1994/1995, the median species numbers and abundances of the
macrofauna were highest in spring (May). After the ice winter 1995/96, the highest
species numbers and abundances were recorded later in the year (July) than in
previous years. In 1994 and 1996, the highest values of macrofaunal biomass were
determined in September, whereas in 1995, the maximum was detected in May.
Strong temporal tluctuations were also found for single species and taxonomic
groups of the macrobenthos at each site. The mudsnail Hydrobia ulvae dominated
at site SP6 in 1994. The dominating polychaetes and oligochaetes occurred with
different frequencies at the sampling dates. However, no clear spatial or seasonal
patterns were observed. After the ice winter, Pygospio elegans and Aphelochaeta
marioni dominated the benthic assemblages in spring 1996 until summer.
A classification of the polychaetes into trophic groups showed a dominance of
deposit-feeding species on almost all transect sites. Only in 1994 and 1995, the
proportion of surface deposit-feeding polychaetes was smaller in the mussel bed
sites SP I +2 than at the other sites. During this time, subsurface deposit-feeders
were more abundant in the mussel bed. The changing frequencies of the taxonomic
and trophic groups were also retlected in a heterogeneous frequency distribution of
the biomass at the sites in the course of the respective years (Table 5.2.4). Only in
the spring of 1996, deposit-feeding polychaetes accounted for most of the benthic
biomass at all sites.
The high temporal fluctuations of the abundances at the transect sites became
also apparent in multivariate analyses of the assemblages. Multi-dimensional
scaling (MDS) showed a clear gradient of benthic assemblages along the transect
from the mussel bed to the sandtlat in a few months only. Besides seasonal intluences and the new settlement of mussels close of the reference site in the sandl1at,
the ice winter 1995/1996 affected the benthic assemblages also. In general, seasonal similarities of the transect sites were more pronounced than spatial similarities of the individual sites during the study period. MDS-analyses of the biomass
data showed that the benthic assemblages of the transect sites were more similar to
each other at most sampling dates. Seasonal similarities of all stations existed in
May and September 1996.
5 Spallal and Temporal DistrIhutlon Pallerns
During the study period, the benthic assemblages along the transect changed as
a result of several events like dislodgement of the mussel beds, variations in mussel densities as well as a new settlement of M. edulis at site SP5 in 1994 (Table 5.2.1). During the same year, a strong spatfall of larvae of Macoma balthica
occurred also. According to the results of the geochemical investigations, site SP5
was characterized by organic matter of high quality. During 1994 and 1995, biomass, average species number and abundances of the macrofauna were clearly
higher at this site than at the other sites of the transect. However, the new settlement of M. edulis, which caused changes in the macrofauna, did not affect other
parameters such as mud content, bacterial cell numbers or POC-concentrations in
the sediment (Sect. 5.2.1). After the ice winter, the mussel beds had disappeared
and no mussels were present until the summer 1996, when a new settlement of
Mya arenaria, Cerastoderma edulis, M. halthica and M. edulis took place in the
transect area.
Spatial and temporal variations of median species numbers, abundances and
biomass of the macrofauna were high (Table 5.2.4). Median values for species
numbers ranged from 2-11 species 20 cm' and varied at each site with season and
between years. In 1994/1995, the median species numbers and abundances of the
macrofauna were highest in spring (May). After the ice winter 1995/96, the highest
species numbers and abundances were recorded later in the year (July) than in
previous years. In 1994 and 1996, the highest values of macrofaunal biomass were
determined in September, whereas in 1995, the maximum was detected in May.
Strong temporal tluctuations were also found for single species and taxonomic
groups of the macrobenthos at each site. The mudsnail Hydrobia ulvae dominated
at site SP6 in 1994. The dominating polychaetes and oligochaetes occurred with
different frequencies at the sampling dates. However, no clear spatial or seasonal
patterns were observed. After the ice winter, Pygospio elegans and Aphelochaeta
marioni dominated the benthic assemblages in spring 1996 until summer.
A classification of the polychaetes into trophic groups showed a dominance of
deposit-feeding species on almost all transect sites. Only in 1994 and 1995, the
proportion of surface deposit-feeding polychaetes was smaller in the mussel bed
sites SP I +2 than at the other sites. During this time, subsurface deposit-feeders
were more abundant in the mussel bed. The changing frequencies of the taxonomic
and trophic groups were also retlected in a heterogeneous frequency distribution of
the biomass at the sites in the course of the respective years (Table 5.2.4). Only in
the spring of 1996, deposit-feeding polychaetes accounted for most of the benthic
biomass at all sites.
The high temporal fluctuations of the abundances at the transect sites became
also apparent in multivariate analyses of the assemblages. Multi-dimensional
scaling (MDS) showed a clear gradient of benthic assemblages along the transect
from the mussel bed to the sandtlat in a few months only. Besides seasonal intluences and the new settlement of mussels close of the reference site in the sandl1at,
the ice winter 1995/1996 affected the benthic assemblages also. In general, seasonal similarities of the transect sites were more pronounced than spatial similarities of the individual sites during the study period. MDS-analyses of the biomass
data showed that the benthic assemblages of the transect sites were more similar to
each other at most sampling dates. Seasonal similarities of all stations existed in
May and September 1996.