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Life in Freshwater

Temperature in Ponds and Lakes

Temperature is one of the major factors affecting freshwater ecosystems. Compared with others systems it has a high heat storage capacity, which means that between the seasons there is a minimum of impact compared with terrestrial ecosystems. However, the main effects of temperature are:

1. Distribution of organisms both seasonally and vertically through the water

2. Behaviour and reproduction of organisms

The reason that organisms are affected by temperature is largely due to the fact that they are cold blooded or poikilothermic. The external environment will determine their internal temperature and therefore metabolic activity.

Above 4 degrees celcius the density of water get progressively less. The most noticable effect of this will be on the plankton. Increases in temperature above 20 degrees will make them considerably less buoyant and they will be more likely to sink. Most aquatic invertebrates are sensitive to changes in temperature and therefore density. Being cold-blooded their metabolic rate will slow down as the temperature drops. Those needing to be active early in the year may be dark in colour. The large red damselfly is one of the first insects to emerge in the spring. It has a great deal of black on the body and a melanic (dark) form is fairly common. This will absorb a maximum of warmth.

Thermal Stratification
The water in a pond and lake is warmed by solar energy. If the pond is very shallow there could be a substantial increase in temperature over a diurnal period with cooling at night. With the high heat storage capacity the larger the water body the less effect the sun will have over the day. However, the upper region of a lake will warm in the sun and, if wind turbulence is low, an underlying cool layer will be present. This is thermal stratification and is very important in determining other abiotic and biotic factors. Remembering that very cold water is at its most dense the warm water at the surface (called the epilimnion) will be least dense and "float" over this substantially cooler, dense water layer (called hypolimnion). The water between the two will show a rapid change in temperature over a relatively short distance. This sharp change in temperature is called the thermocline.

This stratification tends to be present during summer and winter when disturbance is at a minimum. In spring and autumn there is the greatest chance of wind-induced wave action. In turn this creates a greater mixing of the layers and the thermocline is most likely to disappear. The position of the thermocline (the most important physical event in a lake) in any lake will vary according to factors such as size of water body, depth, and latitude.

Adaptations to Changes in Temperature
As mentioned above temperature affects the metabolism of the organism as temperature affects enzyme activity in the body. An enzyme will have an optimal temperature at which it works best. As these enzymes vary it will mean that different organims will have differing abilities to cope with the variations that exist in ponds and lakes. An organism that can tolerate a wide range of temperatures is called a eurotherm whilst those with a narrow tolerance range are stenotherms. Fish have been widely studied, as much as anything for commercial reasons. Trout have been shown to have the ability to reproduce if the water is between about 3-14 degrees C with optimal growth between 7-18 degrees C. Above 24 degrees and it becomes lethal. By contrast a Carp reproduces optimally at around 20 degrees C and optimal growth temperature is between 20-30 degrees C. It can tolerate a wide temperature range between 0 and 32 degrees C. These figures are approximate because many organisms have the ability to acclimatise to wider ranges.

These different tolerances will vary with algae as well as animals.

For all organisms if the temperature becomes too extreme, for example high temperature that could denature enzymes organisms may be able to survive through a sexually produced egg or cyst stage that will lie at the bottom. Daphnia produce a tough egg case called an ephippium although this is often stimulated through low temperatures or short photoperiod in the autumn.

Abiotic factors:



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