 The dysphotic zone is also referred to as the "mesopelagic zone", or the "twilight zone".Its lowermost boundary is at a thermocline of , which, in the tropics generally lies between 200 and 1000 meters.The euphotic zone is somewhat arbitrarily defined as extending from the surface to the depth where the light intensity is approximately 0.1–1% of surface sunlight irradiance, depending on season, latitude and degree of water turbidity.