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I'm trying to figure out the optimal sea depth on an artificial world (a Banks Orbital as it happens, though I think this would also be an important parameter for many other kinds of megastructure). Generally speaking, I think the oceans would not be kilometers deep as they are on Earth, as this would be both expensive and undesirable; making them shallow would be cheaper and better, because biological productivity would be higher with the seafloor nutrients closer to the sunlight from the surface.

I have previously considered depth for shipping. Right now, I am looking at the question of what depth, or range of depths, would maximize biological productivity. I'm really interested in production of edible fish, but overall biological productivity would presumably be a good proxy for that, if that's what figures are available for.

How exactly does biological productivity vary with sea depth? I know it goes very low when it's deeper than 200 meters, that being about as far as sunlight can penetrate at all. Is shallower always better? Is there a threshold such that 'at least this shallow' is optimal? Is there an optimal depth such that any shallower makes productivity start going down again?

(And yes, I'm aware that the builders don't strictly have to care about biological productivity; if you have the technology to build a Banks Orbital, you could use nanofactories or suchlike to produce edible protein. I'm postulating a scenario in which they do care at least somewhat, in which it is one of the inputs to the decision-making process.)

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    $\begingroup$ Are you restricted to a 'natural' ocean? Can you have artificial underwater geothermal vents? Or pipes with macronutrients (like iron, or calcium and phosphorus), or heated water? $\endgroup$
    – user86462
    Apr 7, 2022 at 8:32
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    $\begingroup$ A question related to yours, what temperature zone is the most productive? Tropical, subtropical, temperate or polar? I'd guess tropical? The reason I ask is, aside from intrinsic interest, you might find the optimal depth varies with latitude? $\endgroup$
    – user86462
    Apr 7, 2022 at 8:38
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    $\begingroup$ @SeanOConnor Very good questions! To keep the scope manageable, let's say a 'natural' ocean; no active systems for pumping nutrients into the water. And yes, I would also be interested in knowing how all this varies with temperature. $\endgroup$
    – rwallace
    Apr 7, 2022 at 8:46
  • $\begingroup$ How do you want to measure productivity? Biological productivity per litre of water or Biological productivity per square meter covered? The first might make mangroves a possible solution, the latter would allow for a technically infinitely deep water dept. $\endgroup$
    – vinzzz001
    Apr 7, 2022 at 10:45
  • $\begingroup$ @vinzzz001 Per square meter. Sure, that doesn't instantly disallow arbitrarily deep water – but it does contraindicate it, because we know the deep oceans on Earth, are the equivalent of deserts; they have low productivity per square meter. $\endgroup$
    – rwallace
    Apr 7, 2022 at 11:18

2 Answers 2

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Quoting from this Nature paper

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Due to the impoverishment of low latitude surface waters in N and P, the productivity of the low latitude ocean is typically described as nutrient limited. However, limitation by light is also at work (Figure 2). As one descends from sunlit but nutrient-deplete surface waters, the nutrient concentrations of the water rise, but light drops off. The cross-over from sunlit and nutrient-poor to dark and nutrient-rich typically occurs at roughly 80 m depth and is demarcated by the "deep chlorophyll maximum" (DCM; Figure 2) (Cullen 1982), a depth zone of elevated chlorophyll concentration due to higher phytoplankton biomass and/or a higher chlorophyll-to-bulk carbon ratio in the biomass.

It seems therefore that the peak productivity happens around 80 meters of depth.

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    $\begingroup$ That is helpful, thanks! Though I don't think it's quite saying the same thing. If I understand correctly, it's saying that if the sea is kilometers deep, highest productivity is around 80 meters depth. But that's more like 'best of a bad situation'; it looks from the graph like sunlight is very attenuated at that depth, just that above it in deep water, nutrients are even scarcer. It looks to me like if the sea were only, say, 30 or 20 meters deep, productivity would be considerably greater? $\endgroup$
    – rwallace
    Apr 7, 2022 at 8:31
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    $\begingroup$ @rwallace nutrients are scarcer because the light allows more things to consume them until they are used up. I have no idea why the chlorophyll peaks at 80m instead of at the surface. $\endgroup$
    – user253751
    Apr 7, 2022 at 16:29
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Around 70 meters deep.

Most ocean life is around the coast, or on coral reefs which tend to be close to the surface. This is because coasts and reefs offer lots of minerals and valuable resources, land to hide in, and generally better conditions.

To maximize the productivity, you'll want to have lots of coasts, reefs, and shallow enough water to maximize the amount of light.

That said, as a frame challenge, water is pretty cheap. Most atoms are either hydrogen, helium, or oxygen. There's tons of water out there for any building projects and it's cheaper to have a deep ocean than to have a deep planet, and deep oceans are useful as buffers for planets in terms of storing heat and airborn gases.

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    $\begingroup$ It's true that water is cheap. My reason for thinking deep oceans on a Banks Orbital would be expensive is that they add to the rotating mass, therefore to the force exerted on the unobtainium structural shell, which needs to be thicker to compensate. And we can reasonably postulate that unobtainium is expensive. $\endgroup$
    – rwallace
    Apr 7, 2022 at 9:25
  • $\begingroup$ But you are certainly right that shallow water is more productive, as you say. Doesn't that mean the optimal depth would not be so much 'around 70 meters' as 'not more than 70 meters'? Might better results still not be had from something like, say, 30 meters depth? $\endgroup$
    – rwallace
    Apr 7, 2022 at 9:27
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    $\begingroup$ Water is also much less dense than rocks. seaworld.org/animals/all-about/coral-and-coral-reefs/habitat Most coral reefs are at 46 meters, so you could have it be that deep. $\endgroup$
    – Nepene Nep
    Apr 7, 2022 at 9:29
  • $\begingroup$ Good link! Though again, strictly speaking it says 'less than 46 meters'. $\endgroup$
    – rwallace
    Apr 7, 2022 at 9:38
  • $\begingroup$ Are waters at 70m still as productive without deep water beneath them? $\endgroup$
    – user86462
    Apr 12, 2022 at 10:36

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