Euphotic zone

The euphotic zone, also called the sunlit zone, is the illuminated surface layer where enough light is available to support photosynthesis. It is not a fixed-depth layer: particles, dissolved substances, plankton, water colour and surface conditions affect how quickly light is absorbed or scattered. NOAA describes the upper 200 metres as the ocean’s euphotic or sunlight zone and notes that sunlight may travel farther under suitable conditions, while significant light is rare below that depth. Thus, 200 metres is a useful general guide rather than a universal boundary.

Light allows photosynthetic organisms, including phytoplankton, to convert carbon dioxide and water into organic matter. Phytoplankton form the base of many marine food webs, supporting zooplankton and larger animals, and photosynthesis releases oxygen. The zone also matters to fisheries because many ocean organisms feed, grow or spend time in its comparatively well-lit waters. On continental shelves, the sunlit layer may extend to the seabed; in the open ocean it occupies only a thin upper layer above much deeper water.

Ocean descriptions often distinguish the euphotic zone from dimmer and darker waters below it. NOAA uses the dysphotic, or twilight, zone for approximately 200 to 1,000 metres, where light decreases rapidly and is insufficient for photosynthesis, and the aphotic zone for waters below about 1,000 metres where sunlight does not penetrate. These depth ranges are broad conventions, not sharp physical boundaries; light diminishes gradually with depth.

The terms euphotic zone and photic zone are sometimes used interchangeably in general explanations, but scientific definitions can vary with context. The traditional euphotic concept focuses on light adequate for photosynthesis. A 2025 paper in Communications Earth & Environment argues that studies of the wider photic zone could include biological processes influenced by celestial light even when it is too weak to sustain photosynthesis, and proposes measuring light by biological response. This broader proposal does not change the conventional use of euphotic zone for the photosynthetically illuminated layer.