77
1988). They reported that Cu and Zn in the porewater are controlled by the solubility
of Fe and Mn oxides in the oxic zone, and by metal-sulfides in the reduced zone.
Particulate Cr and Co are mainly bound to Fe/Mn oxides. With the reduction of
these oxides, Cr and Co are simultaneously released into the porewater. Considering
the concentration gradients, a diffusion of dissolved Fe, Mn, Cr, and Co towards the
uppermost sediment layer is expected. In the surface layer, Fe 2+ and Mn 2+ were
reoxidized as Fe/Mn oxides and immobilized. As a result, Cr and Co diffused from
deeper sediment layer are scavenged by freshly formed Fe/Mn oxides. Therefore,
Fe/Mn oxides in the surface layer act as a trap for Cr and Co in the sediments.
4.3.3 Degradation of organic matter and the mobility of heavy metals
The peaks of dissolved Cu and Zn at the sediment-water interface can be explained
either as a result of leaching during the oxidation of metal-sulfides by oxygen in the
overlying water, or as a result of the degradation of biomass which contains these
metals. The seasonal variation of heavy metal profiles at the sediment-water interface
at the site Lauffen is presented in Fig.4.37. Pronounced peaks of Cu and Zn are
measured in the samples from June and October, a period of high sedimentation of
biological materials. Large, rapidly sinking biogenic particles have been recognized
as an important factor on the vertical transport of many elements in the water column.
Evidence from sediment traps studies reflects that the plankton depositing to the
sediments may comprise large particle fluxes of the elements, particularly following
algal blooms (Hamalton-Tayler et al. 1984; Sigg et al. 1987; Morfett et al. 1988;
Gerringa 1990; Batistrieri et al. 1992; Lee and Fisher 1992).
Sigg (1986) reported that mean elemental composition of the phytoplankton
corresponds to the following stoichiometry:
(CH20)I 13(NH3)I 5(H3PO4)Zn0.06Cu0.0osPb0.004Cd0.00005
This stoichiometry reflects the active biological uptake of Cu and Zn, essential
elements for biota. Cd and Pb have no physiological functions, but an adsorption of
Cd and Pb on biological surfaces is possible (Sigg et al. 1987). According to the
relative affinity of trace metals to phytoplankton: Zn >> Cu > Pb >> Cd, the seasonal
changes of Zn and Cu released by the decomposing phytoplankton should be
measured at the surface sediments of the Neckar River. It is possible that small
amounts of Pb and Cd released at the surface sediment layer may become adsorbed
on other particles such as Fe/Mn oxides and clay minerals. Therefore, Pb and Cd in
the porewater would be too low to be detected. Gendron et al. (1986) suggested that
Cd in the Gulf of St. Lawrence is fixed by phytoplankton in the overlying water. The
biological Cd is transported downward with the deposition of phytoplankton and
subsequently released into the water during the aerobic degradation of organic matter.
Similar results are reported by Morfett et al. (1988). In addition, the release of
biogenic metals is conf'wmed by an incubation experiment. Gerringa (1990) found
that dissolved Cu, Cd, Pb, and DOC increased rapidly after addition of easily
degradable organic matter (shrimp). Table 4.9 compares the molar ratio of dissolved
1988). They reported that Cu and Zn in the porewater are controlled by the solubility
of Fe and Mn oxides in the oxic zone, and by metal-sulfides in the reduced zone.
Particulate Cr and Co are mainly bound to Fe/Mn oxides. With the reduction of
these oxides, Cr and Co are simultaneously released into the porewater. Considering
the concentration gradients, a diffusion of dissolved Fe, Mn, Cr, and Co towards the
uppermost sediment layer is expected. In the surface layer, Fe 2+ and Mn 2+ were
reoxidized as Fe/Mn oxides and immobilized. As a result, Cr and Co diffused from
deeper sediment layer are scavenged by freshly formed Fe/Mn oxides. Therefore,
Fe/Mn oxides in the surface layer act as a trap for Cr and Co in the sediments.
4.3.3 Degradation of organic matter and the mobility of heavy metals
The peaks of dissolved Cu and Zn at the sediment-water interface can be explained
either as a result of leaching during the oxidation of metal-sulfides by oxygen in the
overlying water, or as a result of the degradation of biomass which contains these
metals. The seasonal variation of heavy metal profiles at the sediment-water interface
at the site Lauffen is presented in Fig.4.37. Pronounced peaks of Cu and Zn are
measured in the samples from June and October, a period of high sedimentation of
biological materials. Large, rapidly sinking biogenic particles have been recognized
as an important factor on the vertical transport of many elements in the water column.
Evidence from sediment traps studies reflects that the plankton depositing to the
sediments may comprise large particle fluxes of the elements, particularly following
algal blooms (Hamalton-Tayler et al. 1984; Sigg et al. 1987; Morfett et al. 1988;
Gerringa 1990; Batistrieri et al. 1992; Lee and Fisher 1992).
Sigg (1986) reported that mean elemental composition of the phytoplankton
corresponds to the following stoichiometry:
(CH20)I 13(NH3)I 5(H3PO4)Zn0.06Cu0.0osPb0.004Cd0.00005
This stoichiometry reflects the active biological uptake of Cu and Zn, essential
elements for biota. Cd and Pb have no physiological functions, but an adsorption of
Cd and Pb on biological surfaces is possible (Sigg et al. 1987). According to the
relative affinity of trace metals to phytoplankton: Zn >> Cu > Pb >> Cd, the seasonal
changes of Zn and Cu released by the decomposing phytoplankton should be
measured at the surface sediments of the Neckar River. It is possible that small
amounts of Pb and Cd released at the surface sediment layer may become adsorbed
on other particles such as Fe/Mn oxides and clay minerals. Therefore, Pb and Cd in
the porewater would be too low to be detected. Gendron et al. (1986) suggested that
Cd in the Gulf of St. Lawrence is fixed by phytoplankton in the overlying water. The
biological Cd is transported downward with the deposition of phytoplankton and
subsequently released into the water during the aerobic degradation of organic matter.
Similar results are reported by Morfett et al. (1988). In addition, the release of
biogenic metals is conf'wmed by an incubation experiment. Gerringa (1990) found
that dissolved Cu, Cd, Pb, and DOC increased rapidly after addition of easily
degradable organic matter (shrimp). Table 4.9 compares the molar ratio of dissolved