SantaMonica said –“But the atmospheric source cannot keep up with rapid consumption by algae. So the algal growth slows until equilibrium with the source is reached.”
Karimwassef said – “Atmospheric CO2 is sufficient. Why wouldn't it be?
seawater at a pH of 8.2 has almost no CO2, or carbonic acid.
The carbonic acid that is formed, when carbon dioxide hydrates, very quickly equilibrate into the water's carbonate buffer system, converting into both bicarbonate and carbonate by releasing protons (H+). The primary factor that determines the relative amount of each species at equilibrium in seawater
is the pH, with a small temperature effect as well.
Bicarbonate is then the predominant source of CO2 for seawater photosynthesising organisms.
Many marine algae and other organisms take up some measurable portion of the carbon dioxide that they incorporate during photosynthesis.
But, in most cases this process can account for only
a portion of the demand for carbon dioxide.
The rate at which carbon dioxide is used by rapidly photosynthesizing organisms
is fast enough that organisms can deplete the carbon dioxide in the surrounding seawater
faster than it can be replaced by diffusion and other transport mechanisms through the seawater.
So, if the incoming CO2 is rapidly being used up before it is hydrated into carbonic acid, & then consequently bicarbonates, the alkalinity falls (but stabilizes at 125ppm in my case).
http://reefkeeping.com/issues/2006-10/rhf/index.php