Breathability of Atmosphere on Ocean Planet

So I'm wondering if it would be possible for humans to live healthily on a largely Earth-like planet that was covered almost entirely in salt water. Landmasses would all be islands, with only a few reaching sizes of, say, 300,000 km$^2$, and the rest being around half of that or less. These landmasses would mostly appear in groups, with only two or three of these groups comprising most of the land surface on the planet. The islands would be varied ecologically, but the variety of habitats would be similar to Earth (regarding plants, animals, environments, etc). All in all, I don't imagine more than about 2% of the entire planetary surface would be land.

I'm mainly wondering what the atmospheric implications would be, i.e., would humans be able to inhabit these islands in large populations (at least so far as natural resources allowed) and actually breathe? Obviously, there would be far fewer trees given the lack of land, so there would be less oxygen produced via photosynthesis. But if the landmasses were grouped closely together, thus lending their collective oxygen output to each other (if that's how it works?), and there were plenty of trees on many of these islands, and there was a substantial amount of marine plants such as phytoplankton in the oceans producing their own oxygen (they already produce around 70% of the Earth's oxygen), would humans be able to flourish?

• I don't see why not, some sort of equilibrium would have been reached and if there is enough oxygen to sustain humans, there is no need for it to change. If you must you could always postulate land plants that produce much more oxygen than earth ones, but probably the amount of shallow water compared to deep ocean would make more difference. – Kilisi Mar 30 '17 at 0:53
• You may not have noticed, but Earth IS almost entirely (~70%) covered in salt water :-) – jamesqf Mar 30 '17 at 4:33
• IMO @kingledion 's answer is spot on, with just one addition from me: you expect fewer trees. I would expect none whatsoever, at least "none" in the way temperate climate inhabitants tend to think of tree. Without significant landmass, there won't be much evolutionary pressure for large tall trees, because the niche is very small and might not sustain separate ecosystem long enough for that to happen. Instead you should expect a lot Mangroves. Likely going far more to the north, due to more stable climate. – M i ech Mar 31 '17 at 14:36
• M i ech, what about landmasses that are larger, though? Say, approximately the size of Italy, or half the size of Egypt. I know that's still not very big when compared to whole continents, but given the dimensions I provided above, there's certainly room for at least a few larger islands. Would bigger trees grow there? Would they require a particular climate? Or would they still not grow much at all? – C. S. Wright Apr 1 '17 at 15:38

Humans will be fine

Fortunately, 70% of the oxygen generated by plants comes from phytoplankton. 71% of the Earth's surface is water, so water and land photosynthesizers are basically tied in oxygen production. I would not expect much difference between your world and ours.

If you integrate over time, the photoplankton have actually been much more important, since phytoplankton were generating oxygen for around 2 billion years before land plants got into the game. They (the phytoplankton) are responsible for bringing the Earth's atmosphere to its current composition, by generating oxygen and driving off the methane. So, again, there won't me much difference between your world and ours, as far as oxygen goes.

• The 70-71% correlation makes sense to me now, as does the steady introduction of phytoplankton over time. Thanks! – C. S. Wright Mar 30 '17 at 1:10

Earth initially got its oxygen from the Great Oxygenation Event, which was a period in earth history when photosynthetic life began to flourish and change the chemistry of the atmosphere. If this planet underwent something similar then you have all the pieces in place. With a large oceans you have plenty of space for that oxygen producing life to maintain a oxygen rich atmosphere.

• Not exactly. The GOE is when Earth's atmosphere first contained significant quantities of oxygen, but that is not the same oxygen that exists today. Oxygen is very reactive, and if it weren't continuously being replenished, it would quickly (in geological timescale) react with other things. – jamesqf Mar 30 '17 at 4:37
• @jamesqf you may be right, however, it is my understanding that oxygen had only been able to build up in the atmosphere in the first place specifically because it had oxidized everything that it could. – Joe Kissling Mar 30 '17 at 4:42
• @JoeKissling: See the oxygen cycle on Wikipedia. Basically, if photosynthesis stopped the 1.4E18 kg of oxygen in the atmosphere would be gone in about 5000 years if we include both biological and non-biological processes, or 250 000 years if we include only non-biological processes. – AlexP Mar 30 '17 at 14:57
• Correcting my answer then – Joe Kissling Mar 30 '17 at 15:14
• @Joe Kissling: Yes & no. I think (though I'm by no means an expert) it's a matter of reaction rates. There were originally a lot of things that reacted really quickly with oxygen, like dissolved iron (thus the banded iron formations &c). Once those things were oxidized away, the oxygen concentration could build towards its present value. – jamesqf Mar 30 '17 at 17:48

Well, positing photosynthesis in the ocean, I don't see why not. Land masses don't have any kind of special effect on the atmosphere. The weather is a different matter.

The weather would not be fun. Without large land masses to take power away from them, hurricanes will be stronger and last much longer. Any land life better be able to dig into or hold onto the land very tightly.

The one thing that land masses (on Earth) are good for is providing nutrients to the coastal communities as soil gets washed down rivers. Though, you can get the same thing from upwellings from the ocean floor.

The main thing to consider is the general depth of the ocean. On Earth the oceans tend to have more biodiversity where they are shallow (which also corresponds to being near land masses).

• Thanks for the feedback! I suspected the weather would be nastier as well, so it's good to hear I'm on the right track. Good to know about the depth thing, too. – C. S. Wright Mar 30 '17 at 1:09
• With only 2% of land mass I suppose your probabilities or been hit by a hurricane would be fairly low. Hurricanes tend to form at certain latitudes only, so it all depends where you place the islands. – Rekesoft Mar 30 '17 at 9:34
• @Rekesoft, hurricanes are generated in tropical latitudes but they can travel north or south from there. Hurricanes hitting New York and areas farther north are not unheard of. I believe that with very little land, that world may not have the stable air and water currents Earth does. However, that goes beyond my understanding. – ShadoCat Mar 30 '17 at 17:15
• @ShadoCat This may help you with that: medium.com/universe-factory/climate-modeling-101-4544e00a2ff2 – Tim B Mar 31 '17 at 12:01

Such a planet may have oxygen.. but the problem may well be CO2.

On Earth, carbon dioxide is regulated on very long timescales by the balance between chemical weathering and volcanic emission. That requires large land areas and especially large mountain belts. Without those, there may be no mechanism for removing CO2 from the atmosphere on geological timescales, so the planet would tend to 'go Venus'. Water worlds do not have stable climates.

• Certainly there's a mechanism for removing CO2: shellfish, and all the other ocean critters that build shells &c from calcium carbonate and such, then die and create limestone & chalk beds. – jamesqf Mar 31 '17 at 18:18
• Would the volcanic dilemma be resolved if there was a lot of underwater volcanic activity? It would be on a notably higher scale than what we see on Earth. – C. S. Wright Apr 1 '17 at 2:21