Fresh water is an essential for all life forms, both animal and vegetable. Some plants do not like to be swamped by water, others grow in swamps! Anywhere fresh water occurs is usually a good place to look for wildlife but the wildlife one will find varies hugely with the variation in the sub-habitats that occur within this general heading. Some plants thrive in fast moving streams, others prefer the water to be sedentary; some like their roots submerged in water whilst others can endure drying out in drought conditions.
As with all habitat classes there are many variations around a common theme; in this case six common variations in fresh water habitats:
- Chalk streams: most of Dorset's rivers have their source in the chalk hills of north and mid-Dorset where, initially, they are fairly fast flowing streams of pure and slightly alkaline water
- Slow moving rivers: as these streams merge and reach the lower lying land south east of the central chalk ridge they form slow moving, meandering rivers where silt readily builds on inside of bends in the course
- Fen and carr: where rivers have changed course or regularly over flow onto lower lying areas nearby fen can establish (reeds and sedges) or possibly carr (trees such as willow and alder)
- Ditches and streams: on the lower reaches of the main rivers are extensive low lying plains which readily flood in winter and are often drained by streams (natural) or ditches (man-made)
- Lakes: lakes are large areas of deep and still fresh water; in Dorset these are rarely natural and most occur as a remnant of past mineral extraction
- Ponds: many ponds in Dorset are of man-made origin too from village ponds once used to water cattle to remnants of clay or peat digging
Each of these fresh water sub-habitats will have a unique flora and fauna together with a collection or more widely distributed fresh water plants and insects. Various species of dragonflies and damselflies, in particular, tend to vary in requirements and favour specific conditions in which to breed and prosper.
Given the diversity and extent of wildlife around fresh water sites it is not surprising, perhaps, that many nature reserves have some form of fresh water habitat present.