I read that isopycnal diffusion is 107 times larger than diapycnal diffusion and this is the reason why the concentrations in the thermocline and below do not propagate to shallow waters. However, I read that even the diapycnal diffusion is 105 times higher than passive molecular diffusion probably due to eddy diffusion. So I understand that the diffusion barrier of the thermocline is not really a barrier but it is actually that the horizontal eddies are much more prevalent than vertical eddies and that the diapycnal molecular diffusion is not really inhibited but it is in reality enhanced due to the vertical eddies.
What then is the reason that concentrations in thermocline and below do not propagate to shallow waters? Is it mostly because of the plankton present in the shallow photic zone that change the concentration with their metabolism rather than that the thermocline poses a diffusion barrier? In other words, if there were no plankton, would the concentrations in the thermocline and below propagate to the shallow waters (ignoring for precipitation and evaporation)?