106
5 Spalla I and Telllporal DI,trihulIon Patterns
Bacterial exoenzym activities
The highest exoenzyme activities were measured in the sediments underneath the
mussel bed, where the supply of degradable substrate for bacterial mineralization
was highest and the largest surface area for bacterial colonization was present in
form of fine-grained sediments. These sediments also contained the highest bacterial cell numbers (Leu unpub!. data). Within the sediment, extracellular enzymes
were bound to particles and were therefore enriched in fine-grained (silty) sediments.
On a seasonal scale, the activity of exoenzymes (protease-/glucosidase-/phosphatase activity) showed a similar pattern to the seasonal development of the phytoplankton blooms (Leu un pub!. data) This was most pronounced at the reference
site in the sandtlat, and not obvious at the mussel bed.
On the basis the exoenzym activities it turned out that protein-containing highquality substrate was taken up immediately after deposition and transferred into the
food web. In vertical profiles, the highest bacterial exoenzym activities were found
at the oxic sediment surface. The microbial turnover rates and exoenzym activities
decreased with increasing sediment depth (Leu unpub!. data).
Like protease and glucosidase, which had similar seasonal patterns, the activity
of phosphatase also showed a spatial behaviour in 1994 and decreased along the
transect from the mussel bed to the reference site in the sandtlat (Fig. 5.2.6). However, in contrast to the other two exoenzymes, the phosphatase activity rose
slightly during the study period and differed in its seasonal development from the
other two studied exoenzyme activities (Leu unpub!. data). This can be explained
by the fact that increased activities of protease and glucosidase were related to an
enhanced supply and mineralization of settled phytoplankton blooms, whereas high
activities of phosphatase indicate a lack of dissolved phosphate.
Influence on the elemental composition
The knowledge of the element composition of different individual components and
their dependence on the particle size distribution of the tidal tlat sediments is important for the interpretation of biotic and abiotic geochemical processes. The
intluence of early diagenetic processes on the sediment geochemistry can be
shown most clearly for the element Mn, since it reacts very sensitive to modifications of the redox environment. Under reducing conditions Mn is remobilized and
released into the interstitial water. When this dissolved Mn reaches oxic conditions
via upward diffusion, tidal pumping or other processes, it is precipitated as Mnoxi-/hydroxide or adsorbed onto particles. In this case, the activity of bacteria plays
a substantial role, since oxidation and reduction of Mn is mainly controlled by
microbial processes. Thus, an enrichment of solid Mn oxide occurs frequently in
the oxidized layers of the sediments in nearshore areas. But in the vertical sediment profiles taken on the Swinnplate this effect could not always be observed due
to the occurrence of Mn-containing heavy minerals (ilmenite ((Fe, Mg, Mn)Ti0 1 ))
(Fig. S.2.7a). Only with consideration of the Mn-proportion bound to heavy minerals by a standardization on Ti (Fig. S.2.7b), the typical enrichment of Mn at the
sediment/seawater interface could be proven in all studied depth profiles. Seasonal
5 Spalla I and Telllporal DI,trihulIon Patterns
Bacterial exoenzym activities
The highest exoenzyme activities were measured in the sediments underneath the
mussel bed, where the supply of degradable substrate for bacterial mineralization
was highest and the largest surface area for bacterial colonization was present in
form of fine-grained sediments. These sediments also contained the highest bacterial cell numbers (Leu unpub!. data). Within the sediment, extracellular enzymes
were bound to particles and were therefore enriched in fine-grained (silty) sediments.
On a seasonal scale, the activity of exoenzymes (protease-/glucosidase-/phosphatase activity) showed a similar pattern to the seasonal development of the phytoplankton blooms (Leu un pub!. data) This was most pronounced at the reference
site in the sandtlat, and not obvious at the mussel bed.
On the basis the exoenzym activities it turned out that protein-containing highquality substrate was taken up immediately after deposition and transferred into the
food web. In vertical profiles, the highest bacterial exoenzym activities were found
at the oxic sediment surface. The microbial turnover rates and exoenzym activities
decreased with increasing sediment depth (Leu unpub!. data).
Like protease and glucosidase, which had similar seasonal patterns, the activity
of phosphatase also showed a spatial behaviour in 1994 and decreased along the
transect from the mussel bed to the reference site in the sandtlat (Fig. 5.2.6). However, in contrast to the other two exoenzymes, the phosphatase activity rose
slightly during the study period and differed in its seasonal development from the
other two studied exoenzyme activities (Leu unpub!. data). This can be explained
by the fact that increased activities of protease and glucosidase were related to an
enhanced supply and mineralization of settled phytoplankton blooms, whereas high
activities of phosphatase indicate a lack of dissolved phosphate.
Influence on the elemental composition
The knowledge of the element composition of different individual components and
their dependence on the particle size distribution of the tidal tlat sediments is important for the interpretation of biotic and abiotic geochemical processes. The
intluence of early diagenetic processes on the sediment geochemistry can be
shown most clearly for the element Mn, since it reacts very sensitive to modifications of the redox environment. Under reducing conditions Mn is remobilized and
released into the interstitial water. When this dissolved Mn reaches oxic conditions
via upward diffusion, tidal pumping or other processes, it is precipitated as Mnoxi-/hydroxide or adsorbed onto particles. In this case, the activity of bacteria plays
a substantial role, since oxidation and reduction of Mn is mainly controlled by
microbial processes. Thus, an enrichment of solid Mn oxide occurs frequently in
the oxidized layers of the sediments in nearshore areas. But in the vertical sediment profiles taken on the Swinnplate this effect could not always be observed due
to the occurrence of Mn-containing heavy minerals (ilmenite ((Fe, Mg, Mn)Ti0 1 ))
(Fig. S.2.7a). Only with consideration of the Mn-proportion bound to heavy minerals by a standardization on Ti (Fig. S.2.7b), the typical enrichment of Mn at the
sediment/seawater interface could be proven in all studied depth profiles. Seasonal