Assertion: Oceans are a low producitivy ecosystems despite occupying about 70% of the earth,s surface.
Reason: In aquatic ecosystems, productivity water depth.

Answer

A

Explanation

Productivity levels of an ecosystem depend upon plant species inhabiting a particular area, their photosynthetic capacity, availability of nutrients, sunlight, moisture and a variety of other environmental factors. The annual net primary productivity of the whole biosphere is approx. 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 % of earth's surface, oceans contribute only 32 % of the total productivity (55 billion tons out of 170 billion tons). Thus, oceans are a low productivity ecosystems. It is because in oceans, productivity is limited by light which decreases with increasing water depth.