[alexander_busek]

If oxygen pressure is higher at the surface, it diffuses to lower levels independent of temperature differences.

...but NOT independent of convection, right? And temperature differences would promote convection, right? Water reaches it max. density at 4.5 °C, so heating the uppermost layer of (oxygen-rich) water having a temperature of, say, 1 °C would cause it to sink, no?

Regards,

[reasonisfaith]

I think the gradient in oxygen pressure causes movement of oxygen molecules towards equilibrium independent of convection. But it’s been many years since I studied this, so please forgive me if what I say is wrong.

[miniTAX]

.but NOT independent of convection, right? And temperature differences would promote convection, right?
temperature difference triggers convection only if high temperature is at the bottom. In oceans, high temperature is at the top because the heat source (sun) is at the top, so there is no convection in oceans. It's like trying to boil a pot of water from the top, there will be no convection cell. There is though localized mixing from wind, conveyor belts currents...