124
:; Spatial and Temporal Distribution Patterns
flats (Hild 1997). This indicated that the filtration and biodeposition of the mussels
does not change the inorganic composition of the sediments.
The use of biomarkers could show the source of the material ingested and deposited by the mussels as well as seasonal variations in the input of organic matter
to the sediment. However, estimating the origin of organic matter on the base of
biomarkers supplies only an indirect impression of the degradability and availability for macroorganisms (Dauwe 1999, see Sect. 5.2.3.2).
Bacteria and mussels can derive a significant part of their nutritional requirements by poorly degradable material such as humic substances, chitin and cellulose
(Kreeger et al. 1988; Kirchner 1995; Schmidt & Ionasdottir 1997). Kosfeld (1989)
analysed the composition of organic matter in mussel faeces as approx. 60 %
quickly degradable (within 6-8 weeks) and approx. 40 % of slowly degradable
material (over 20 weeks). In mussel beds, the degradation of organic matter is
accelerated, e.g. by the digestion of the mussel. The release rates of dissolved
nitrogen and phosphorus components from the sediments of the mussel beds into
the overlying water are enhanced (Nixon et al. 1976; Murphy & Kremer 1985;
Doering et al. 1987; Dame & Dankers 1988; Dame et al. 1989; Asmus et al. 1990;
Asmus & Asmus 1991: Smaal & Prins 1993). Particularly the release of nitrogen
in the form of ammonia from the mussel bed exceeds that of other benthic communities in the Wadden Sea, by far (Asmus 1994). The increased release of nutrients from the mussel beds promotes the growth of macroalgae which attach to the
mussel shells, while low N- and P-concentrations in the water column during
summer limit the growth of phytoplankton (Asmus & Asmus 1991).
The effect of the mussels on the transformation of organic matter could be followed by comparing the patterns of amino acids and pigments before and after the
ice winter (Sect. 5.2.1). Mussels used specific organic matter of high quality like
amino acids for the production of their biomass. In the mussel bed, a higher content of fresh organic matter was available in comparison to the sandflat. However,
this material was modified in its quality by the mussels through a depletion of
amino acids and an enrichment of degradation products of pigments (Behrends
1997). The composition of amino acids in marine organic matter changes systematically in the course of the degradation of organic matter (Dauwe 1999). This
author suggests that the special role of alkaline amino acids in the geochemistry of
sediments and as part of the diet for heterotrophic organisms should receive more
attention.
The input of POC-enriched biodeposits in the mussel bed sediments and the surrounding area increased the mineralization and composition of organic matter
relative to the reference site. Due to the increased contents of fine-grained material
and POC in the biodeposits and the associated intensified microbial mineralization,
the anoxic depth horizon in the mussel bed sediment was closer to the surface than
in sandtlat sediments.
Biogenic structures are important sites for intensified bacterial decomposition
(Forster 1991) and in turn the activity of the microorganisms influences the
physico-chemical gradients in the sediment (redox processes). At the RPD-Iayer, a
stimulation of benthic activities, both of microorganisms and of bacteria-feeding
meiofauna, was observed by Fenchel & Riedl (1970). In sediments with a high
supply of POC, such as mussel beds, a defined RPD-layer develops close to the
sediment surface. In contrast, the RPD-Iayer of sandy sediments with a low supply
:; Spatial and Temporal Distribution Patterns
flats (Hild 1997). This indicated that the filtration and biodeposition of the mussels
does not change the inorganic composition of the sediments.
The use of biomarkers could show the source of the material ingested and deposited by the mussels as well as seasonal variations in the input of organic matter
to the sediment. However, estimating the origin of organic matter on the base of
biomarkers supplies only an indirect impression of the degradability and availability for macroorganisms (Dauwe 1999, see Sect. 5.2.3.2).
Bacteria and mussels can derive a significant part of their nutritional requirements by poorly degradable material such as humic substances, chitin and cellulose
(Kreeger et al. 1988; Kirchner 1995; Schmidt & Ionasdottir 1997). Kosfeld (1989)
analysed the composition of organic matter in mussel faeces as approx. 60 %
quickly degradable (within 6-8 weeks) and approx. 40 % of slowly degradable
material (over 20 weeks). In mussel beds, the degradation of organic matter is
accelerated, e.g. by the digestion of the mussel. The release rates of dissolved
nitrogen and phosphorus components from the sediments of the mussel beds into
the overlying water are enhanced (Nixon et al. 1976; Murphy & Kremer 1985;
Doering et al. 1987; Dame & Dankers 1988; Dame et al. 1989; Asmus et al. 1990;
Asmus & Asmus 1991: Smaal & Prins 1993). Particularly the release of nitrogen
in the form of ammonia from the mussel bed exceeds that of other benthic communities in the Wadden Sea, by far (Asmus 1994). The increased release of nutrients from the mussel beds promotes the growth of macroalgae which attach to the
mussel shells, while low N- and P-concentrations in the water column during
summer limit the growth of phytoplankton (Asmus & Asmus 1991).
The effect of the mussels on the transformation of organic matter could be followed by comparing the patterns of amino acids and pigments before and after the
ice winter (Sect. 5.2.1). Mussels used specific organic matter of high quality like
amino acids for the production of their biomass. In the mussel bed, a higher content of fresh organic matter was available in comparison to the sandflat. However,
this material was modified in its quality by the mussels through a depletion of
amino acids and an enrichment of degradation products of pigments (Behrends
1997). The composition of amino acids in marine organic matter changes systematically in the course of the degradation of organic matter (Dauwe 1999). This
author suggests that the special role of alkaline amino acids in the geochemistry of
sediments and as part of the diet for heterotrophic organisms should receive more
attention.
The input of POC-enriched biodeposits in the mussel bed sediments and the surrounding area increased the mineralization and composition of organic matter
relative to the reference site. Due to the increased contents of fine-grained material
and POC in the biodeposits and the associated intensified microbial mineralization,
the anoxic depth horizon in the mussel bed sediment was closer to the surface than
in sandtlat sediments.
Biogenic structures are important sites for intensified bacterial decomposition
(Forster 1991) and in turn the activity of the microorganisms influences the
physico-chemical gradients in the sediment (redox processes). At the RPD-Iayer, a
stimulation of benthic activities, both of microorganisms and of bacteria-feeding
meiofauna, was observed by Fenchel & Riedl (1970). In sediments with a high
supply of POC, such as mussel beds, a defined RPD-layer develops close to the
sediment surface. In contrast, the RPD-Iayer of sandy sediments with a low supply