Page 376 - Soil and water contamination, 2nd edition

Patterns in surface water 363

Switzerland during winter months. This atmospheric inversion enhances the atmospheric
+
deposition at the lower altitudes. Furthermore, nitrification of manure -derived NH also
4
produces acidity , which enhances the weathering of the carbonaceous bedrock, and so
increases the runoff of Ca, Mg, and alkalinity to the lakes in areas of intensive agriculture.
Differences in geology and land use in different subcatchments of a lake can also give
rise to within-lake variation of water composition. A good example is the distribution of
chlorophyll concentrations in Loch Lomond, Scotland, reported by George and Jones (1987)
and George (1993). Loch Lomond is the largest freshwater lake in the UK: it is 36.4 km long
and at its deepest point is 190 m deep, albeit the average depth is only 37 m. The northern
part of the lake is long (22.3 km), narrow, and deep (average just over one km), but the
southern basin is much wider (maximum width of 8.8 km) and shallower (Figure 18.18a).
There are pronounced differences in geology between the subcatchments of the northern
and southern basins of Loch Lomond. Base-rich rock comprises 98 percent of the sub-
catchment area of the southern basin, whereas it only occupies approximately 3 percent of
the sub-catchment area of the northern basin. Related to the catchment geology there is also
a distinct difference in land use in both subcatchments (see Figure 18.18a), with arable fields
concentrated on the base -rich rock in the southern part of the catchment. Consequently,
the southern part of the lake receives bigger loads of solutes – including nutrients from the
inflowing streams – than the northern part. This explains the higher electrical conductivity
(EC ) of the water in the southern part of the lake (Figure 18.18c and e). The increased
supply of nutrients promotes the growth of algae , which becomes expressed in increased
chlorophyll concentrations in the southern part of Loch Lomond (Figure 18.18d and f).
The difference in depth between the northern and southern parts of the lake emphasises the
differences in EC and chlorophyll concentrations.
Figure 18.18 also demonstrates that wind exerts an important influence on the spatial
distribution of solutes and particulates in lakes . On 14 July 1978, the wind was blowing
strongly from the north-west along the main axis of Loch Lomond. As a consequence, the
EC and chlorophyll concentrations increased gradually from north to south (Figure 18.18c
and d). In contrast, on 12 October the wind blew lightly from the east (i.e. perpendicular
to the main axis of the lake) and there was a sharp increase in EC and chlorophyll
concentrations between the two basins (Figure 18.18e and f).

18.4.3 Vertical variation during summer stratification
Deep lakes in the temperate climate regions with distinct cold and warm seasons become
thermally stratified during summer, which is manifested in the formation of two water layers:
a warm upper epilimnion , and a cool, lower hypolimnion (see Section 3.4.4). Differences in
temperature and light conditions, and the resulting differences in biological activity, cause
distinct vertical patterns of concentrations of substances, notably of oxygen and nutrients, to
emerge in the stratified lake water. Figure 18.19 presents an example of the vertical variation
in oxygen concentration, pH , redox potential and N and P compounds during summer
stratification in the mesotrophic Lake Dudinghausen, located in Mecklenburg-Vorpommern
(northern Germany) (Selig et al., 2004). This lake has a surface area of 18.8 ha and a
maximum depth of 15.2 m. The vertical water sampling was performed at the deepest point
of the lake. From the temperature profile in Figure 18.19 it can be seen that the epilimnion
is approximately 6 m deep. The thermocline , the transition zone between the epilimnion
and the hypolimnion, extends from 6 to 9 m depth. The hypolimnion encompasses the zone
below 9 m depth.
The well-mixed epilimnion is rich in oxygen due to reaeration from the atmosphere
and the production of oxygen through photosynthesis by algae . The pH in the epilimnion
is high, due to photosynthetic activity. Dead algae or other organic matter produced in the

10/1/2013 6:47:49 PM
Soil and Water.indd 375 10/1/2013 6:47:49 PM
Soil and Water.indd 375