Useful Lake Management Definitions
This is a measurement taken at the State Lab of Hygeine in Madison. They count the total number colony forming units (cfu) of
E. coli bacteria per 100 mL of lake water. We calculate the geometric mean of at least five samples taken over a 30-day period.
A safe, five-week geometric mean is below 126 cfu per 100 mL of water.
Escherichia coli (E. coli)
These are a group of bacteria that normally live in the intestines of humans and animals. E. coli can be one of the most
abundant bacterium in the human body depending upon diet. Although most strains are harmless, several are known to
produce toxins that can cause diarrhea. One particular strain, 0157:H7, can cause severe diarrhea and kidney damage.
Anyone, of any age, can become infected; but children are more likely to develop serious complications.
A note on bacteria counts:
Bacteria counts are extremely variable in any one place or time. High bacteria counts may be caused by waterfowl feces,
strong rains producing high amounts of polluted runoff, or even a child with a dirty diaper in the water. These sources of
bacteria are only transient, and may dissipate in a matter of hours. Because it takes 48 hours to process the water samples,
they are not a good indicator of what the current conditions are at any one beach, at any given time. Therefore, in making
recommendations on swimming safety, data taken over a five-week period is used so that we are actually measuring trends
in bacteria levels (rather than trends in parental diaper changing habits). We also measure bacteria levels in creeks that
empty into beaches, so that we can pinpoint sources. Corresponding weather data, such as prevailing winds and recent
precipitation, are also useful.
A secchi depth is a standard measure of water clarity. It is obtained by dropping a black and white plate into the water until
it can no longer be seen. The depth at which you cannot see the disc is known as the secchi depth. A picture of a secchi disc
can be seen on the right.
Inverse stratification is a phenomenon that occurs in winter in temperate lakes. This only occurs after ice has covered the l
ake's surface. In this instance, ice, which is less dense than water, floats on the surface. Just below the ice is water at or near
zero degrees Celcius. This water is actually slightly less dense than water at four degrees Celcius--despite being cooler.
(Water is most dense at around four degrees C.) Therefore, colder, less dense water (0 C) is trapped above warmer, denser
water (4 C). This is not to imply that the entire lake remains stagnant, in this type of set up, with neither heat entering or
eaving the system nor any mixing occurring. Many factors, including amount of snow cover over the ice and ice break-up
nd shifting, will alter this pattern of inverse stratification.
Spring turnover occurs following ice break-up. Ice deterioration is usually a gradual, slow process; as, while some early spring
days rise above freezing, most nights remain below freezing. Eventually, the ice is permeated by air columns and/or
saturated with water. Warm rains speed up this process, and the complete loss of ice cover is realized--often with the help of
a strong wind. Following the break-up, the water temperature throughout the lake column is nearly constant at around four
degrees Celcius--this is the temperature of maximum density for liquid water. In this situation the entire lake column is able
to mix. The formerly oxygen depleted waters below the ice are replenished by the winds of spring.
As spring progresses, the surface waters of Geneva Lake are heated more rapidly than that heat can be distributed by mixing.
This is especially true on warm, calm days. As the surface waters warm, they also become less dense than the water below.
This density difference soon becomes sufficient to prevent circulation within the water column and the lake is divided into
three regions. The lowest stratum is called the hypolimnion. Stratification not only prevents the exchange of heat between
the hypolimnion and the warming surface waters, but also prevents the exchange of oxygen. These deep waters remain cool
and gradually become increasingly void of oxygen as it is used up by respiration and decomposition throughout the summer.
In addition, continuing decomposition of organic matter results in the accumulation of inorganic nutrients in the hypolimnion.
These nutrients are, of course, trapped in the hypolimnion by stratification. The surface stratum is referred to as the epilimnion.
This layer is more or less uniformly warm and circulates throughout the summer. Here, contact with the atmosphere keeps
oxygen plentiful. However, the nutrients available to plants and algae can become limited later in the summer. Between the
epilimnion and hypolimnion is a region characterized by thermal discontinuity, or rapidly changing temperatures. This region
is called the metalimnion.
As the days become shorter, and the air becomes cooler, the warm surface waters of summer begin to lose their heat.
Eventually, when the temperature difference between the surface waters and deeper waters is minimal, the winds of autumn
succeed in mixing the lake's waters and ending summer stratification. When this happens, the oxygen-depleted, nutrient-rich
waters of the hypolimnion are mixed with the oxygen-rich, nutrient-starved waters of the epilimnion. This process is called fall
turnover. This mixing process can result in algal blooms, as algae are no longer limited by a lack of nutrients in the epilimnion.
The mixing period will continue until the lake is covered by a sheet of ice.
The epilimnion is the uppermost stratum that forms in a lake during summer stratification. It is warmed by the sun and mixed
by the wind. Therefore, temperatures throughout the stratum are usually relatively constant. Mixing also serves to replenish
the waters with oxygen from the atmosphere. Because of this, oxygen is not limiting in the epilimnion. However, nutrient
demand may exceed supply in late summer as aquatic vegetation and algae use up the available nutrients.
The metalimnion separates the surface waters of the epilimnion from the deep waters of the hypolimnion during summer
stratification. The metalimnion is characterized by rapidly changing temperatures. This drastic change in temperature is what
separates the upper reaches of the lake from the lower reaches. The warmer waters of the epilimnion are also much less
dense than the cooler waters of the hypolimnion. The density gradient is what actually prevents mixing.
The hypolimnion is the deepest of the three strata that develop during summer stratification. Here the waters remain cool
throughout the summer. Cold-water fishes and a variety of invertebrates, which are specially adapted to survival in this
environment, occupy these lower reaches. Respiration by these organisms, and the decomposition of organic matter that
occurs here, contribute to use up the available oxygen in the isolated waters of the hypolimnion. By late summer, dissolved
oxygen levels are low and nutrient levels are high. These potential problems are rectified with fall turnover, and the isolation
of the hypolimnion ends.
The thermocline forms within the metalimnion during summer stratification. It is the plane of maximum temperature
decrease with respect to depth.