Lake stratification

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 triggers responsible for the gas releases from either lake are unknown, but one cause could have been a large landslide entering the lake and causing the lake stratification to be disrupted and allowing local oversaturation to initiate the gas release. Both disasters occurred in August, when stratification is... 

Epilimnion. The uppermost layer of a lake during the summer period of lake stratification this layer is warmer than underlying waters and is generally well mixed. Stratification occurs only in lakes of sufficient depth, as determined by the local temperature and wind conditions (see also hypolimnion). 

Hypolimnion. The lowermost layer of a lake during the summer period of lake stratification this layer is cooler than the overlying epilimnion. 

Unlike a temperate lake, stratification in the ocean does not completely break down in the winter and only a few nutrients come up due to exchange between deep and surface water layers. More important sources of P to the photic zone (surface oceanic layer) are the major upwelling regions. Under oceanic currents and wind influence the open oceanic waters exchange with waters from the continental margins accompanied by the corresponding input of nutrients. 

In actuality, temperature cannot be considered constant if the preceding calculation is performed for large altitude differences. In the Standard Atmosphere, temperature decreases at the rate of approximately 6.5 °C per 1000 m (3.5 °F per 1000 ft) up to an altitude of about 11,300 m (37,000 ft), the lower bound of the stratosphere. At that height, the temperature becomes nearly constant at —55 °C (-67 °F). The constant temperature of the stratosphere greatly inhibits vertical mixing, thus leading to its name (recall lake stratification). Vertical temperature changes are discussed further in Section 4.2. 

The principal focus here is on lakes with only a brief discussion of mixing in rivers. The first and simplest generation of contaminant fate models treat lake stratification as simple homogenous or well-mixed units. Contaminant concentrations in the... 

The environmental sampling of waters and wastewaters provides a good illustration of many of the methods used to sample solutions. The chemical composition of surface waters, such as streams, rivers, lakes, estuaries, and oceans, is influenced by flow rate and depth. Rapidly flowing shallow streams and rivers, and shallow (<5 m) lakes are usually well mixed and show little stratification with... 

Fig. 5. Typical thermal stratification of a lake, reservoir, or poorly mixed estuary in summer which, because of density differences, estabUshes discrete 2ones...

Most lakes affected by eutrophication will also have significant amounts of phosphorus in their sediments, which can be recycled into the water column (Section 4). The control of this source can be achieved by treating the sediments with iron salts or calcite to bind the phosphorus more tightly into the sediments. These methods have been used to some effect, but consideration has to be given to the quality of the materials used and whether or not the lake can become de-oxygenated in the summer. In the latter case this can be overcome by artificial de-stratification. 

Aeration of the hypolimnion (lower, colder layer of water in a stratified lake) without disruption of stratification has been used in deep lakes. This has the advantage of not increasing the temperature of the hypolimnion and prevents the advection of nutrient-rich water into the epilimnion (upper, warmer layer of water in a stratified lake). Oxygen injection is preferred in order to prevent the build up of nitrogen super-saturation which is toxic to fish. "... 

In the first stages of the development of an Action plan all control options are considered. In the case of lakes, this process is aided by a PC-based expert system , PACGAP, which looks at the physical and chemical characteristics of the lake to determine the most likely option for control. Once further, more detailed information has been collected on the lake s nutrient inputs and other controlling factors, amore complex interactive model can be used (Phytoplankton Response To Environmental CHange, PROTECH-2) to define the efficacy of proposed control options more accurately. This model is able to predict the development of phytoplankton species populations under different nutrient and stratification regimes. 

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. ... 

Treating a body of water as a completely mi.xed system can be a valuable approach for estiniating the effects of hunuui actii ities. It can be applied to a number of pollutants, including suspended and dissohed substances, as well as to heat balance coniputations. As noted eiulier in this section, the ertical behai ior of lakes is of particular importance because surface and bottom waters e.xhibit quite different quality during periods of stratification. The estimation of vertical mixing is tlicrcfore of importance and, for some situations, a simple method can be used based on the completely mi.xed analysis. 

Fig. 14-5 Typical distribution of P and temperature in a temperate lake in summer. Thermal stratification restricts exchange between surface and deep wafers. Phosphorus is depleted in the surface waters by the sinking of biologically produced particles.

As cooling occurs in the late fall and early winter, the thermal stratification breaks down, permitting mixing of the deep and surface layers. This allows the surface layers to be replenished with P. During the winter months, biological productivity in a temperate lake is limited by the availability of light rather than nutrients. 

Navarro E, Bacardit M, Caputo L, Palau T, Armengol J (2006) Limnological characterization and flow patterns of a three-coupled reservoir system and their influence on Dreissena polymorpha populations and settlement during the stratification period. Lake Reservoir Manage 22 293-302... 

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

Before fall turnover, there is zero oxygen concentration in the sediments and in the water above the sediments of Lake Harriet. At fall turnover, stratification of the lake is broken down and the water overlying the sediments abruptly reaches approximately Co. You are interested in how fast the sediments will respond to the higher oxygen concentration. To determine, this you must answer two questions (1) What is the oxygen profile in the sediments over time (2) What is the flux rate across the sediment-water interface over time ... 

One could try to determine r such that the mean concentration C(, calculated with the realistic air-water exchange velocity, corresponds to the measured value. In fact, PCE in Greifensee is never at steady-state. Thus, rather than trying to optimize rj, in Illustrative Example 21.6 we will analyze the dynamic behavior of the PCE concentration in the lake under the influence of changing stratification regimes. 

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. 

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