5.2 Biogeochemical Processes and Interactions
127
characterization of the composition of this material is therefore necessary in the
future.
5.2.3.3
Spatial and Temporal Variations of the Interactions
The input of POC-rich, fine-grained material into sandy sediments has substantial
influences on the geochemical and biological processes (turnover rates of organic
matter degradation, early diagenesis, element mobilization and fixation by microbial activity; Lochte 1993). These can be subject to seasonal variations and irregular special events (dislodgement of the mussels, new settlement or spatfall of larvae, ice winter, macroalgal development, mud deposition and phytoplankton
blooms). For example, the proportion of biodeposits in the sediments was controlled by the filtration activity of the mussels, which was highest during phytoplankton-blooms from late spring to summer (Kautsky & Evans 1987). A further
factor was the activity of microorganisms, which was linked to the supply of organic matter and the ambient temperature.
The parameters examined along the transect were characterized by high variations. The results of the MDS-analyses of the macrofauna investigations agreed
with the data on biomarkers (Sect. 5.2.1). The reference site in the sandtlat (SP6)
was clearly distinguished from the other sites on the transect influenced by biodeposits. In the analysis of the biomarkers, the biodeposit-enriched sites resembled
each other closely. Microbial tests on the functional diversity showed a high seasonal variation in the sediments outside of the mussel bed, whereas the bacterial
populations in the mussel bed sediment had more site-specific properties.
The RPD-Iayer in coastal marine sediments shows seasonal dynamics (J0rgensen 1980), being millimetres to centimetres thick in winter, and in summer at times
of increased POC deposition and elevated temperatures less than a few millimetres
thin. Differences in the oxygen supply and sediment profiles between the transect
sites were more clearly pronounced in the second half of the year, probably due to
the increasing biogenic intluences. The investigations on the Swinnplate showed
that nutrient supply (e.g. in the form of phytoplankton blooms) leads to a direct
stimulation of benthic activities and the signals of an input of organic matter were
reflected in the geochemical parameters.
However, seasonal variations as well as small-scale fluctuations and deviations
overshadowed the ideal pattern of an evenly decreasing gradient of POC- and
mud-concentrations. For example, these deviations were caused by macroalgal
growth particularly on the sites with mussels. In August 1995, large-scale deposition of mud occurred in the entire study area and was responsible for the similarity
of oxygen conditions at all sites of the transect. In July, August and September
1995, mats of the macroalgae Enteromorpha spp. developed, which likewise exerted a more or less pronounced influence on the oxygen regime of the different
transect sites, because they changed the benthic-pelagic exchange. Depending on
the location where the algae occurred or where they were buried, they obstructed
or blocked the oxygen diffusion across the sediment surface as well as the filtration
activity of the mussels. Furthermore, the current velocity was reduced by the
patchy macroalgal vegetation, which led to an additional physically-controlled
sedimentation of POC-rich fine-grained material. When the macroalgae died, the
127
characterization of the composition of this material is therefore necessary in the
future.
5.2.3.3
Spatial and Temporal Variations of the Interactions
The input of POC-rich, fine-grained material into sandy sediments has substantial
influences on the geochemical and biological processes (turnover rates of organic
matter degradation, early diagenesis, element mobilization and fixation by microbial activity; Lochte 1993). These can be subject to seasonal variations and irregular special events (dislodgement of the mussels, new settlement or spatfall of larvae, ice winter, macroalgal development, mud deposition and phytoplankton
blooms). For example, the proportion of biodeposits in the sediments was controlled by the filtration activity of the mussels, which was highest during phytoplankton-blooms from late spring to summer (Kautsky & Evans 1987). A further
factor was the activity of microorganisms, which was linked to the supply of organic matter and the ambient temperature.
The parameters examined along the transect were characterized by high variations. The results of the MDS-analyses of the macrofauna investigations agreed
with the data on biomarkers (Sect. 5.2.1). The reference site in the sandtlat (SP6)
was clearly distinguished from the other sites on the transect influenced by biodeposits. In the analysis of the biomarkers, the biodeposit-enriched sites resembled
each other closely. Microbial tests on the functional diversity showed a high seasonal variation in the sediments outside of the mussel bed, whereas the bacterial
populations in the mussel bed sediment had more site-specific properties.
The RPD-Iayer in coastal marine sediments shows seasonal dynamics (J0rgensen 1980), being millimetres to centimetres thick in winter, and in summer at times
of increased POC deposition and elevated temperatures less than a few millimetres
thin. Differences in the oxygen supply and sediment profiles between the transect
sites were more clearly pronounced in the second half of the year, probably due to
the increasing biogenic intluences. The investigations on the Swinnplate showed
that nutrient supply (e.g. in the form of phytoplankton blooms) leads to a direct
stimulation of benthic activities and the signals of an input of organic matter were
reflected in the geochemical parameters.
However, seasonal variations as well as small-scale fluctuations and deviations
overshadowed the ideal pattern of an evenly decreasing gradient of POC- and
mud-concentrations. For example, these deviations were caused by macroalgal
growth particularly on the sites with mussels. In August 1995, large-scale deposition of mud occurred in the entire study area and was responsible for the similarity
of oxygen conditions at all sites of the transect. In July, August and September
1995, mats of the macroalgae Enteromorpha spp. developed, which likewise exerted a more or less pronounced influence on the oxygen regime of the different
transect sites, because they changed the benthic-pelagic exchange. Depending on
the location where the algae occurred or where they were buried, they obstructed
or blocked the oxygen diffusion across the sediment surface as well as the filtration
activity of the mussels. Furthermore, the current velocity was reduced by the
patchy macroalgal vegetation, which led to an additional physically-controlled
sedimentation of POC-rich fine-grained material. When the macroalgae died, the