Why are oceans said to have "low productivity" in terms of photosynthesis?

Question

80% of the world's photosynthesis takes place in the ocean. Despite this, oceans are also said to have low productivity - they cover 75% of the earth's surface, but out of the annual 170 billion tonnes of dry weight fixed by photosynthesis, they contribute to only 55 billion tonnes. Are not these two facts, which I have come across separately, contradictory? If oceans fix 80% of the total $\\ce{CO2}$ fixed by photosynthesis on earth and release 80% of the total $\\ce{O2}$ released by photosynthesis on earth, they should have accounted for 80% of the dry weight produced as well. Is there any way to reconcile these facts? In any case, if 80% photosynthesis occurs in oceans, that hardly seems *low* productivity - then why *are* oceans said to have low primary productivity (a host of reasons are also given for this - that light is not available at all depths in oceans, etc.)? A large amount of photosynthesis taking place should mean a large productivity!

Answer 1

This is a very deep (and very interesting/thought provoking) question. To put it simply, it depends upon the scale used to measure primary productivity. When it comes to measuring net primary production( or NPP) per unit area, estuaries and swamps have the largest NPP. However, if you are measuring the average total amount of NPP in the world, oceans would have the highest NPP, since like you had mentioned, oceans cover 75% of the earth's surface.

I am no expert in this field, but you might find this website helpful since it goes into more depth regarding what I said:

https://globalchange.umich.edu/globalchange1/current/lectures/kling/energyflow/energyflow.html

Good luck!