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Life in Freshwater

What is the Freshwater Habitat?

Physical Properties of Water
Different Types of Freshwater
Non-natural Water Bodies incl Acid Rain

Physical Properties of Water:

All freshwater habitats are dominated by the physical properties of water. The molecule is made up of a single oxygen atom with two hydrogen atoms attached. The former is slightly negatively charged whilst the latter is slightly positive. As a consequence, unlike any other solvent on earth, water can attract, dissolve and hold mineral ions in solution. A perfect way of providing nutrients to plants.

Water is viscous and this will produce resistance to animals moving through it whether this is a tiny insect larva, fish or an otter. Hence there may be a need to improve streamlining. Moving water will transport the organism and so those that want to remain in the same area will need to avoid the faster currents (see Invertebrate Drift). Moving water creates drag on plants trying to anchor themselves. If the current is especially fast silt and mud will be removed and there will be little substrate in which roots can grow. In fact plants may not be evident and a closer inspection (including scraping the rock and placing the debris under the microscope) is needed to see algae and non-flowing plants attached.

The range of temperatures found in the majority of habitats is such that water rarely is found in anything other than liquid. In fact water has a high thermal capacity, meaning that it needs a great deal of energy to heat it but also retains it. This means that living in water creates a stable environment. This stability lasts for much of the year although with the on-set of winter the drop in temperature causes the water to increase its density (as in all liquids). Unusually, water has a useful property in that it is at its most dense at 4 degrees Celcius. A continuing drop in temperature actually decreases the density so that as the water freezes it floats above the denser liquid water. For organisms living in ponds by living at the bottom this may help them survive the winter. Flowing water requires a much lower temperature
A stream frozen from the edge
A stream frozen from the edge with the ice on top. The fastest water is still flowing as it has not frozen

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Different Types of Freshwater

Essentially there are two main types, Static Water (called lentic) and Flowing Water (called lotic). However, this is still vague. For example, static water could be any size, a puddle left over from the last rain shower up to Lake Baikal (photo below) in Russia, the deepest freshwater lake in the world at 1620 metres deep. It also holds the greatest volume of water in the world with over 300 streams feeding it. Incidentally Lake Superior in North America has the greatest surface area.

Lake Baikal (photo Vladstudio)
Lake Baikal (photo Vladstudio)

Like lentic systems, flowing water is also varied, from a tiny hill stream bubbling along the edge of a peat bog to a navigable river like the Rhine in Germany. The largest (greatest volume) river system in the world and the second longest is the River Amazon in South America (the Nile is the longest at 6741 kilometres). A few stats: it is navigable for 3700 ks upstream and is 6440 ks at its longest. However, the drainage basin is almost 6 million square kilometres.

Niagra Falls in winter 
Niagra Falls in winter: located between USA and Canada. These are falls on the Canadian side and the mist of water generated freezes on the surrounding vegetation. The water here is cutting back the land at the rate of nearly 1.5 metres a year

Iguassu Falls
Iguassu Falls: located between Brazil and Argentina. The volume of water flowing over the main cataract (the Devil's Throat, 72 metres high) could fill St Paul's Cathedral in a fraction over 0.5 second. The dark marks in the middle of the spray are swifts that specialise in nesting behind the falls and are flying through it!

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Non-natural water Bodies

These are water created by humans. Typically they will fit within the static water section but some may not be obvious. For example where does a canal fit in? Dug out and created as a transport system several hundred years ago they may now be use for amenities such as narrow boat holidays, canoeing or nature reserves for the wildlife. They need careful management as the process of succession will quickly choke them with vegetation. This is because they fit more to the definition of a pond than a river.

A managed canal
A managed canal

What may appear to be a canal could be a drainage channel in a wetland area or on Dartmoor a leat. These are ways of drawing off water for use in industry or drinking. Either way they are channelled and can be fast flowing like an upland river; quite different to the canal.

Reservoirs are often created by damming the head of a valley and allowing water to collect from the natural process of run-off . They are deep and have more in keeping with a lake. They may be used for amenities, i.e. water sports, but may also be stocked with trout for anglers. Stocking may be the only way of ensuring enough fish as there will usually be insufficient food and unsuitable abiotics for breeding.

Acid Lake in Norway
Acid Lake in Norway

Before the latter part of the twentieth century the lakes in Scandinavia were rich in aquatic life including vegetation and a diversity of animals. The invertebrates provided food for trout and salmon but by the 1980's and 1990's this started to become a rarity. In the recent decades sulphur dioxide gases produced by industries burning fossil fuels has been released into the atmosphere. Primarily this has come from countries like the UK. This acidic gas dissolves in the clouds to fall, many miles from where it was produced, as acid rain. This greatly increases the acidity of the lake killing many of the species living there. This is coupled with the release of aluminium from the soil as the acid rain percolates through and drains to the lakes.

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Looking for a next step?
The FSC offers a range of publications, courses for schools and colleges and courses for adults, families and professionals that relate to the freshwater environment. Why not find out more about the FSC?

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