Mudflats occur on low-energy coastlines. Mud and silt, often from nearby rivers, protect the burrowing organisms living in these ecosystems from common shoreline stressors (e.g. high temperatures and desiccation) and predatory shorebirds, crabs and fish. These shorelines are critical stopovers and foraging grounds for migratory shorebirds. Primary productivity is mostly from diatoms (single-celled algae) that rely on tides. Oxygen can be low where sediments are very fine or burrowing or other disturbance is limited.