Surface Pond Film

You step out your door, glance at your pond and see what appears to be an oil slick on the surface of your pond.  What causes such a film or “scum” on the surface of a pond?

Surface films have several sources.  I will introduce you to some of the more common sources and, of course, present unusual sources to fill your curiosity.

The most common source of surface film is surface tension combined with any number of dusty debris items suspended in it.  Surface tension causes items to remain on the surface instead of sinking to the bottom of the pond as they would normally do.  Some insects use surface tension to “walk” on water.  Surface tension can be relieved with light doses of non-ionic surfactants applied to the pond from shore.  The debris can consist of dust, fine sand, pollen, etc.

Another type of surface film is caused by certain algal species.  Planktonic algae, such as diatoms and blue-green algae, can produce a greenish colored bubbles and film on the surface.  As the bloom continues, more algae will appear in the water column as well.  These can be controlled with products such as Crystal Plex, applied every two weeks until the algae is under control.

Another algal species, Euglena, appears as an orange-red film on the water’s surface once the colonies mature.  After it appears as a red film, no treatment will cause it to dissipate.  It will only go away with cooler water temperatures in late summer or fall.  Preventative treatments need to be made in the spring and applied every two weeks to control the Euglena in its green stage.  Crystal Plex in higher doses or high doses of copper sulfate will aid in controlling this nuisance before it turns red.

Other films are caused by the decomposition of dead algae and organic matter.  This can occur at any time you have a temperature change or a large algae kill.   Also, dead fish can release oils into a film.

In rare instances, water run-off from streets can carry engine oils into a pond and produce an oil slick. 

There are several causes of surface films and “oil slicks” on ponds.  Be sure to examine the film thoroughly before pursuing a solution to your problem.