Question

Despite of occupying $70$ of surface the productivity of ocean is less. Why?

Answer 1

Following are the reasons for low primary productivity of Oceans:
1. Main producers are small floating autotrophic plants which are less vascular than their terrestrial counterparts.
2. Light cannot penetrate beyond certain depth in Oceans.
3. Oceans are not as rich as land in mineral composition.
4. Terrestrial plants are advanced in terms of their photosynthetic capacity.