Statement I: Because the oceans occupy about 70 per cent of the earth’s surface, most of the annual net primary productivity of the biosphere comes from the oceans.
Statement II: Primary productivity depends on the plant species inhabiting an area, environmental factors, availability of nutrients and the photosynthetic capacity of plants, so it varies in different types of ecosystems.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Only statement II is correct.
Primary productivity is the rate at which plants produce biomass (organic matter) by photosynthesis in a given area over a time period. Gross primary productivity minus the loss in respiration gives the net primary productivity (NPP), the biomass available to consumers.
Statement I: Because the oceans occupy about 70 per cent of the earth’s surface, most of the annual net primary productivity of the biosphere comes from the oceans. — False. The oceans do cover about 70 per cent of the surface, but their productivity is low. Of the 170 billion tons of organic matter made in the biosphere each year, the oceans give only 55 billion tons, less than a third. The larger share comes from land.
Statement II: Primary productivity depends on the plant species inhabiting an area, environmental factors, availability of nutrients and the photosynthetic capacity of plants, so it varies in different types of ecosystems. — True. These are the factors the book lists. Since they differ from place to place, primary productivity varies from one type of ecosystem to another.
NCERT Extract:
Productivity – “… 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.”