Page 378 - Soil and water contamination, 2nd edition

Patterns in surface water 365

epilimnion are readily decomposed in the epilimnion, but some sinks to the hypolimnion .
Here, the continued decomposition of organic matter depletes the oxygen and lowers
the redox potential (Eh) while regenerating dissolved inorganic nutrients and carbon
dioxide . Because the presence of the thermocline prevents oxygen from diffusing into the
hypolimnion, the oxygen saturation drops gradually from 100 percent to 0 percent. In Lake
Dudinghausen this occurs between 5 and 7 m depth (Figure 18.19). In the hypolimnion,
oxygen production occurs only if light penetrates below the thermocline. In lakes where light
does not penetrate below the thermocline, as apparently is the case in Lake Dudinghausen,
there is no internal source of oxygen.
Summer stratification also causes typical vertical profiles of N and P
compounds. Nitrate occurs in relatively high concentrations in the epilimnion ,
but in the thermocline and the deeper hypolimnion it drops to zero due to
+
denitrification by denitrifying bacteria. In contrast, NH concentrations are
4
very low in the epilimnion, but high in the hypolimnion. In the epilimnion,
+
-
NH released by the decomposition of organic matter is oxidised to NO .
4 3
+
During stratification, nitrification of NH does not occur in the hypolimnion because of
4
3-
the lack of oxygen . The reactive phosphorus (RP (< 0.45); mainly consisting of ortho-PO )

4
is relatively low in the epilimnion due to uptake by algae . In contrast, the RP (< 0.45)
concentrations are relatively high in the hypolimnion as a consequence of P release from the
anaerobic sediments. The release of P from anaerobic sediments will be further elaborated
upon in Section 18.4.4. Oxygen depletion in the hypolimnion and reducing conditions at
the sediment /water interface may, in addition, result in iron , manganese and related trace
elements being released from the bed sediments (Balistrieri et al., 1991; Thomas et al., 1996).

Figure 18.19 Vertical distribution of temperature , oxygen saturation, pH , Eh, dissolved inorganic N and P
compounds in Lake Dudinghausen, northern Germany, in September 2000. Adapted from Selig et al. (2004).

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