Despite occupying about 70% of the surface area of earth, the productivity of the oceans is low because-

Nutrient availability is low at the bottom of the ocean as less sunlight penetrates into it.

The correct answer is Option (1) → Nutrient availability is low at the bottom of the ocean as less sunlight penetrates into it.

Primary productivity depends on the plant species inhabiting a particular area. It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems. The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 per cent of the surface, the productivity of the oceans are only 55 billion tons. Rest of course, is on land. 

The low productivity in the ocean, despite its vast size, is primarily due to low nutrient availability in deeper waters, which is linked to lack of sunlight for photosynthesis.

• Sunlight only penetrates to a certain depth (about 200 meters), and beyond that, it becomes dark. Since plants (like phytoplankton) require sunlight to perform photosynthesis, the availability of light is a key factor limiting productivity.
• Nutrient cycling in the ocean is also a challenge. Most nutrients (like nitrogen, phosphorus) are concentrated in the upper layers due to mixing, and the deeper parts of the ocean have lower concentrations of these nutrients, which limits the growth of marine life in those areas.