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

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!

• It'll help if you can provide where you found those two statistics (80% of the world's productivity takes place in the ocean and 55/170 million tonnes of dry weight is produced by the oceans)
– C_Z_
Commented Dec 8, 2015 at 17:58
• Actually both were my [high school level] textbooks. But <a href="books.google.co.in/…> is a reference for the 170 billion tonnes figure. Commented Dec 12, 2015 at 7:12
• BioNumbers suggests that the 170 and 50 Gt values are roughly correct, as measured by the papers cited. Commented May 9, 2016 at 6:11

First, we have to know which are the most important criteria for photosynthesis to occur; these are: light, CO2, water, nutrients. docenti.unicam.it/tmp/2619.ppt Second, the productivity, you are talking about, it should be called "primary productivity" and it is calculated, dividing the amount of carbon converted per area (m2) by the time. ww2.unime.it/snchimambiente/PrPriFattMag.doc

So, thanks to the fact that oceans occupy the larger area of the world, the marine microorganisms can convert lots of inorganic carbon into organic (principle of photosynthesis). A big problem in the oceans is availability of nutrients; these tend to deposit or react with water or other chemical compounds, even though the marine photosynthetic organisms are essentially found on the surface, where, of course, light is present. This reduces as a consequence the photosynthetic productivity potential of oceans.

• You should provide some references to support your answer. Commented Sep 10, 2016 at 17:02

If oceans fix 80% of the total CO2CO2 fixed by photosynthesis on earth and release 80% of the total O2O2 released by photosynthesis on earth, they should have accounted for 80% of the dry weight produced as well.

First, what is meant by "O2 released"? Does it mean "O2 released from the oceans into the atmosphere, where it contributes to a growing surplus"? That can't be the case since the amount of O2 in the atmosphere is pretty constant, and there is evidence that it is significantly lower than in Jurassic times. Overall the global O2 sinks must balance the O2 sources, or if anything must slightly exceed them, resulting in the current gradually increasing atmospheric CO2 levels at the expense of O2 levels.

So by "released" we have to just mean "released by the process of photosynthesis, at the point of its operation".

Oceans fix 80% of the total CO2 fixed by photosynthesis, yes, but they also unfix it at a similar rate. For every algal cell that's photosynthesising, there's one that's dead or dying and being consumed by bacteria (which consume O2), or that's consuming oxygen itself in order to keep its metabolic processes operating at night. So the NET amount of O2 released by the oceans is something close to zero.

Now we have to ask what we mean by "productivity" in this context. If a molecule of CO2 gets fixed due to algal activity but then almost immediately gets unfixed again, does that count as "productivity"? But, blink and you'd miss it! Even if you don't blink, it's hardly likely to be measurable. The dry weight of the algae at the end of the of the process is the same as at the beginning. so if we define "productivity" as "increase in dry weight of algae" then the productivity is zero.

For algal photosynthesis to have an enduring effect on global CO2 or O2 levels, the fixed CO2 has to become incorporated into something less fleeting than algae. Something like cod or hake, which as a bonus can be harvested and placed on tables. "Productivity" usually refers to the power of the oceans to replenish the stocks of these things post-harvest, and that is indeed low when compared to the ability of the land to produce repeated harvests.

It would be a different story if we were to regard algae as potentially suitable for mass harvesting, so that their ability to grow like wildfire in the presence of fertilizer runoffs from the land was regarded as "productivity" rather than as a profound nuisance. But this is not the case.

In other words, we tend to define "productivity" in terms of what is useful to us as a species, and algae are generally not useful.