1. The conditions of land and in ocean are not same. Primary productivity which is the biomass or dry weight produced by the plants per unit area during the photosynthesis is different for oceanic ecosystem.
2. In ocean, sunlight does not penetrate uniformly at all depths. It is thus the main limiting factor which decreases the rate- of photosynthesis.
3. Decrease in rate of photosynthesis decreases the growth of phytoplankton and then that of the zooplankton. Aquatic plants such as algae are also affected due to lack of sunlight.
4. Minerals and nutrients is also a limiting factor based on location of the oceans.
5. Therefore, there is less productivity of the oceans to about 55 billion tons as compared to productivity on land which is about 170 billion tonnes.
