Sunlight in the Deep — Light in the Ocean
Quick Fact
Sunlight can only penetrate to approximately 200 metres in the clearest ocean water — below this depth, the ocean is in permanent twilight or darkness.
About This Species
Sunlight is the engine of ocean life. The photic zone — the layer of ocean where sufficient sunlight penetrates for photosynthesis — extends to approximately 200 metres depth in clear tropical waters. Below this lies the twilight zone (200–1,000 m), where bioluminescence replaces sunlight as the primary light source. Below 1,000 metres, the ocean is in permanent darkness. The distribution of sunlight determines where phytoplankton can grow, which in turn determines where all other ocean life can exist.
Why It Matters
The photic zone supports the phytoplankton that produce approximately 50% of the Earth's oxygen and form the base of the marine food web. Changes in ocean clarity caused by pollution and climate change affect the depth of light penetration and the productivity of the entire ocean.