More than 400 species of Spirogyra are known to be in existence and most are found free-floating in freshwater environments. The algae are particularly abundant in nutrient-rich environments and large populations can be an indication of over-fertilization caused by contaminated storm water. At night or when it is overcast, it may be difficult for the casual observer to determine the level of algae in a pond or other body of water. However, on sunny days, oxygen bubbles produced during photosynthesis cause Spirogyra to rise to the surface of the water, often in thick, tangled mats.