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Most people knows that the ocean overall is a carbon sink, but a big part of the sink is due to chemical equilibrium (via dissolution) with the atmosphere (for ref: https://link.springer.com/article/10.1007/s10021-005-0036-3). Not all the (chemically) dissolved CO2 is consumed by primary producers in the ocean (simple deduction from ocean acidification).

In terrestrial ecosystems, we have a term called net ecosystem production (NEP) which is defined as gross primary production minus the total (autotrophic + heterotrophic) respiration in an ecosystem (generally a meaningful timeframe should be at least on an annual basis). I wonder is there any research done regarding the NEP of the ocean? (I am sure there are some out there but I cannot find a conclusive summary of the entire ocean or at least a big part of the ocean). I am assuming this would be harder to measure than terrestrial ecosystems considering how big the ocean is and the big confounding factor of physicochemical processes in the ocean.

Edit: I found somebody trying to answer the same question 20 years ago (Del Giorgio, P. A., & Duarte, C. M. (2002). Respiration in the open ocean. Nature, 420(6914), 379). But in it, it said the answer remained uncertain. I am assuming there would be advances since then.

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  • $\begingroup$ The ocean is a sink and a source of carbon, it depends on the circumstances. E.g. as Antarctic ice shelfs retreat, they expose surfaces for sinking carbon. Otoh, the north Atlantic can switch between sink and source. Maybe a search for "Primary Productivity" helps further ? There is a plethora of work, a summary will be difficult to find because many factors, biotic or not, play a role. $\endgroup$ – user18607 Jan 5 at 14:04
  • $\begingroup$ I posit that one could estimate the capacity of carbon removal attributed purely to physical and chemical means (like from a lab experiment using sterile sea water and induced wave and wind action OR some math derivable from physics/fluid equations). Then it can be compared to the actual carbon removal capacity of the ocean and I guess we can then do some complicated math and modeling that can provide some insights based on ocean models (even if it may be inaccurate, at least they could generate some numbers possibly). $\endgroup$ – y chung Jan 5 at 23:03
  • $\begingroup$ I am also looking at ocean oxygen concentration trend which attacks the problem from a different angle. $\endgroup$ – y chung Jan 5 at 23:16
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Yes, the marine biosphere is a carbon sink, and a very important one. It is known that the oceans produce more oxygen and biologically consume more carbon dioxide than all the tropical rainforests. This is done by microscopic blue-green algae, cyanobacteria and phytoplankton of many kinds. Also by green algae such as seaweeds, by sea grass meadows, and by coral reefs which are built of calcium carbonate synthesised by the corals themselves. Sea grass is an interesting plant, related to land grasses but able to survive and prosper in shallow seas where there is plenty of light and suitable soil. It is very important for the marine ecosphere. As on land, there are marine deserts where there is very little biological activity, and other areas that are teeming with life.

The question arises of how much of this captured carbon remains in the sea. The answer is most of it, but it is impossible to give exact figures. Some of the marine biomass is removed by various means, fishing for example, but that does not mean that all the carbon is returned to the atmosphere. Sea birds consume a lot of fish, and much of their guano is deposited on land, and some of them die on land, but again not all the carbon is returned to the atmosphere. There are enormous deposits of guano on the coast of South America and some Pacific islands, which are mined for fertiliser. The bodies of dead marine creatures, and sometimes live ones, are often consumed by other marine creatures. A dead whale, for example, provides a feast for all sorts of marine carnivores and scavengers. Much of the marine biomass eventually sinks to the ocean floor, where it becomes part of the ooze and sediment.

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    $\begingroup$ Thank you for your answer. However, do you have any reference to back you up because your answer is qualitative in nature but carbon sink/source is a quantitative measure. I would intuitively think that because the majority of the ocean is not in the photic zone and that life is also teeming in the dark, respiration and decomposition would also run rampant. It is a good point you mentioned that some of the fixed carbon in the ocean is consumed by terrestrial consumers which added to the difficulty of arriving at a number (but that still sound like sth that can be estimated). $\endgroup$ – y chung Jan 4 at 17:52
  • $\begingroup$ For obvious reasons there is a lot more life in the sunlit zone of the ocean than in the dark depths. Photosynthetic life is confined to the well illuminated zone. Vertebrate life in the darkdepths is also dependent on surface photosynthesis. Food debris rains down from above and is consumed by vertebrates and invertebrates alike. There are examples of both kinds, squid and vertebrates, which spend the day in the depths but rise to the surface at night in order to feed. A small amount of primitive life forms depend on fumaroles which supply heat and chemicals on the deep ocean bed. $\endgroup$ – Michael Walsby Jan 4 at 18:52
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    $\begingroup$ You still don't understand my point. An ecosystem does not require carbon fixation > carbon release to exist (except in primordial Earth and pioneering community) at any time-scale. Also, environment changes over time and there are time lags for equilibrium to adjust (depending on mixing and reaction rate). Your argument still does not convince me. FYI, many mature forests (especially in the tropics) are net carbon sources. $\endgroup$ – y chung Jan 4 at 19:06
  • $\begingroup$ All life requires carbon to exist, and almost the only way of fixing that carbon is by photosynthesis. The exception is the tiny amount of anaerobic bacteria such as exist around deep sea fumaroles and support a small amount of life which directly or indirectly feeds on them. $\endgroup$ – Michael Walsby Jan 4 at 19:15
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    $\begingroup$ You have decomposition of dead organic matter (which is a large pool for established ecosystems) going on as well in an ecosystem, it is not just photosynthesis of autotrophs and respiration in consumers...Thank you for your input anyway. $\endgroup$ – y chung Jan 4 at 19:18

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