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On a habitable world completely covered in water, with no land above sea-level, there would presumably be storms of biblical proportions. Suppose this world has oceans so deep that the deeper layers just have too much pressure for life of any kind to survive - from the planet's organisms' point of view, the sea would essentially be bottomless.

Without any seabed for shelter/anchorage, but also without any land to be beached on, would gigantic storms, waves etc. be seriously harmful to underwater creatures? When I say "seriously harmful", I mean so damaging as to make the very existence of complex sunlight-zone life on the world dubious.

Two points for further clarification:

• The animals do not need to surface to breathe.


• Yes, they could potentially take shelter in some kind of floating reef/microbial mat, but ignore that for the purposes of the question.

Photosynthesis can take place at depths of about 200m; so, as long as sustained storms do not significantly disturb life at those depths, then life would be fine.

To understand your risks, a wave only causes major disturbances down to about 1/2 the height of the wave, meaning you'd need storms to consistently produce 400m waves to kill off all life. However, in the open ocean (under Earth's gravity), wind can generally not produce a wave taller than 10m because as the wind whips it up bigger and bigger, the wave will begin to crest and gravity will force it to collapse in on itself. For this reason, waves bigger than 10m are almost always causes by seismic activity or from being pressed up on by a continental shelf. So, in your world, even if you had massive super storms constantly raging at the surface, just 5m down you'd have relatively calm water without any continental shelves to exasperate things giving you a 195m save zone for photosynthetic life.

So to answer your question, storms will not inhibit life.

That said, with no readily available access to ground minerals, your world may not be able to sustain life for completely different reasons. If your planet does not have underwater mountains that reach within 200m of sunlight or some other mechanism for uplifting significant mineral content into the upper layers of the ocean, it's unlikely for you to have the diversity of elements you would need for life.

The climate of a planet is primarily driven by the energy imparted to it by its parent star and the difference in atmospheric circulation driven by the poles not rotating and the equator rotating at high speed. The most serious storms would probably occur on rapidly rotating planets with high solar input, but there is a limit to how high wind blown waves can get.

If there is no need to surface to breathe creatures on such a world would be able to avoid the effects of storms simply by diving down a few hundred metres and in most cases by diving just a few tens of metres.

It is hard to believe that wind-blown waves would be able to build to such intensity that this strategy would not work. Adding too much energy to the atmosphere would have the effect creating chaotic conditions in which it would be difficult for sufficiently large well-formed waves to exist as they would be hit by many other waves and variable wind conditions.

No.

• Earth has storms of biblical proportions that happen over its oceans. Perhaps a dolphin or whale is occasionally affected, but that's it. Those storms are a much greater danger to those of us dependent on its surface. Generally speaking, I doubt it matters how big a storm gets.




• Your ocean bottom won't be smooth just because there's no surface land. You'll have ridges, sinkholes, canyons, areas of shallow water and areas of deep water. You'll have currents, waves, and tides (assuming a moon). You'll (presumably, it's your world) have coral reefs. If you draw a shape encompassing our own oceans starting at a depth of, say, 2 meters and encompassing everything deeper, you'd have your world in a nutshell. In other words, there will be plenty of places to hide if you can't get deep enough.




• Your sea life will have evolved on that planet (I assume, you didn't say if they were transplanted or not). They will have adapted to whatever storms your world can throw at them just as all life on Earth has done. Can Mother Nature throw a curve ball that takes out a few creatures (like a wildfire on land)? Sure! But I don't think that's what you're asking about. I believe you're asking about within-the-statistical-norm storms — the kind life would have adapted to. (This is why I don't believe it matters how large your storms get.)




• Earth has depths were no life can live, but it also has a very wide band of depth where life thrives, and at the bottom of that band are depths that wouldn't know a storm was raging no matter how biblical its proportions. Frankly, I'd doubt anything deeper than 100 meters would ever now a storm was in process.