Oceans are vast spreads of water with great depths. Enough sunlight does not penetrate the open oceans. Thus, photosynthesis is limited. Thus, open oceans do not contribute to productivity of ecosystems.
Although the contribution of open oceans to productivity of ecosystem is less, it does contribute to overall productivity of the biosphere. This is because of its huge size and the variety of plant and animal species found in them. Aquatic ecosystem has the maximum variety of species of both plants and animals.