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Imagine you're on vacation in Greece. It's a nice sunny day, so you decide to go for a swim. The bottom of the sea is full of some greenish-brown ugly seaweed, every stone is covered by some fuzz and all around are scattered these things that look like poo and many more things.

That's why I decided to do something. I took control over the world and now I will destroy all life in the oceans! (Muhahahahahaaa!)

Just a little problem - I don't know how. I want to destroy all (or as much as possible) visible life of Earth's oceans in less than, let's say, 10 years

Bonus points for an answer:

  • Not destroying all life on Earth, leaving terrestrial life as untouched as possible (anyway it will slowly die out without the ocean life support)

  • Destroying even deep-ocean and bacterial life

  • Cleaning up after the wipeout and making the seas accessible to humans

  • Doing this as fast as possible

I thought of various methods, like poisoning the sea, but I'm not sure, what to do.

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    $\begingroup$ Sorry, destroying life in the ocean will affect the terrestrial life as well. $\endgroup$ – JDługosz Aug 22 '16 at 21:49
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    $\begingroup$ If you will destroy everything, even bacteria, you will not need to worry about cleanup. Nothing would rot anyway. $\endgroup$ – Mołot Aug 22 '16 at 21:57
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    $\begingroup$ Remove all water from the sea. Then there is no more life in the sea, because even if there is any life left, they are on land! $\endgroup$ – SOFe Aug 23 '16 at 3:22
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    $\begingroup$ "How can I wipe out life in oceans?" Please don't :( $\endgroup$ – inappropriateCode Aug 23 '16 at 11:40
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    $\begingroup$ you just need a hell of a lot of pool robots and detergent. But you might find it easier to build a very large pool and keep that clean. $\endgroup$ – Burki Aug 23 '16 at 15:33
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The answer is (fortunately maybe?) no, you can't. The ocean is home to some of the Earths most hardcore extremophiles that can withstand conditions we couldn't perceive. What is poison to one species is essential to life for another.

The largest extinction event in the globes history was actually caused by oxygen, not lack of oxygen...known as https://en.wikipedia.org/wiki/Great_Oxygenation_Event (oxygen catastrophe is how I know it) started when the first photosynthesis plants started creating oxygen. With nothing to consume the oxygen, this process went out of control and killed the vast majority (98%?) of life on the planet (oxygen is actually toxic to this life). Even if you were to rid the ocean of free oxygen and kill the majority of that life, these little guys https://en.wikipedia.org/wiki/Obligate_anaerobe would be in seventh heaven and begin taking the place of those that died off.

That will follow with any attempt to rid the ocean of life. No matter how extreme you make the conditions in the ocean, there is already an extremophile ready and waiting to thrive in these new conditions.

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This isn't hard science...

The Great Dying at the end of the Permian was a period when most sealife perished. It was caused by anoxia: for various reasons large part of the ocean had no oxygen in them.

You release a microbe that breeds prodigiously, produces an oily secretion, that floats (and rapidly builds up to a great thickness: several metres of rancid oil) and an acid waste product that raises the pH of the oceans. The microbe rapidly uses up the oxygen in the oceans, producing acetic acid, and the oily secretion prevents oxygen from getting in.

Sea life needs oxygen, so the seas are killed.

But then, the population of the microbes crashes as they use up the resources of the sea. The oily surface reacts with the air, becoming denser and sinking to the bottom (where it festers, releasing toxins that poison the sea bed). The surface,however is now clear, but dead.

The death has been dramatic due to the incredible speeds of growth of your microbes. The seas are dead, and we are still here.

We die soon though, since we have just knocked out the most important plants that regenerate oxygen: phytoplankton. So the climate is knocked right out of kilter. Global warming follows since there is nothing to absorb the CO2. The carbon cycle is badly disrupted. Food becomes scarce. Human megadeath occurs.

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    $\begingroup$ And yet, there will still be a lucky few, even in the anoxic oceans. $\endgroup$ – JohnWDailey Aug 22 '16 at 22:15
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    $\begingroup$ This doesn't kill the purple bacteria. But then not much does. Perhaps Domestos? $\endgroup$ – James K Aug 22 '16 at 22:19
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Some of the creatures that live in the Ocean are so durable that the only way you would wipe them out is to dry up the Oceans and irradiate the remaining puddles. But that has a serious effect on terrestrial life.

So my next best thought would be to bio-engineer a series of highly invasive super-creatures (generally micro-organisms) with a kill switch. The aim of at least one of those creatures is to wipe out the lower end of the food chain by either breeding into the main food source(propagating the kill switch) or killing its competitors and damaging its own food-source (so even if the species dies off competitors still cannot survive.) It would probably take many long years but when the creatures have ran their course (when they have completely taken over a link in a food chain) you just trigger the kill switch. Killing the bottom of the food chain off means that the creatures higher in the food chain also die.

Because there are a large variation of creatures at the bottom of the food chain, you would need quite a few variations of predators/invasive-creatures to do the job.

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