Environmental Engineering Reference
In-Depth Information
METHOD 6.1.
How Do Limnogists Sample Water from Lakes?
The main consideration in determining how to sample water from a lake is
what information is desired. If the information can be obtained with a sub-
mersible sensor (such as for temperature, dissolved oxygen, pH, and con-
ductivity) results are obtained most easily without removing samples. How-
ever, for many chemical and biological parameters, water must be removed
from known depths with a minimum of contamination or turbulence.
Several devices are available that allow water to be sampled from
depth. Many different types of limnological equipment are lowered into the
water on ropes or cables. The devices are generally triggered with a weight
that is dropped down the line to the sampling equipment. This weight is
called a messenger.
Van Dorn bottles and Kemmerer bottles are devices used to remove
lakes rarely if ever mix because they contain dissolved compounds in the
hypolimnion that stabilize density layers.
Several conditions cause meromictic lakes. Seasonal temperature regimes
can be constant enough (mainly in tropical areas) that lakes rarely mix. The
temperature difference in tropical lakes does not need to be as great to form
a stable stratification as in the temperate zone because the water temperatures
are higher and a greater relative difference in density occurs for each degree
difference in water temperature (Fig. 2.3). For example, the density difference
is greater between 20 and 25°C than between 10 and 15°C water.
Salinity differences can also cause stable stratification. In this case,
more saline water can sit below cooler surface waters when the salinity-
caused density difference is greater than the temperature-related differ-
ences. Several conditions can cause such salinity differences. In tropical
lakes that are stratified for long periods of time, the nutrients enter the sur-
face waters from rivers. These nutrients enter the biomass of the planktonic
food web, and when organisms die they sink. The sinking organisms slowly
release nutrients and a portion is transported to the hypolimnion. Slowly,
the salinity of the hypolimnion increases and the stratification is stabilized.
In arid regions, evaporation can lead to increases in dissolved salt con-
centrations. Fresh river water flowing into the lake will remain on top of
the denser saline water. A fresh surface lake is a common occurrence in
closed basins where saline lakes form. Such was the case in the Dead Sea,
where a stable stratification was maintained for nearly 300 years until wa-
ter diversions for human uses reduced inflow to the lake and the dilute sur-
face layer disappeared in the 1970s (Gavreili, 1997).
Fjord lakes can also have saline waters below freshwater. In this case,
a glacial valley is formed below sea level. As the glacier recedes, saline ma-
rine water floods the valley. The floor of the valley rebounds from the
weight of the glacier and if a raised portion exists at the end of the valley
(e.g., a terminal moraine), the saline ocean water can be isolated. Fresh-
water flows on top of the saline water and a lake forms with saline water
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