Are my diurnal changes in alkalinity due to coral consumption and/or pH changes

[Randy Holmes-Farley]

Don't forget corals actually make use of kH in the process of photosynthesis. So more will be consumed.

Well, that's not so clear.

photosynthesis itself consumes no alkalinity. That is not why alk is consumed during the day.

It is not clear if the higher demand for alk during the day is because the pH in the bulk water is higher (due to photosynthesis raising pH) or if it relates to internal processes where corals use the higher internal pH to driver more calcification (which does use alk).

it may be a combination of both of these in a typical reef tank.

 

[J1a]

Well, that's not so clear.

photosynthesis itself consumes no alkalinity. That is not why alk is consumed during the day.

It is not clear if the higher demand for alk during the day is because the pH in the bulk water is higher (due to photosynthesis raising pH) or if it relates to internal processes where corals use the higher internal pH to driver more calcification (which does use alk).

it may be a combination of both of these in a typical reef tank.

I agree the alkalintiy consumption is a result of complex interactions of many processes happening at the same time.

I think bicarbonate ion is a source of inorganic carbon in marine systems. But I would also agree that the degree of impact of this process most certainly varies from one aquarium to another.

Inorganic carbon uptake for photosynthesis by the symbiotic coral-dinoflagellate association II. Mechanisms for bicarbonate uptake

Mechanisms of HCO3− uptake as a source of dissolved inorganic carbon (DIC) for photosynthesis by the intracellular symbiont, Symbiodinium sp. were stu…

www.sciencedirect.com

 

[Randy Holmes-Farley]

I agree the alkalintiy consumption is a result of complex interactions of many processes happening at the same time.

I think bicarbonate ion is a source of inorganic carbon in marine systems. But I would also agree that the degree of impact of this process most certainly varies from one aquarium to another.

Inorganic carbon uptake for photosynthesis by the symbiotic coral-dinoflagellate association II. Mechanisms for bicarbonate uptake

Mechanisms of HCO3− uptake as a source of dissolved inorganic carbon (DIC) for photosynthesis by the intracellular symbiont, Symbiodinium sp. were stu…

www.sciencedirect.com

Yes, bicarbonate is the source of the carbon used in photosynthesis by most corals, but they spit back out the OH-, thus not consuming the alkalinity that was "attached" to the CO2 they actually consume:

HCO3- ---> OH- + CO2 (internal to the coral)

OH- ---> back to the bulk water
CO2 ---> consumed in photosynthesis

I discuss these issues in extensive detail here:

Photosynthesis and the Reef Aquarium, Part I: Carbon Sources by Randy Holmes-Farley - Reefkeeping.com

 

[J1a]

Yes, bicarbonate is the source of the carbon used in photosynthesis by most corals, but they spit back out the OH-, thus not consuming the alkalinity that was "attached" to the CO2 they actually consume:

HCO3- ---> OH- + CO2 (internal to the coral)

OH- ---> back to the bulk water
CO2 ---> consumed in photosynthesis

I discuss these issues in extensive detail here:

Photosynthesis and the Reef Aquarium, Part I: Carbon Sources by Randy Holmes-Farley - Reefkeeping.com

Very comprehensive discussion

So yes, the net effect is zero once the OH reequilibrated.

 

[Sabellafella]

I am using kalkwasser continuously, which can drive the pH to 8.5 - 8.6 during the day and keeps it at 8.2 during the night. The alkalinity measures 8.2 dkh in the morning and 7.9 dkh in the evening. Ignoring that pH will also affect the alkalinity measurement, where did that 0.3 dkh go to? Was it consumed by the corals, or is it possible that it fell irreversably out of solution because of the high day-time pH?

Well the rise in your ph during the evening indicates corals/stuff photosynthesising. Of course you would see a fluctuation in alkalinity, lower in the afternoon, higher during the lights off. That's If alkalinity is supplied through the 24 hour period.