Many algae are planktonic, i.e. they float or actively swim in the open water of lakes, ponds or puddles, and some have the ability to accumulate at the surface of still or slow-flowing waters. Planktonic algae are often referred to as “phytoplankton”. These groups have various mechanisms to prevent them sinking too deep (where there may be insufficient light for photosynthesis) such as gas vacuoles within the cells, propulsion mechanisms like flagellae (whip-like tails in various groups) or raphes (in diatoms) allowing these groups to swim/glide through the water, or simply the combination of small size and neutral buoyancy.

Planktonic algae that discolour the water are often seen as undesirable, but they are important components of aquatic food chains, providing food for zooplankton and small invertebrates, that in turn provide food for important fish stocks. Planktonic algae are also used extensively in wastewater treatment ponds because they can absorb nutrients and undesirable contaminants, binding them into less harmful organic matter.

Many algal groups live around the shallow margins of lakes, wetlands and ponds, including groups that are not well suited to open-water habitats. Many types of filamentous algae can attach to (or become entangled amongst) the bed or to aquatic plants in these shallow zones where there is always sufficient light for photosynthesis. These algae create the green or brown “fuzz” often seen on submerged surfaces in marginal habitats.