A pond suddenly turns red on its surface with the arrival of summer. Picture by Robert Dryja

The red surface breaks into small squares when blown among grasses growing in the water. Photo by Robert Dryja

By ROBERT DRYJA
Los Alamos

One of the interesting things about exploring the natural world is to observe how things change over time. Some changes are expected. The change of the seasons from winter to spring, to summer, and then fall are one example. The changes in plants as they grow over the summer is another example.

But some changes can be sudden and unexpected. Imagine hiking by a small pond in the forest from week to week and year to year. The pond goes through an annual cycle of grasses growing along its border. The pond changes in depth depending the summer rains. Then something totally unexpected happens.  

The pond suddenly turns an intense red color. A brief look from the distance gives the impression that it is the water that has turned red. A closer look shows that the redness is only on the surface of the water. The water below is still a typical muddy grey color. Further, the redness has two patterns to it. One pattern is a result of the wind blowing across the surface. The red material, whatever it is, is being pushed to one side. The surface looks as if is painted and curves can be seen in the paint. A second pattern is a result of the grasses growing out of the water. The red material beaks apart into small squares among the grasses. What is causing this redness?

One possibility is based on the time of year. A great variety of grasses and plants have been flowering in the spring time. These have created pollen that has been scattered across the landscape. Recent rains then may have washed the pollen down into the pond. Perhaps it is pollen floating on the surface of the pond that is creating the redness. But why is the redness only occurring at this pond? Other ponds a short distance away have not changed color.

A second possibility is based on the droppings left by elk and deer from the prior winter and spring. A large number of droppings may have decomposed with the warmth of summer, becoming a fertilizer.   These then were washed by the rains into the pond. The fertilizer could have been the nutrient for algae that then multiplied greatly in number. But algae typically are green, not red in color.

The third possibility is based on a single celled creature that is a combination of plant and animal.  Euglena have a tail that they use to swim with, similar to a fish. This tail is called a flagellum. They also can photosynthesize like algae. The color of certain species of euglena is red-like and these can live in highly fertilized water. They also may prefer to swim to the surface of a pond in order to receive as much light as possible for photosynthesis. A layer then develops when a large number gathers at the surface. Viewing with a microscope confirms that this is the species that is making the pond’s surface look red.

A microscope shows that the redness is part of a single celled creature that is a blend of animal and plant. A tail called a flagellum is coming out on the top side. The green spots are a part of photosynthesis. A shell-like covering makes a circle around the body of the cell. Photo by Robert Dryja

This red colored pond is one of several ponds that are in line. It is receiving all of the nutrient because it is the first upstream pond. Water is not flowing out of this pond and so it is keeping all of the nutrients impounded for itself.

The bright red color lasts only a few days. The color becomes reddish-brown after a week and disappears entirely in two weeks. The surge in nutrients allows for a surge in the number of euglena. As the nutrients are consumed the number of euglena decreases.