What organism is commonly referred to as "pond scum"?

"Pond scum" is a colloquial term that primarily refers to certain types of algae, particularly filamentous algae that can grow in thick mats on the surface of water bodies. These algae can form greenish masses that resemble scum, hence the name. Planktonic-type filamentous algae thrive in nutrient-rich waters, often leading to blooms that can cover large areas of ponds and lakes. These blooms are typically a result of excess nutrients, such as phosphorus and nitrogen, entering the water, which promotes the rapid growth of the algae.

In contrast, the other options represent different types of aquatic plants or organisms. Chara is a type of calcium carbonate-algae that can be found in freshwater but doesn't typically create the "scum" effect associated with filamentous algae. Duckweed and water hyacinth are both floating aquatic plants, but they are distinct from the algae that comprise what is commonly referred to as pond scum. These plants do not exhibit the same characteristics or behavior and do not contribute to the same kind of surface coverage that filamentous algae do.

Understanding these differences clarifies why planktonic-type filamentous algae are correctly identified as "pond scum."