I have made a bird bath in my garden with a large plastic receptacle fashioned from a plant-pot saucer to hold the water. It has proved to be successful, with many birds using it each day, but I have found that algae build up very quickly on the surface covered by the water. The plastic receptacle was bought new from the shop and the water I put in it is good drinking water, so where do the algae come from? The birds bathe in it and drink the water without any obvious ill effects.
鈥� There are two issues here. Where do the algae come from, and how do they survive and grow?
Freshwater algae are well adapted to distribute and establish themselves in new, often temporary, ponds and puddles. Most green alga species can make tough-walled resting spores when conditions get difficult, usually when their habitat dries up. These dry resting spores can survive a long time and are small enough to be picked up and carried by the wind or in mud that has dried onto the feet of birds. Any newly filled bird bath will very rapidly be colonised, with the spores revitalising as soon as they enter water.
Advertisement
鈥淩esting spores of algae can be picked up and carried by the wind or in mud on the feet of birds, and rapidly colonise any newly filled bird bath鈥�
But the algae also need nutrients to grow, and wouldn鈥檛 survive well in distilled water. Ordinary drinkable tap water, processed to be free of toxins and micro-organisms, will still contain quite high concentrations of plant nutrients such as nitrates and phosphates. Other sources include dust carried in the air and, of course, general muck on the bodies of the birds bathing in the bath.
For these reasons, repeatedly topping up a garden pond with tap water usually leads to the unsightly green algal overgrowth your correspondent has reported.
So it is absolutely normal for algae to grow in bird baths, which are just another pond habitat for them. The good news is the algae are harmless and won鈥檛 worry the birds.
Stephen Head, Director of pond conservation, The Water Habitats Trust, Oxford Brookes University, UK
鈥� Given light and water containing dissolved nutrients, a microscopic amount of algae or their spores will grow to a visible film before too long.
But where does the microscopic amount of algae come from to start with? It might have been on the receptacle, which was not sterilised, or in the water, which also does not have to be sterile to be drinkable. Or it could have come from the surrounding environment, either airborne or carried in by birds.
Those who work in offices that have water coolers equipped with large, clear plastic bottles will know that drinking water in a plastic container closed to the environment can grow algae if it is left in sunlight for too long. In fact, I once drank a glass of such water before I noticed. Although it tasted slightly different, I subsequently suffered no obvious ill effects.
Ian Williamson, London, UK