I have a garden pond that suffers from 鈥減ea-soup鈥� algae. A thunderstorm recently flooded the nearby road, overflowing into my garden and the pond. The next morning the algae had gone. What was in the floodwater that managed to clear the algae?
鈥淭he pea soup may look repellent, but it is often enthralling to view under a microscope鈥�
鈥� 鈥淧ea-soup鈥� algae are generally single-celled species, plus others that clump in twos and fours. Populations explode when sunshine and nutrients combine in warm conditions 鈥� Daphnia and similar organisms that graze on algae are barely able to thin the ranks. The resultant pea soup may look repellent, but is often a healthy community that is enthralling to view under a microscope. To clear the water, key nutrients such as phosphates must be reduced.
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Floodwater might wash out a pond or cause the algae to die through pollution. Dead cells will sink or form a scum, leaving the rest as clear water.
Possible toxic pollutants include roadside herbicides, salt or tarry phenolics from asphalt on recently surfaced roads. Furthermore, heavy traffic sprinkles roads with exhaust dust and rubber powder. Tyre rubber contains toxic organic compounds and metals such as zinc, with the dust having a huge surface-to-volume ratio that permits rapid release of poisons into the water. In fact, it is a common (although ill-advised) practice to put old tyres into ponds to clear them of unwelcome organisms. However, it actually increases the nutrient supply and invites cyanobacterial blooms that are far less welcome than any pea soup.
Jon Richfield, Somerset West, South Africa
鈥� Algae do well in a neutral to slightly alkaline environment, at a pH of about 7 or 8. At this level they take up nutrients such as phosphate and nitrate easily.
All rain is acidic. Carbon dioxide makes up about 0.04 per cent of air and it dissolves in clouds and falling rain to form carbonic acid, giving natural unpolluted rain a pH of about 5.5. Air-polluting gases such as the oxides of sulphur and nitrogen further reduce the pH of rain.
Normal amounts of rainfall on the surface of a pond would decrease its pH, but it is buffered by carbonates and bicarbonates 鈥� naturally occurring alkaline chemicals in water.
However, a deluge of rainwater would have defeated the pond鈥檚 buffering capacity, lowering its pH.
An acidic pH can damage algal cell walls, thus causing the overnight algal demise in this case. Because pond plants use up the excess carbon dioxide during photosynthesis, the pond鈥檚 pH will start to return to normal.
But pond owners should not be tempted to use acid to get rid of algal blooms because it could have disastrous consequences for pond flora and fauna.
David Muir, Science Department, Portobello High School, Edinburgh, UK
鈥� As photosynthetic organisms, algae will tend to cluster close to the surface of the pond to capture maximum light (just as trees in a dense woodland have most of their leaves at the top). So, although the water may appear as 鈥減ea soup鈥�, it is probably much clearer towards the bottom.
The storm water pouring into the pond causes it to overflow. And the overflow comes from the top layers of water. It is also possible the storm water, having fallen from a height and being colder than the pond water, sank rapidly, enhancing the effect. So perhaps it is nothing in the storm water itself. Within a week, I鈥檒l bet it turned just as pea-soupy as it was before.
Alistair Scott, Gland, Switzerland
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This article appeared in print under the headline 鈥淧ond skimmer鈥�