5.2 Biogeochemical Processes and Interactions
101
Just like the element composition, the POC-content of the sediments was linked
to the particle size distribution. The relationship between the POC-content (% of
weight) and the mud content as well as the POC-concentration (% of volume) and
the mud-content was elaborated by Delafontaine et al. (1996). These authors put
special emphasis on the distinction between content (per unit of weight) and concentration (per unit of volume) for parameters determined in sediments and exemplified this for POc. Specifying a value as concentration takes the compaction and
water content of the sediment into account. In the sediments of the backbarrier
tidal flats, a linear correlation existed between the proportion of mud and the POCcontent, but not between the proportion of mud and the concentration of POc.
Contrary to the POC-contents, which had their maximum with a mud content of
100 %, the highest POC-concentrations occurred in sediments with a mud content
of 60-70 %.
The finding of Delafontaine et al. (1996) is important for the "availability" of
organic matter for organisms (but see section on quality of organic matter below)
and for the calculation of mass balances on the base of sediment volumes. Their
results showed that during the turnover of sand, substantial and previously underestimated amounts of POC were set in motion. The POC-contents were lower in
sand than in mud, but due to the reallocated sediment masses in total, larger quantities of POC (approx. 40 %) were moved with the sand fraction.
Formation of gradients along the transect
The studies on the Swinnplate took place under the assumption that a gradient
developed with the discharge of fine-grained material from the mussel bed with the
ebb flow and a pronounced gradient indeed existed in 1994 at the beginning of the
investigations. However, due to changes at single transect sites during the study
period (macroalgal growth, mussel dislodgement or settlement), this gradient was
not as pronounced later. In 1994, POC concentrations as well as mud and carbonate contents decreased continuously from SPI (mussel bed) to SP6 (sandtlat)
(Flemming, Bartholoma, Delafontaine, Kroncke, unpub!. data). This situation
lasted until the ice winter destruction of the mussel bed. After the ice winter
1995/96, the former gradient was destroyed by dislodgement of the mud packages
and the death of the mussels (Table 5.2.2). Compared to the strongly fluctuating
values in the sediment of the former mussel bed, the POC contents at the reference
site in the sandflat (SP6) showed little variation over time (Kri"incke, unpubl. data).
The contents of total extractable protein and amino acids also showed a continuous decrease along the transect in the direction of the sandflat in 1995
(Behrends 1997). During that year, the average protein contents increased. However, while the total contents of amino acids decreased along the transect from SP I
to SP6, the proportion of amino acid carbon (AAC) on the entire POC increased in
the same direction (Behrends 1997). This result indicates that, relative to POC, the
amino acids were not enriched in the sediments of the mussel bed, since the mussels take in particulate amino acids. An outline of the spatial and temporal variations of the parameters along the transect is given in Table 5.2.2.
101
Just like the element composition, the POC-content of the sediments was linked
to the particle size distribution. The relationship between the POC-content (% of
weight) and the mud content as well as the POC-concentration (% of volume) and
the mud-content was elaborated by Delafontaine et al. (1996). These authors put
special emphasis on the distinction between content (per unit of weight) and concentration (per unit of volume) for parameters determined in sediments and exemplified this for POc. Specifying a value as concentration takes the compaction and
water content of the sediment into account. In the sediments of the backbarrier
tidal flats, a linear correlation existed between the proportion of mud and the POCcontent, but not between the proportion of mud and the concentration of POc.
Contrary to the POC-contents, which had their maximum with a mud content of
100 %, the highest POC-concentrations occurred in sediments with a mud content
of 60-70 %.
The finding of Delafontaine et al. (1996) is important for the "availability" of
organic matter for organisms (but see section on quality of organic matter below)
and for the calculation of mass balances on the base of sediment volumes. Their
results showed that during the turnover of sand, substantial and previously underestimated amounts of POC were set in motion. The POC-contents were lower in
sand than in mud, but due to the reallocated sediment masses in total, larger quantities of POC (approx. 40 %) were moved with the sand fraction.
Formation of gradients along the transect
The studies on the Swinnplate took place under the assumption that a gradient
developed with the discharge of fine-grained material from the mussel bed with the
ebb flow and a pronounced gradient indeed existed in 1994 at the beginning of the
investigations. However, due to changes at single transect sites during the study
period (macroalgal growth, mussel dislodgement or settlement), this gradient was
not as pronounced later. In 1994, POC concentrations as well as mud and carbonate contents decreased continuously from SPI (mussel bed) to SP6 (sandtlat)
(Flemming, Bartholoma, Delafontaine, Kroncke, unpub!. data). This situation
lasted until the ice winter destruction of the mussel bed. After the ice winter
1995/96, the former gradient was destroyed by dislodgement of the mud packages
and the death of the mussels (Table 5.2.2). Compared to the strongly fluctuating
values in the sediment of the former mussel bed, the POC contents at the reference
site in the sandflat (SP6) showed little variation over time (Kri"incke, unpubl. data).
The contents of total extractable protein and amino acids also showed a continuous decrease along the transect in the direction of the sandflat in 1995
(Behrends 1997). During that year, the average protein contents increased. However, while the total contents of amino acids decreased along the transect from SP I
to SP6, the proportion of amino acid carbon (AAC) on the entire POC increased in
the same direction (Behrends 1997). This result indicates that, relative to POC, the
amino acids were not enriched in the sediments of the mussel bed, since the mussels take in particulate amino acids. An outline of the spatial and temporal variations of the parameters along the transect is given in Table 5.2.2.