Discuss the main reason for the low productivity of ocean.

Explanation

The primary productivity is very low in ocean ecosystem comparitively with terrestorial ecosystem.
Unlike terrestorial ecosystems, the majority of primary production in the ocean is performed by feel living microscopic organisms called phyto plancton, large autotrophs such as the sea grasses and macro algae or seaweeds with are generally confined to the littoral zone.
The sunlight zone of the ocean is called the photic zone or euphotic zone, it is a thin layer upto 10 to 100 m near the Ocean.s surface where there is sufficient light for photo synthesis to occur. Light attenuted down the water column by its absorption or scattering by the water itself. Net-photosynthesis in the water column is determained by the interaction between the photic zone and mixed layer. Into the deep water of ocean (Demersal) there is nolight penetration for photo synthesis.
Another factor relatively recently discovered to play a siginficant role in oceanic primary production the micronutient iron. .
The factors limiting primary production in ocean" are also very different from those on land.
However, the availability of light, the source of energy for photosynthesis, and mineral nútrients, building the blocks for new growth, play crucial role in regulating primary production in the ocean.