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Flow patterns, temperature, algal growth and oxygen

Physical parameters like water depth, stream velocity, temperature and dissolved oxygen provide the opportunity to elucidate interrelations between flow patterns, algae growth and oxygen.

Learning goals

Background information

To evaluate the self-purification and self-remediation capacity of a river it is important to look on the diversity of flow patterns. Each of them has its specific ecological function in providing different physical conditions.

Ponds provide low flow velocities and act as retention and sedimentation zones. Since there is less water exchange, the temperature will be more influenced by local climatic conditions than in fast flowing parts of the river. Thus the water will get warmer in the summer and cool more in the winter. Ponds are the first parts of the river which will freeze in case of minus degrees.

Rapids have high current velocity and a low retention time of the water. Thus the water temperature will be mainly determined by the conditions in the river sections above. It can significantly differ to the air temperature, and will be cooler in the summer and warmer in the winter season.

Habitats with high velocities provide a higher aeration rate and thus higher oxygen levels. More oxygen is dissolved at lower temperatures. Therefore you will normally find the lowest oxygen level in ponds in the summer season and the highest at the end of high turbulent sections in the winter season.

However, the warm and low turbid conditions in ponds also favour the growth of algae, especially in nutrient rich rivers. In the daytime algae provide an additional oxygen source with the photosynthesis. Therefore it is possible that you will measure the highest oxygen levels in a pond with intensive algae growth, despite its physical conditions which would rather result in a low oxygen level. In the nighttime however, the algae will consume oxygen for the dissimilation and oxygen levels will be significantly lowered.

Time requirement

The experiment will take 2-4 hours and is recommended to be planned as a half day excursion. If you find ponds with algal growth, try to also arrange a monitoring session during the nighttime. This can be much fun! The time requirement will also be highly dependent on how far you need to travel to come to a suitable river location.

Material requirement

Let's start

To measure the water depth, try to get to the deepest place of the particular section or flow pattern and measure it with the help of a stick and a meter.

To measure the water velocity, define a 10 m section of the river with the help of the 10 m cord. Drop a floating object, for example a piece of cork, into the river at the start of the section and measure the time it requires to reach the end. Repeat the measurement three times and calculate the speed according to the formula speed (m/s) = section length (m) / average time (s).

To measure the air temperature, hang the thermometer in the shade and read the temperature after 3-5 minutes. For the water temperature, dip the thermometer 10 cm into the water and read the temperature after 3-5 minutes.

To measure the oxygen, take a beaker of water from the river, dip the electrode into it and read the value. If the value fluctuates, wait some seconds until it stabilizes.

It is much more difficult to quantitatively assess the algal growth. For a first approximation it is sufficient to measure it qualitatively with indicators like the colour of water and ground and the turbidity:

See and feel

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