Biology · Ecosystems · NEET
NCERT gives the exact numbers. The annual net primary productivity (NPP) of the whole biosphere is about 170 billion tons (dry weight) of organic matter. Oceans, despite covering about 70% of Earth's surface, make only 55 billion tons. The rest (about 115 billion tons) is produced on land. So oceans give less than one-third of total NPP while holding most of the area. NEET loves these two exact figures: 170 and 55.
Light is the main limiting factor for productivity in aquatic ecosystems. This is a direct NCERT fill-in-the-blank. Sunlight is absorbed and scattered as it goes down through water, so only the top thin layer (the photic zone) gets enough light for photosynthesis. Below that, it is dark, so producers cannot grow there. Nutrients like nitrogen and phosphorus are also often in short supply in open ocean water, which lowers productivity further.
Size does not decide productivity — rate of photosynthesis per unit area does. On land, plants get full sunlight and grow tall in layers (stratification), and nutrients are held in the soil. In the ocean, only the surface layer gets light, and open-ocean water is often poor in nutrients. So even though the ocean is bigger, most of its water volume is a dark, nutrient-poor zone where producers cannot photosynthesise. That is why land makes 115 billion tons and oceans only 55.
Only the top sunlit layer, called the photic (euphotic) zone, is productive. Here phytoplankton and algae get enough light for photosynthesis. Below this, in the deep dark water (aphotic zone), there is no light, so almost no primary production happens. Since the productive zone is thin compared to the huge dark deep ocean, overall ocean productivity stays low.
Both light and nutrients limit ocean productivity. Light is the primary limiting factor stressed in NCERT for aquatic systems. But open-ocean surface water is also often low in nutrients such as nitrogen and phosphorus, because these sink to the deep water. Where nutrients are supplied (like coastal upwelling or river mouths/estuaries), productivity rises sharply. Most of the open ocean lacks this, so it stays low.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because light and nutrients are limiting — sunlight reaches only the thin top layer, so the huge dark deep ocean cannot photosynthesise, giving only 55 of 170 billion tons of NPP.
About 70% of Earth's surface, as stated in NCERT. Yet they contribute only 55 billion tons of the 170 billion tons total biosphere NPP.
About 170 billion tons (dry weight) of organic matter per year. Of this, oceans make 55 billion tons and land makes the rest (about 115 billion tons).
Light is the main limiting factor stressed in NCERT, but low nutrients (nitrogen, phosphorus) in open-ocean surface water also lower productivity.
Land is more productive overall (about 115 billion tons vs 55 billion tons for oceans), even though oceans cover more surface area, because land plants get more light and nutrients.
