Pond with algal scum

This is Mirror Lake on the University of Connecticut campus during a summer bloom of the green alga Hydrodictyon.

Hydrodictyon reticulatum is fascinating because its cells form a net that is visible to the naked eye. The genus name Hydrodictyon means “water net” and the specific epithet reticulatum means (somewhat redundantly) “netlike”. Green algae are phototrophic, meaning they generate their own food using energy from the sun and carbon dioxide from the air. As a by-product of this process, known as photosynthesis, oxygen is given off, and it is quite common to see bubbles of oxygen trapped by algae forming the scum on the surface of ponds. Hydrodictyon can reproduce very quickly, especially when nutrient levels in the pond are elevated. This alga is a very common cause of extensive algal blooms.

The photo above was created by threading a wire through a piece of Hydrodictyon and hanging it up like a sheet on a clothes line! This makes it clear why the common name is “Water Net.”

The final photo is a closeup, showing how the algal cells are connected end-to-end to form the net.