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Originally Posted by DNA
Two months ago I had Ca at 350 and Alk at 5 and almost no dinos left.
Now it's Ca at 380 and Alk at 7 and the dinos are blooming again.
This is quite a thin line.
The levels took only a week to reach where they are now and they have hit the titanium wall.
Looking at my logs for the last 4 years the only times they have peaked across the wall is with chemicals on top of my oversized Calcium and Kalkwasser reactors.
I measure my Calcium reactor occasionally and last time it read:
Alk at 29 and flow 30ml/minute
Is anyone able to solve this mystery?
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Chemistry isn't my thing, but this has been at the back of my mind for a while, so I'll take a stab at it.
Given that you haven't reported precipitates in your system or your sand bed turning to concrete, the only other sink for all the missing calcium that I can think of would be biological... Are you still getting the dusting of tiny shells and calcareous debris you previously reported, or the barely visible haze in the water that you thought might be coccolithophores?
Biological calcification has evolved again and again in algae because the following chemical reactions tie the calcium cycle to photosynthesis, allowing primary producers to alleviate CO2 limitation...
calcification (requires an alkaline environment)
Ca2+ + CO2 + H2O ----> CaCO3 + 2 H+
bicarbonate conversion (requires an acidic environment)
2 H+ + 2 HCO3- ----> 2 CO2 + 2 H2O
photosynthesis ("CH2O" represents sugar)
CO2 + H2O ----> CH2O + O2
combine the above 3 reactions and the CO2, H2O, and H+ all cancel out
Ca2+ + 2 HCO3- ----> CaCO3 + CH2O + O2
There are dinos that can calcify, but they only seem to do it on special occasions -- some pelagic dinos can form calcareous dormant forms that seem to be their version of the cysts formed by benthic and shallow water coastal species, and some cyst-forming species form calcareous cysts. In both cases, the point of the exercise seems to be to up-armor the dinos when they're dormant and unable to flee or defend themselves.
But generally speaking, dinos aren't calcifying organisms, which means that they must be escaping CO2 limitation by using bicarbonate for photosynthesis:
H+ + HCO3- ----> CH2O + O2
Trouble is, the H+ required for this normally comes from H2O or HCO3- which leads to surplus OH- and CO3- and rising pH. And of course, drawing down CO2 levels raises alk by triggering the dissociation of HCO3- into OH- and CO2 as the system maintains carbonate equilibrium.
My guess is that the dinos are creating a high-alk microenvironment around them which favors calcification, and judging by the amount of calcium you're pumping into your system and the tiny shells you're seeing, there are organisms taking advantage of it. When you draw down alk far enough, you put this part of the dinoflagellate holobiont out of business. Without calcification reactions releasing protons (H+ ions), the dinos lose a major source of protons for bicarbonate photosynthesis and must rely more on CO2, limiting their growth.
Interestingly, even though photosynthesis creates the high pH conditions, calcification has been observed to persist in reef sands in the wild for up to 7 hours after photosynthesis ceased due to darkness. In the morning, it only took about an hour for the chemistry in the surface layer of the sand to tip back to an alkaline environment favoring calcification.