Lake Greifen

Figure 1. Schematic representation of the processes influencing the cycles of trace elements in Lake Greifen. During stagnation the lake is divided into an oxic epilimnion and an anoxic hypolimnion. Three different stages can be distinguished in time and space ...

This chapter discusses the chemical mechanisms influencing the fate of trace elements (arsenic, chromium, and zinc) in a small eutrophic lake with a seasonally anoxic hypolimnion (Lake Greifen). Arsenic and chromium are redox-sensitive trace elements that may be directly involved in redox cycles, whereas zinc is indirectly influenced by the redox conditions. We will illustrate how the seasonal cycles and the variations between oxic and anoxic conditions affect the concentrations and speciation of iron, manganese, arsenic, chromium, and zinc in the water column. The redox processes occurring in the anoxic hypolimnion are discussed in detail. Interactions between major redox species and trace elements are demonstrated. 

The surface area of Lake Greifen is 8.5 X 10 m2, and the volume is 150 X 106 m3 its average depth is 17.7 m, with a maximum of 32.2 m. The residence time of water is 1.1 years. Thermal stratification lasts about from May to December, and lake overturn usually takes place in December-January. An anoxic hypolimnion develops during summer stagnation from about June to December. 

Figure 3. Seasonal variations of the major redox species in the water column of Lake Greifen at the depth 30-31 m 02, NH4 +, Mn(II), Fe(II), and S(-II) are represented as a function of time. Fe(II) and S(-II) are found only at the end of the stagnation time (October-November 1989 and 1990). S(-II) was quantified only in the fall 1990 samples.

Figure 4. Concentration—depth profiles in the water column of Lake Greifen particulate manganese (Mnpan) and Oj during summer stagnation (September 20,1989) Mn(Il) and particulate manganese (Mn pan) at the end of stagnation...

Sulfide. Sulfide appears in the water column of Lake Greifen only at the end of stagnation time and in the deepest water layers. This distribution indicated the biological reduction of sulfate in sediments. The reactions involved in the sulfur cycle are described in ref. 65. The occurrence of sulfide indicates a very low pe (p < 0) sulfide is an efficient reductant for many elements, including Fe(III), Mn(IV), As(V), and Cr(VI). The occurrence of sulfide also implies the possible precipitation of solid sulfide phases of various elements and the formation of dissolved complexes (21-23). 

Figure 6. Seasonal variations of As(III)and As(V) in the water column of Lake Greifen mixed lake (January 24,1990) summer stagnation (August 30, 1989) end of stagnation (October 19, 1989, and November 14, 1990). On November 14, 1990, S(-II) is shown for comparison with As(III).

The oxidation of As(III) to As(V) by oxygen is a rather slow process (68, 72). The oxidation of As(III) to As(V) was, however, shown to occur readily by reactions with manganese oxides (19, 20, 73). For the conditions encountered in Lake Greifen, the oxidation of As(III) by manganese oxides is likely to be an important oxidation mechanism. The role of iron oxides in... 

The observations in Lake Greifen are in line with other studies of As(III) and As(V) at oxic-anoxic boundaries (16, 17, 76). In Saanich Inlet (16) and in Lake Pavin (17), increasing As(III) concentrations were found below the 02-H2S boundary, although the reduction of As(V) was not complete. A similar situation is encountered in Lake Greifen, in which sulfide is only an intermittent species. The formation of reduced and methylated As species in the upper layers of various lakes has also been described in ref. 76. 

The seasonal variations of Cr(VI) in the water column of Lake Greifen (83) are illustrated by Figure 7. In the mixed lake, the concentration of Cr(VI) is uniform throughout the water column (approximately 2.5 nM). During stagnation, the concentration of Cr(VI) increases in the epilimnion and decreases in the hypolimnion. This separation can be explained by a slow removal process in the bottom waters, while at the same time Cr(VI) enters only the epilimnion and is cut off from the hypolimnetic waters. In the late stagnation time (November 1989), the concentration of Cr(VI) close to the sediment-water interface decreases. Cr(III), however, cannot be detected in solution. 

These results are in broad agreement with the findings in ref. 11, which indicate the reduction of Cr(VI) in the presence of H2S. In Saanich Inlet H2S is always present in the deeper water, whereas in Lake Greifen an intermediate situation is observed with the predominance of Mn(II) in the hypolimnion. The method used for the determination of Cr(III) in ref. 11 would probably include colloidal Cr(III) the present study attempted to determine dissolved Cr(III). 

Figure 10.4. in a eutrophic lake. Titration curves of Lake Greifen water with... 

Figure 10.10. Zn and Cu in an eutrophic lake (Lake Greifen). Depth-concentration profile is given for Zn in the water column (in January the water is mixed because of winter circulation). (Adapted from Kuhn et al. 1993.)...

Figure 10.13. Reciprocal interaction of phytoplankton and Cu in eutrophic lake (Lake Greifen). Variations are plotted of chlorophyll, assimilated C, p[Cu], and log([Cu]r/ [Cu ]) over time in 1990. Chlorophyll and assimilated C represent averages from the values of 0-5 m depth p[Cu] and log([Cu]77[Cu ]) are the measured values at 5 m depth. (From Xue and Sigg, 1993.)...

From such types of experiment, steady-state concentrations of various photooxidants have been determined for Lake Greifen by Hoignd and collaborators... 

Figure 15.14 gives some pore water profiles from Lake Greifen. The idealized redox sequence leads to a picture of vertically separated processes (see Section 8.6). 

Zn and Cu in the Water Column. Total concentrations of dissolved Zn (<0.45 pm) in the water column of Lake Greifen were 10-40 nM total concentrations of dissolved Cu (<0.45 pm) were 5-20 nM. Systematic variations of total dissolved Zn were observed over the seasonal cycle, with a... 

Figure 2. Concentration versus depth profiles of dissolved Zn and dissolved Cu (<0.45 p.m) in the water column of Lake Greifen in the mixed water column (January 9, 1991) and at the end of summer stagnation (September 19, 1990).

Figure 4. Concentration versus depth profiles of total dissolved Zn [Zn(tot.) Zn2, and voltammetrically labile Zn (Zn lab.) in the water column of Lake Greifen on April 14, 1992. [Zn2 ] amounts to 5-10% of total dissolved Zn.

Figure 5. Overall sedimentation rates (a) and sedimentation rates of organic C, P, Zn, Cu, and Mn (b-f respectively) versus time (at 15- and 28-m depth). Lake Greifen overturn occurs in December-January.

Zn and Cu contents in these particles are well correlated with each other (r = 0.956 over all samples), with an average Zn Cu ratio = 3.8 in the summer samples this ratio is up to 8. These relationships are similar to previous observations of correlation in settling particles from Lake Zurich (4). However, the correlations of Zn and Cu with organic C and P are less clear for Lake Greifen. 

Figure 6. Ratios of Zn P and of Cu P in the settling particles (trap at 15-rn depth) from Lake Greifen as a function of time.

These Zn P and Cu P ratios (or the corresponding Zn C and Cu C ratios) may be compared to those observed in other systems and in algae (Table III). The composition of the settling particles in Lake Zurich is quite similar to that in Lake Greifen this similarity is reflected in these ratios. 

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