Algae, including both phytoplankton and periphyton, are the principle photosynthesizers in ponds and lakes. Phytoplankton are found drifting in the water column of the pelagic zone. Many species have a higher density than water, which should cause them to sink inadvertently down into the benthos. To combat this, phytoplankton have developed density-changing mechanisms, by forming vacuoles and gas vesicles, or by changing their shapes to induce drag, thus slowing their descent. A very sophisticated adaptation utilized by a small number of species is a tail-like flagellum that can adjust vertical position, and allow movement in any direction. Phytoplankton can also maintain their presence in the water column by being circulated in Langmuir rotations. Periphytic algae, on the other hand, are attached to a substrate. In lakes and ponds, they can cover all benthic surfaces. Both types of plankton are important as food sources and as oxygen providers.