Freshwater oceans

[someone_else]

In a discussion with a friend about a (silly) cause for planetwide destruction, he said:
"suddenly oceans become fresh water" (not 100% pure distilled water, mind me)

And that's cool. In a swift move, you kill most stuff living in the oceans and cause panic and madness in the ones feeding from that.
Assuming human civilizations manage to survive this someway, we went on thinking on how would look like a post-apocalyptic scenario set in there.

The first thing that I thought is about salt. We and most living beings need salt (NaCl) to live, around 3 grams or so per day for your average Joe.
I went on wildly speculating that the oceans would become a huge salt sink and that that will slowly eat up the nutrients and salt coming from the rivers, and that such loss would eventually change/destroy life on the shores and close to any river.

Am I correct? If not, what would happen on living beings on land (I mean mostly the ecosystem)?

[spartasman]

Well considering that algae that lives in saltwater oceans supplies a great deal of the Earths air supply, I imagine we'd all suffocate to death eventually. Also, something about changing the oceans currents and global weather bla bla bla.

[Broomstick]

I think the lack of algae generated oxygen will become a problem long before the land is sufficiently depleted of salt so as to cause nutritional problems. There's quite a bit of salt in typical dirt - salt washing from the land is believed to be a major source of salt in the oceans in the first place.

With the collapse of fisheries there could be massive starvation in parts of the world heavily dependent on seafood.

Anywhere near the shore is going to reek from the scent of decay as dead sealife washes up. Even what isn't killed outright by the sudden change in salinity will probably choke on decay gasses and bacteria. Much of which will deplete the water of oxygen.

IF humanity survives that they'll face a much-depleted world. Massive die-off of the human population, probably, especially if oxygen drops so low we have to extract and concentrate what's left from the atmosphere to stay alive. At that point you're looking at most of the animals dying off. We'd all wind up vegetarians, more or less, but the eco-system will be in such chaos it's an interesting question as to how much edible food will be produced.

If you get massive, massive amounts of things like methane due to all the dead stuff decaying you're looking at something that makes current global warming look trifling. If massive die-off of landlife results in a higher reflectivity you could wind up with temperatures plunging. You could have one of those effects follow the other.

Basically, a truly mass extinction event, maybe on par with the Permian-Triassac Event which is believed to have killed of 96% of marine species and 70% of all land species at the time due to a huge die-off in sea life, sharply diminished oxygen, oodles of stuff like methane gas screwing up the atmosphere, and possible global warming/cooling in response. It wouldn't happen all at once, but one the ball got rolling it wouldn't be stoppable. It would be interesting to debate if humanity's technology is sufficiently advanced to permit groups to living in artificial biospheres until the Earth is compatible with human life again or if the Earth would have to be re-terraformed or, for the transhumanists, if humanity could be altered to live in the new environment.

[General Zod]

Since the ecosystem is so interdependent, it seems to me like the planet would be completely fucked. I'd expect global die-offs due to a food shortage and climate change within a century aside from the few lucky species that manage to adapt.

[Molyneux]

Humanity's gone, fast, as well as probably most other land-based life; we don't do too well without oxygen. Maybe some few of us manage to work up enclosed sources of oxygen in time to survive, but I doubt it. Maybe if we were twenty years down the line, we'd have a better chance.
Pretty much everything in the oceans either dies, or suffers major population drops.
Freshwater life starts spreading out to colonize the new frontier, but they may be hampered by the massive atmospheric changes and the presence of god only knows how many tons of dead sea life there are.

[Broomstick]

However, the carrion eaters, particularly those that do OK in low oxygen environments, will do well. At least in the short term.

Long term you'll see some generalists survive (hello, rats and cockroaches!) which, combined with technology, might enable some of the human species to survive as we, too, are generalists. The problem is we're BIG for animals, and in these sorts of events big animals don't do as well as small animals.

[Darmalus]

Wouldn't the death of the oxygen producing life, combined with the surviving bacteria eating the dead stuff and using up what oxygen was already there, result in a sterile, oxygen-free ocean?

[Broomstick]

Even if the ocean is very low oxygen (it wouldn't be entirely oxygen free, as photosynthesis would undoubtedly continue on land) the ocean contains anaerobic life that could colonize the anoxic areas. Granted, we're talking about exotic (to us) pond scum but it's still life. Not sterile at all. Just very different.

[Modax]

The amount of oxygen in the atmosphere is enormous. According to wiki, there is about 10^18 kg. that's 100,000,000 kg of oxygen per human and human sized animal. since it we can live breathing less than 20% oxygenated air, (thats why CPR works) it would not get used up right away. I'm guessing it would take years. (if there was quickly some kind of world wide moratorium on non-essential combustion)

[Broomstick]

Oh, sure - we don't all drop dead instantly. It's quick on geologic terms, that's all. That's why we might be able to survive using technology, at least in small, isolated groups. There would be time to set up refuges if we weren't total dumbshits. Given the average level of stupidity in the world it's far from guaranteed, though.

[General Zod]

Freshwater life starts spreading out to colonize the new frontier, but they may be hampered by the massive atmospheric changes and the presence of god only knows how many tons of dead sea life there are.

I'd expect you'd have a lot of carrion eaters thriving in the new environment. That plus natural decomposition ensures that the corpses won't be too big of a problem.

[LaCroix]

Sorry, but wouldn't enough freshwater algae be washed into the ocean by rivers? They now die off once the salt levels are too high, but as they are now flooding into a usable water body, they will multiply quickly. I say these 'new' algae will claim the ocean long before the oxygen level will be massively decreasing. Also, the greenhouse effect of such a scale of decay will improve the living conditions for algae massively.

Fish will migrate into the ocean, crocodiles will follow and feast upon the carrion, and then a new ecosystem will form. We will see a new kind of marine flora and fauna, but the world won't end. I don't know how much sea fish is used per annum fo human consumption, but I doubt it will lead to a worldwide starvation. Famine? Yes, but not a human extinction sized one.

[General Zod]

Sorry, but wouldn't enough freshwater algae be washed into the ocean by rivers? They now die off once the salt levels are too high, but as they are now flooding into a usable water body, they will multiply quickly. I say these 'new' algae will claim the ocean long before the oxygen level will be massively decreasing. Also, the greenhouse effect of such a scale of decay will improve the living conditions for algae massively.

Fish will migrate into the ocean, crocodiles will follow and feast upon the carrion, and then a new ecosystem will form. We will see a new kind of marine flora and fauna, but the world won't end. I don't know how much sea fish is used per annum fo human consumption, but I doubt it will lead to a worldwide starvation. Famine? Yes, but not a human extinction sized one.

I don't think there's enough freshwater algae to mitigate the devastating effects quickly enough. You're also forgetting climate change; with the oceans such an important regulator you're looking at massive crop die-offs. If humanity somehow survives it would be at a fraction of its current population.

[LaCroix]

I don't think there's enough freshwater algae to mitigate the devastating effects quickly enough. You're also forgetting climate change; with the oceans such an important regulator you're looking at massive crop die-offs. If humanity somehow survives it would be at a fraction of its current population.

Well, my pool goes from chlorine clear water to algae pond in less than a month, and that's without direct migration path.

Also, we are not talking about oceans gone, but oceans without salt - that won't change their climate regulating properties... Even better, you end up with useful water instead of worthless brine. The difference in irrgation possibilities for formerly not arable land alone is anything but trivial. Pipelines are much easier than desalination.

[General Zod]

Well, my pool goes from chlorine clear water to algae pond in less than a month, and that's without direct migration path.

An ocean is considerably larger, though.

Also, we are not talking about oceans gone, but oceans without salt - that won't change their climate regulating properties... Even better, you end up with useful water instead of worthless brine. The difference in irrgation possibilities for formerly not arable land alone is anything but trivial. Pipelines are much easier than desalination.

I wouldn't be so sure about that. The ocean's salinity is supposedly tied into global carbon dioxide levels.