Corona on a water surface
Text: Claudia Hinz

Water corona on 07/19/1998
Photo: © Heino Bardenhagen, Helvesiek / Germany

Water corona of acorn polls
Photo: © Heino Bardenhagen, Helvesiek / Germany

On August 8, 1996, Eero Savolainen could observe a bright, strange phenomenon around the reflection of the sun beneath the surface of a little pond in Joensuu/Finland. Colours and appearance were similar to the corona which sometimes can be observed in thin clouds in the sky. These coronae were not round however, but of an unsymmetrical shape as the one known from pollen coronae only up to then. So it could not be explained as an effect of light diffraction on water droplets from the beginning, and another reason was searched for. The search for more phenomena of that kind achieved success very soon. Just two days later more water coronae could be found on several lakes and ponds. Water samples were taken, the analysis of which showed the surprising result that the substances causing the coronae must have been Labrador spores (Chrysomyxa ledi) in two cases and freshwater seaweed in the others. Further observations finally confirmed the theory that the coronae on water surfaces are caused by light diffraction on microorganisms.

On July 19, 1998, Heino Bardenhagen was the first person who observed water coronae also in Germany. Contrary to the observations made in Finland in lakes and ponds without any exceptions, these coronae appeared on the surface of older puddles. Continuous observations of these puddles showed that they are changing subject to the sun elevation. Also some conditions could be determined which are favourable to the formation of water coronae:

  1. The lake, pond or puddle has to be protected from the wind.
  2. The soil should be clayey.
  3. The water should contain a high concentration of suspended matter and nutrients (which can be recognized by the dirty colour of the water).

In books on seaweed we could find information showing that seaweed formation in puddles having formed in nutrient - containing soils begins just after 12 hours.

In May 1999, special observations were made. Heino Bardenhagen observed coronae on the water which he supposes to having been caused by oak tree pollen. He writes: “Just beside the puddle there was a young oak tree, about 5 meters tall, which was in full bloom when the observation was made. The matter which caused the corona was dispersed on the water surface in streaks, a fact that points out that the phenomenon had been caused by air brought into the water. In that case the concentration of pollen was higher near the oak tree. Other kinds of pollen can probably be excluded as the wind from the west could only have brought birch-tree pollen from an even longer distance (about 80 m) to the puddle.”

Should this idea be confirmed by water analyses later, a possible new reason for the formation of water coronae has been found in the pollen. Therefore I point out here that, if water coronae are observed, a water sample should be taken in any case and been tested for the matter causing the corona formation.