Despite occupying about 70 per cent of the surface, the productivity of the ocean is
less. Why?

Hint: Productivity, also known as the primary productivity refers to the production of dry weight or biomass by the plants per unit area through the process of photosynthesis. In the oceans, maximum productivity occurs in the benthic zone.

Complete answer:
The earth comprises 70 per cent ocean and 30 per cent land. Sunlight is the major limiting factor in the oceans. As the depth of the ocean increases, the intensity of light decreases, thus decreasing the photosynthetic rate. This decrease in rate of photosynthesis will eventually decline the development of plants and animals thriving in the aquatic environment. Phytoplanktons carry out the maximum primary production inside the oceans. Moreover, Land is a rich source of mineral deposition therefore; the nutrients and minerals are also scarce in the oceans thus retarding the growth and development of aquatic organisms. Also, in the oceans, some small sized, floating autotrophic plants serve as the major producers. But these plants are less vascular in comparison to the terrestrial plants due to which the productivity is also low. Besides, the terrestrial plants have developed advanced structures thus enhancing their capacity of photosynthesis.

Note: The productivity of land is 170 billion tons while the productivity of the ocean is only 55 billion tons. Coral reefs, sea grasses and brown algal beds are the systems in marine waters offering maximum productivity. In cool waters, dense forests are formed by the kelps (brown algae belonging to genera Macrocystis and Laminaria) in the sublittoral zone of the rocky coasts.