:'2 Biogeochemical Processes and Interactions
125
of POC lies deeper within the sediment and is wider. Here, the microbial biomass
and activity are primarily limited by the availability of the organic matter.
In the presence of oxygen, the organic matter could be oxidized completely via
aerobic microbial respiration. Under anoxic conditions and with a lack of suitable
electron acceptors, only a partial and incomplete degradation of the organic matter
occurs. The products of fermentation are low-molecular organic acids, which could
be used further by other groups of anaerobic bacteria as substrates for anaerobic
respiration like sulphate-reduction or methanogenesis. The combination of both
processes leads finally to the complete mineralization of the degradable organic
matter and to the breakdown into its inorganic constituents. Anaerobic metabolic
pathways are generally less efficient in their energy yield compared to aerobic
pathways. For example, sulphate reducing bacteria have an about 80 % lower energy gain from the metabolized substrate than aerobic bacteria (Stumm & Morgan
1996).
Organic matter in the form of phytoplankton was clearly changed by the food
uptake of the mussels. The mussels use predominantly those components of the
filtered organic matter which are easy to degrade, like e.g. amino acids and pigments. Easily degradable, fresh organic matter of animal and plant origin was
preferentially degraded by heterotrophic bacteria, whereas refractory material,
could be stored and preserved in the sediment for longer time periods.
5.2.3.2
Regulation of Benthic Distribution Patterns by Biogeochemical
Parameters
The dependence of benthic distribution patterns on geochemical parameters, food
availability and quality of the organic matter was examined in detail in ELA WA T.
It was shown that the production of biodeposits changed the benthos composition.
In the sediments enriched with biodeposits the location of the boundary between
oxic and anoxic sediment layers shifted due to the enrichment of POC and mud.
This intluenced the micro-, meio- and macrofauna living in the sediment. Most
heterotrophic organisms (with the exception of anaerobic bacteria), depend on
oxygen and cannot survive in an oxygen-free environment over a longer time period.
The distribution of microorganisms found in this study was mainly linked to the
substrate supply. The highest total cell numbers and exoenzyme activities of bacteria were found in the mussel bed sediments. Here, apart from the sufficient supply
of degradable substrates, the highest settlement surface for bacteria was available
in form of fine-grained sediments. Laboratory experiments also indicated that the
enzyme activity of the microorganisms was closely connected to the grain size of
the sediment.
Ecological effects of low oxygen concentrations on macrobenthos were described by Diaz & Rosenberg (1995) and Levin et al. (1991). Not only the oxygen
deficiency, but also the enrichment of metabolic endproducts (H 2 S, NH 4 +), which
are released into the pore water from the microbial mineralization, are toxic for
macroorganisms at higher concentrations (Giblin et al. 1995) or, in the case of S2,
even in trace levels.
125
of POC lies deeper within the sediment and is wider. Here, the microbial biomass
and activity are primarily limited by the availability of the organic matter.
In the presence of oxygen, the organic matter could be oxidized completely via
aerobic microbial respiration. Under anoxic conditions and with a lack of suitable
electron acceptors, only a partial and incomplete degradation of the organic matter
occurs. The products of fermentation are low-molecular organic acids, which could
be used further by other groups of anaerobic bacteria as substrates for anaerobic
respiration like sulphate-reduction or methanogenesis. The combination of both
processes leads finally to the complete mineralization of the degradable organic
matter and to the breakdown into its inorganic constituents. Anaerobic metabolic
pathways are generally less efficient in their energy yield compared to aerobic
pathways. For example, sulphate reducing bacteria have an about 80 % lower energy gain from the metabolized substrate than aerobic bacteria (Stumm & Morgan
1996).
Organic matter in the form of phytoplankton was clearly changed by the food
uptake of the mussels. The mussels use predominantly those components of the
filtered organic matter which are easy to degrade, like e.g. amino acids and pigments. Easily degradable, fresh organic matter of animal and plant origin was
preferentially degraded by heterotrophic bacteria, whereas refractory material,
could be stored and preserved in the sediment for longer time periods.
5.2.3.2
Regulation of Benthic Distribution Patterns by Biogeochemical
Parameters
The dependence of benthic distribution patterns on geochemical parameters, food
availability and quality of the organic matter was examined in detail in ELA WA T.
It was shown that the production of biodeposits changed the benthos composition.
In the sediments enriched with biodeposits the location of the boundary between
oxic and anoxic sediment layers shifted due to the enrichment of POC and mud.
This intluenced the micro-, meio- and macrofauna living in the sediment. Most
heterotrophic organisms (with the exception of anaerobic bacteria), depend on
oxygen and cannot survive in an oxygen-free environment over a longer time period.
The distribution of microorganisms found in this study was mainly linked to the
substrate supply. The highest total cell numbers and exoenzyme activities of bacteria were found in the mussel bed sediments. Here, apart from the sufficient supply
of degradable substrates, the highest settlement surface for bacteria was available
in form of fine-grained sediments. Laboratory experiments also indicated that the
enzyme activity of the microorganisms was closely connected to the grain size of
the sediment.
Ecological effects of low oxygen concentrations on macrobenthos were described by Diaz & Rosenberg (1995) and Levin et al. (1991). Not only the oxygen
deficiency, but also the enrichment of metabolic endproducts (H 2 S, NH 4 +), which
are released into the pore water from the microbial mineralization, are toxic for
macroorganisms at higher concentrations (Giblin et al. 1995) or, in the case of S2,
even in trace levels.