Lentic (still water) communities can vary greatly in appearance, anything from a small temporary puddle to a large lake is capable of supporting life to some extent. The type of life which is supported will depend greatly on the biotic and abiotic components of the freshwater ecosystem explained on previous pages of the tutorial.
The creation of many of today's long standing freshwater lentic environments are a result of geological changes over a long period of time, notably glacial movement, erosion, volcanic activity, and to an extent, human intervention.
The consequence of these actions results in troughs in the landscape where water can accumulate and be sustained over time. The size and depth of a still body of water are major factors in determining the characteristics of that ecosystem, and will continually be altered by some of the causes mentioned above over a long period of time.
One of the important elements of a still water environment is the overall effect that temperature has on it. The heat from the sun takes longer to heat up a body of water as opposed to heating up dry land. This means that temperature changes in the water are more gradual, particularly so in more vast areas of water. When this freshwater ecosystem is habitable, many factors will come into play determining the overall make up of the environment which organisms will have to adapt to.
As with osmosis, temperature will even out across a particular substance over time, and this applies to a still body of water. Sunlight striking the water will heat up the surface, and over time will create a temperature difference between the surface and basin in the body of water. This temperature difference will vary depending on the overall surface area of the water and its depth.
Over time, two distinctly different layers of water become established, separated by a large temperature difference and providing unique ecological niches for organisms. This process is called stratification, where the the difference in temperature between surface and water bed are so different they can easily be distinguished apart. The surface area is deemed the epilimnion, which is warmed water as a result of direct contact with sunlight. The lower layer is deemed the hypolimnion, found below the water surface, and due to increased depth, receives less heat from the sun and therefore results in the colder water underneath.
The next page will continue to look at how heat from the sun affects the water and how other factors also come into play.