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.)