Light - Marine Ecosystems

Light is a limiting factor in the ocean as it contributes significantly to organic production and distribution of marine life. The ocean is divisible on the basis of the light penetration into two horizontal zones:

1. The lighted photic or euphotic zone extending from the sea surface up to a depth of 200 metres, where sufficient light reaches to support photosynthesis. This photic zone, also called the epipelagic one, is characterised by sharp gradients or light, temperature and salinity. The amount of light reaching the lower level of this zone rarely exceeds 0.0001% of what is received at the surface. Below this is the

2. Aphotic or lightless zone which distinguishable into three further sub-zones

• Mesopelagic which extends from 200 metres to 1.0(10 metres. This zone is in semi-darkness as very little light penetrates it. Here temperature gradient is more even and gradual, without much seasonal variation. Further this zone contains a minimum of oxygen and a maximum of nitrates and phosphates. Next is the
• Bathypelagic zone which extends from 1,000 metres to 2,000 metres and where darkness IS virtually complete, tor humans though some fishes and crustacean do respond to dim light. The third and the lower most zone is the
• Abyssopelagic zone where permanent darkness prevails and where temperature is uniform at 3°C and hydrostatic pressure is enormous.