Algae was added in the chapter: Meet Swampy
Algae is an obstacle that appears in Where's My Water? and Where's My Water 2.
PhysicsAlgae is a plant that usually proves as an obstacle for Water. The consequence being Water causes algae to grow. However, it can be combated by Poison Water, which dissolves it, and Ooze, which turns it into rock. Steam passes through it, and Mud sits on top of it. Balloons cannot pass through it. It also is part of the main conflict in Cranky's Story, the problem being that Cranky cannot eat his food because it is covered with algae.
Algae plays a main role as the antagonist in the episode, Rising Tide, where in its first form, is slug-like and gets Swampy to give it water. Later on, the algae falls into the river, allowing it to grow to an enormous size, taking the form of a giant alligator which captures Cranky. However, Swampy then saved the day and used a pipe containing poison water to extinguish the algae beast.