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Life in Freshwater

Light in Ponds and Lakes

For much of freshwater the primary source of energy is light. However, the levels of light under water will vary according to many factors: 1. the time of day and the season 2. the depth 3. the turbidity of the water (how clear the water is) 4. amount of cloud cover 5. altitude of the lake.

1. The angle of the sun to the water surface is all important and this of course varies with the seasons and the time of day. On the equator in the middle of the day light rays may hit the water at precisely right angles. This allows very little light to be reflected from the surface and almost 99% absorbance by the water. However as the angle becomes more acute, especially in temperate climes, the amount reflected increases and that passing into water diminishing. At 45 degrees it is about 90%. At 10 degrees barely 25% of light enters the water.

2. If the water is crystal clear (e.g. oligotrophic lake) light may penetrate over 40 metres down into a lake. However the intensity will be extremely low, just a few percent of what was at the surface. In fact the penetration of light drops very rapidly to begin with so that at 10 metres depth already two thirds of the light has disappeared. Intensity is just one aspect, wavelength of light is another. On land, yellow-green light (the middle bands of light) are of least importance to plants whilst blue and red light (the two extremes of the visible spectrum) are the most important. This latter group penetrate water quickly but do not go deep. Green penetrates the furthest. This means that the green plants need to remain near the upper regions of the photic zone as that is where the light is most suitable for photosynthesis. Some plants, especially algae, will be able to survive lower down as they produce specialised accessory pigments to pick up the wavelengths of light that will pass into the deeper areas. They appear brown or even black. The best region for macrophyte growth will therefore be around the edge, called the littoral region, as in the seashore.

3. Turbidity may be generated by organic matter in the form of detritus from decay or it could be in a peaty area where humic acid flows into the lake. This can create very dark water. Alternatively high nutrient levels will encourage high densities of plankton to grow.

Abiotic factors:



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