Stratification in lakes

The flux of DOC from terrestrial landscapes to surface runoff has wide-ranging consequences for aquatic chemistry and biology. DOC affects the complexation, solubility, and mobility of metals (Perdue et al., 1976 Driscoll et al., 1988 Martell et al., 1988 see Chapter 8) as well as the adsorption of pesticides to soils (Senesi, 1992 Worrall et al., 1997). Formation of trihalomethanes when drinking water is disinfected with chlorine, a worldwide threat to water supplies, is also linked to DOC concentrations (Siddiqui et al., 1997). DOC attenuates ultraviolet-B (UV-B) radiation and thus provides some protection to aquatic biota from exposure to harmful UV radiation (e.g., Williamson and Zagarese, 1994). Finally, DOC affects the heat balance and thus stratification in lakes, which is an important constraint for aquatic organisms with limited habitats (Schindler et al., 1996, 1997). 

Stratification in estuaries is in some respects similar to stratification in lakes, although in estuaries the density difference is primarily due to the difference in salinity between freshwater and ocean water, instead of being primarily due to temperature differences, as in most lakes. Freshwater has a density of approximately 1.00 g/cm3, whereas ocean water has a density of approximately 1.03 g/cm3 due to dissolved salts [primarily sodium (Na+), chloride (Cl-), calcium (Ca2+), and sulfate (SO4 ). This is a much larger density difference than that which occurs due to temperature differences in surface waters hence, the stratification may be very strong. Whatever its cause, stratification always inhibits the vertical transfer of dissolved chemicals from layer to layer. 

T. Powell, M.H. Kirkish, P.J. Neale, P.J. Richerson (1984). The diurnal cycle of stratification in Lake Titicaca Eddy diffusion. Verh. Int. Verein. Limnol, 22, 1237-1243. 

The coupling between the hydrological forces and chemical processes —H->C—is responsible for transport of solutes, and this coupling is also to some extent reflected in the water-flow-rate dependence of some mineral dissolution reactions. The reverse coupling between chemical forces and hydrological processes—C- H—is seen in such phenomena as chemical density stratification in lakes, evaporative mixing caused by solute concentration increase in a surface water layer, and chemical density-driven water currents. 

Loss of ice cover and strengthening of summer stratification in lakes. 

Figure 2.25 Stratification in Lake Vanda, Antarctica (Goguel and Webster, 1990, courtesy of the New Zealand Antarctic Record).

The most commonly used physical method for long-term eutrophication control in lakes is that of artificial destratification. This method is well tried and understood and uses either jetted water or compressed air bubbles to break down the lake stratification in the summer months. Algal growth is also affected by an increase in circulation. This is due to the artificial shading effect which results from the algae spending less time near the surface and consequently less time in the light. This technique also reduces the redox-dependent phosphorus release from sediments because the sediment surface remains aerobic. 

The major reasons for the beluu ior of vertical temperature in water bodies are the low thermal condnctii ity and the absorption of heat in the first few meters. As tlie surface waters begin to heat, transfer to low er layers is reduced and a stability condition develops. The prediction of thermal behavior in lakes and reser oirs is an important power plant siting consideration and also is a major factor in preienting e.xcessive thermal effects on sensitive ecosystems. Furthermore, the extent of thermal stratification influences the vertical dissolved ox)gen (DO) profiles where reduced DO often results from minimal exchiuige with aerated water. ... 

Thermal stratification in reservoirs ( lake-type versus river-type )... 

In Illustrative Example 21.5 we discussed the behavior of tetrachloroethene (PCE) in a stratified lake. As mentioned before, our conclusions suffer from the assumption that the concentrations of PCE in the lake reach a steady-state. Since in the moderate climate zones (most of Europe and North America) a lake usually oscillates between a state of stratification in the summer and of mixing in the winter, we must now address the question whether the system has enough time to reach a steady-state in either condition (mixed or stratified lake). To find an answer we need a tool like the recipe for one-dimensional models (Eq. 4, Box 12.1) to estimate the time to steady-state for multidimensional systems. 

Figure 5. Profiles of ion activity products (pIAP values) of FeS measured in Lake Kinneret sediments after the end of an algae bloom (May 30, 1988), during the stratification period (October 24, 1988), and after overturn (January 5, 1989). Straight lines correspond to solubility products of various FeS phases according to the reaction FeS + H+ Fe2+ + HS. (Based on data...

Setaro and Melack (1984) hypothesized that the seasonal changes in nutrient limitation in Lake Calado were related to differences in stratification and mixing. At high water, when the lake was deep and stratified, phosphorus-rich particulates... 

Fig. 1.6. Bacterial ant biochemical stratification in sediments of Lake Vechten, The Netherlands. Data on bacterial populations from Cappenberg (1974a).

In the Lake Vechten sediments referred to above, stratification of the two groups of orgcmisms appeared to be due Isurgely to the sensitivity of methane producers to HjS which became inhibitory above about 0.1 mM (Cappen-berg, 1975). In many sediments, however, H2S does not accumulate to a significant extent due to its fixation as iron sulfides and its diffusion from the sediment. Winfrey emd Zeikus (1977), for example, reported that, in Lake Mendota sediments, the amount of sulfide which could be added before free HjS appeared in the pore waters was about 25 times the concentration of sulfate required to inhibit methanogenesis. 

During thermal stratification of a eutrophic lake, the hypolimnion would be essentially anoxic and manganese would occur primarily as Mn(II)), as found in Lake Mendota (U.S.A.) by Delfino and Lee (1968) and Nordbytjer-net (Norway) by Hongve (1974). Microbial activity certainly is responsible for reduction of Mn(IV) to Mn(II) under such conditions (Dubinina et al., 1974). The presence of dissolved sulphide and carbonate in the hypolimnion may result in the precipitation of MnS and MnCOs if conditions are suitable (Morgan and Stumm, 1965b Delfino and Lee, 1968 Hongve, 1974). 

Stuiver, M., 1967. The sulfur cycle in lake waters during thermal stratification. Geochim. Cosmochim. Acta, 31 2151—2167. 

Thermal stratification is common in lakes located in climates with distinct warm and cold seasons. Although many variations are possible, the classic temperate zone lake begins a period of summer stratification when heat from solar radiation preferentially warms the uppermost water, decreasing its den-... 

FIGURE 2-8 Density versus temperature curve for water. Maximum density occurs at 4°C thus, stratification in a lake can occur in winter with bottom waters near 4°C and less dense surface waters closer to 0°C. In summer, if stratification occurs, the warmer water will be at the surface. Note that a given spread in water temperature conveys a larger density contrast between the waters (and hence a more stable stratification) at higher temperatures than at lower temperatures. The density of ice is much less than the density of liquid water (note the broken scale for ice density). 

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