Summer stagnation

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 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...

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 bacterial population faced with the "autumn-input" was derived from an anoxic population (fermentative bacteria, sulfate reducers) which prevailed during summer stagnation. Within this population the input of freshly produced organic material caused a drastic shift. Bacteria primarily reacted with a strong increase in cell volume (biomass production). Deviating from its "normal" distribution (cf. above), the size spectrum was dominated by medium and large-size cells. Following the final breakdown and sedimentation of the autumn phytoplankton bloom, the bacteria subsequently responded with cell division (increase in cell number). 

The following concentrations were measured in a 35-m deep lake during summer stagnation ... 

Figure 8.23. Titrating a lake with settling biota (see Example 8.18). In the lower part of the lake (summer stagnation), progressively more reducing conditions (decreasing pe) develop. The figure illustrates schematically how the relative concentrations of redox constituents change with decreasing pe.

Figure 1. Concentration versus depth profiles of dissolved Zn (<0.45 pm) in the water column of Lake Greifen on two days during summer stagnation, in comparison to depth profiles of the nutrients P and Si.

Results on total dissolved copper were less systematic than for Zn. Some analytical difficulties were encountered because of the concentration range, which is close to the detection limit of the method used. No depletion from the epilimnion was observed during summer. On the contrary, higher Cu concentrations were observed in the epilimnion than in the hypolimnion during summer stagnation. 

During the summer stagnation with high productivity from June to September, a significant fraction of the settling material consists of calcium car-... 

Table II. Correlation Matrix for the Concentrations in Settling Particles during Summer Stagnation...

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