5.2 Biogeochemical Processes and InteractIons
95
5.2
Biogeochemical Processes in Tidal Flat Sediments and
Mutual Interactions with Macrobenthos
M. Villbrandt, A. Hild & S. Dittmann
Abstract: In this chapter results from investigations on biogeochemical processes
and their interactions with macrofauna and bacteria in two tidal flats of the East
Frisian Wadden Sea are presented. On the Swinnplate, the intluence of a mussel
bed (Mytilus edulis) on the surrounding sandflat, the inhabiting fauna as well as
interactions between macrozoobenthos, microorganisms and geochemical parameters were examined. Samples were taken along a transect from the centre of a
mussel bed towards the sandtlat. Part of the biodeposits produced by the mussels
was transported by the ebb current into the surrounding sediments. Thus, not only
the grain size composition (mud content of the sediment) and the PaC-content
(particulate organic carbon) of the sediments changed along the transect, but also
other physico-chemical sediment characteristics. These changes in the sediment in
turn affected the composition of benthic assemblages.
The proportion of fine-grained material in the biodeposit-containing sediments
was subject to seasonal tluctuations. During winter, the mud content decreased due
to intensified erosion by increased wind and wave activity. In summer, the deposition of fine-grained biogenic sediments rich in pac increased due to higher biological activity and calm weather conditions. The chemical composition of the
sediments was predominantly coupled to the particle size distribution and not directly to the activity of the mussels. During winter, the content of Mn, P and Cu in
the suspended matter was coupled to the distribution of the clay minerals (Alcontent). In contrast to that a strong tluctuation occurred in summer, which was
attributed to early diagenetic processes due to microbial activity.
The input and decomposition of organic matter derived from plant material in
the surface-sediments of the mussel bed could be documented and followed using
specific organic biomarkers (fatty acids, sterols). Thus, it was possible to differentiate between signals of phytoplankton and macroalgal blooms or bacteria in the
sediment of the mussel bed and to allocate these to specific events.
During the investigations on the intluence of organic matter availability on biotic activities it turned out that not only the amount, but also its quality was important. Organic constituents of high quality such as photosynthetic pigments,
hydrolYl.able amino acids and fatty acids decreased with increasing distance from
the mussel bed towards the sandtlat. The composition of pigments and their degradation products (mainly phaeophorbides) in the biodeposits were related to seasonal inputs of diatoms and/or macroalgae. Some amino acids were enriched in the
sediment underneath the mussel bed, but the relative contribution of amino acidderived carbon to total pac was reduced, indicating a selective depletion of amino
acids by the mussels. Investigations in a mesocosm confirmed that M. edulis selectively eliminated amino acids and pigments from the water column and converted them into biomass.
The spatial and temporal distribution of microbial biomass and activity was
closely coupled to the availability of fresh, labile organic matter and followed the
95
5.2
Biogeochemical Processes in Tidal Flat Sediments and
Mutual Interactions with Macrobenthos
M. Villbrandt, A. Hild & S. Dittmann
Abstract: In this chapter results from investigations on biogeochemical processes
and their interactions with macrofauna and bacteria in two tidal flats of the East
Frisian Wadden Sea are presented. On the Swinnplate, the intluence of a mussel
bed (Mytilus edulis) on the surrounding sandflat, the inhabiting fauna as well as
interactions between macrozoobenthos, microorganisms and geochemical parameters were examined. Samples were taken along a transect from the centre of a
mussel bed towards the sandtlat. Part of the biodeposits produced by the mussels
was transported by the ebb current into the surrounding sediments. Thus, not only
the grain size composition (mud content of the sediment) and the PaC-content
(particulate organic carbon) of the sediments changed along the transect, but also
other physico-chemical sediment characteristics. These changes in the sediment in
turn affected the composition of benthic assemblages.
The proportion of fine-grained material in the biodeposit-containing sediments
was subject to seasonal tluctuations. During winter, the mud content decreased due
to intensified erosion by increased wind and wave activity. In summer, the deposition of fine-grained biogenic sediments rich in pac increased due to higher biological activity and calm weather conditions. The chemical composition of the
sediments was predominantly coupled to the particle size distribution and not directly to the activity of the mussels. During winter, the content of Mn, P and Cu in
the suspended matter was coupled to the distribution of the clay minerals (Alcontent). In contrast to that a strong tluctuation occurred in summer, which was
attributed to early diagenetic processes due to microbial activity.
The input and decomposition of organic matter derived from plant material in
the surface-sediments of the mussel bed could be documented and followed using
specific organic biomarkers (fatty acids, sterols). Thus, it was possible to differentiate between signals of phytoplankton and macroalgal blooms or bacteria in the
sediment of the mussel bed and to allocate these to specific events.
During the investigations on the intluence of organic matter availability on biotic activities it turned out that not only the amount, but also its quality was important. Organic constituents of high quality such as photosynthetic pigments,
hydrolYl.able amino acids and fatty acids decreased with increasing distance from
the mussel bed towards the sandtlat. The composition of pigments and their degradation products (mainly phaeophorbides) in the biodeposits were related to seasonal inputs of diatoms and/or macroalgae. Some amino acids were enriched in the
sediment underneath the mussel bed, but the relative contribution of amino acidderived carbon to total pac was reduced, indicating a selective depletion of amino
acids by the mussels. Investigations in a mesocosm confirmed that M. edulis selectively eliminated amino acids and pigments from the water column and converted them into biomass.
The spatial and temporal distribution of microbial biomass and activity was
closely coupled to the availability of fresh, labile organic matter and followed the