A rainbow on the surface of the water of a swamp? It almost seems like the setting of a fairy tale, but it is a phenomenon that actually occurs. These are the so-called “rainbow swamps” or “iridescent swamps” with colored stripes ranging from blue, to green, to purple, to golden yellow. The origin of this temporary phenomenon of iridescence it’s completely natural: typically occurs at dawn or dusk in the autumn months when sunlight interacts with a thin oily film produced by the decomposition of organic material present above the marsh water. Conceptually, it is the same phenomenon that is also observed in other situations of iridescence on thin patinas like on the surface of a soap bubble or on a puddle of oil from a car.
Why Some Swamps Are Iridescent: The Explanation of the Phenomenon
Rainbow marshes were once thought to be the result of urban waste, contamination or pollution, but this is not the case: it is a natural phenomenonThe iridescence that can affect some marshes is due to the interaction between sunlight and the marsh surface under certain conditions.
The leaves and bark of trees (usually cypresses) present in the swamp fall and they rotenriching stagnant water with compounds derived from the decomposition and degradation of organic material. Responsible for decomposition are mainly anaerobic bacteria of the genus Pseudomonas, normally present in these environments, which break down organic matter. These bacteria contribute to the formation of a oily film above the swamp water, because they produce surfactants which lower the surface tension of the water and allow the oil film to expand.
It is this oily film that causes the rainbow effect when hit by the sun’s rays. A similar effect can be observed when a puddle contains some oil that has leaked from a car. This phenomenon is called in technical jargon thin film interference and occurs in the presence of a very thin film that separates two materials. In this case, part of the sunlight (which contains all the colors of the rainbow) is reflected by the oily film upwards and part is refracted to the lower edge of the film, to then be reflected upwards again. If the film is very thin, these two rays are practically superimposed, therefore they interfere with each other. For some wavelengths of sunlight, this interference strengthens the corresponding colors, while for others it weakens the corresponding colors. It is this geometric game that produces the colored bands that give certain swamps the effect of a rainbow on the water!
The effect becomes more intense as organic matter accumulates and the water becomes increasingly stagnant and swampy.
When and where to observe the “rainbow swamps”
The conditions for the rainbow effect in swamps are not found everywhere, and therefore this type of iridescence is not observable in all swamps in the world. There are some places in particular where it is easier to find this phenomenon, and in some where the conditions are perfect and this event becomes a regular appointment.
Rainbow marshes are especially visible throughout the autumn season and to thebeginning of winterwhen it increases organic material load lost from the trees and pouring into the stagnant water of the swamp. During these seasons, moreover, theangle of refraction of light is perfect for the phenomenon to reveal itself, especially atsunrise and end of the day. Furthermore, the perfect occasion is to see them in a sunny day that was preceded by a rainy day not too strong.
As for places where this phenomenon is known to be repeated over time, we have to fly to the USAin particular in Carolina (in Congaree National Park), in Virginia (in the Great Dismal Swamp), in Georgia (in the Okefenokee Swamp) and in Florida (in Everglades National Park).
