Water management and algae control
In a sealed pond, transpiration can cause a fall in the water level of more than 5 mm during a hot summer day. The more the mains-water has to be refilled, the greater nutrient levels become. Generally, water with low hardness and nutrient content should be preferred to prevent algal growth, just as in swamp beds with bog and fen vegetation. The refilling of water can work automatically, when the supply pipe is controlled by a floating switch.
In newly filled ponds, it is quite normal that after some weeks the water becomes murky because of floating algae. After a while, natural predators, especially water fleas, develop that eat the algae. In the long term, the nutrient-level should decrease, so that floating algae will not be so abundant. To prevent persistent problems with floating algae, the following recommendations are made for artificially sealed ponds:
– use of water with low hardness and low nutrients
– avoiding lime-containing and nutrient-rich substrate and fill material; hard limestone
without porous structures can be used
– nutrient-enriched surface water from outside the pond should not overflow the edges in
bigger amounts and get inside
– use sufficient helophytes along zones 2 to 4 and hydrophytes in zone 5—floating leaved
plants cover and shade the water surface, this weakens the vitality of algae.
If trees shade parts of a pond, the effect of cooling and reduced light intensity causes deterioration in conditions for algae. Leaves falling into the pond can cause moderate nutrient enrichment, but this is often overestimated. Pine trees are recommended pond shaders because their needles are very poor in nutrients and lead to an acid reaction when falling into the water.
Even in oligotrophic ponds, filamentous algae can cause unpleasant displays. There are species with a rough surface (Cladophorales) and others with a slimy surface (Zygnemales). Both are able to assimilate hydrogencarbonate, leading to an increase in pH in their surroundings. Acidifying the water can destroy the algae effectively, but it also has catastrophic influences on other living organisms in the pond. So the adding of acid into the water should be carried out carefully and only in cases of emergency. Filamentous algae need a fixed surface to anchor themselves with rhizoides, this works best above lime – containing structures. From there they grow through the water body. Besides the avoidance of lime, the following are possibilities for controlling filamentous algae:
– manually removing filamentous algae with a rake or a small net mounted on a long
handle—together with the plant mass, nutrients, especially phosphorus (algae are prone to a considerable consumption of phosphorus), are withdrawn
– water movement or blown-in air can increase and optimise the level of dissolved CO2,
which seems to reduce the growth of filamentous algae
– barley straw is said to have an algae-repressing effect—though the reason for this effect
is still disputed, practical experience strongly suggests that this is very effective (Newman 1997)
– several herbicides are available that prevent algae growth but damage to the water
plants can occur (Hafner and Eppel 2002)
– dyes which produce a turquoise colouration of the water—if its artificiality is not a
problem, this is a very effective way to control algae.