What causes very repeatable ALK swing over a 24 hour cycle?

[SDJustin]

I had a tank 'semi crash' about 9 months ago and I'm pretty sure it was due to an Alk swing (to low) because my calcium reactor was offline for ~3 days. Because of this, I'm very aware about Alk variance and have been trying to tune my Alk dosing to flatten out the curves. One hypothesis is that if I send more Alk (Sodium Corbonate) into the system in the morning to be available to the corals during zooxanthellae photosynthesis, and less at night when they are just relying on respiration, it will help flatten the variance. My lights start at 7am and turn off at 7pm, with the first and last hour being dramatically lower intensity / blue spectrum. This is with the normally scheduled 12am, 6am, 12pm, 6pm 4 times daily Neptune Trident testing schedule. I've since taken the calcium reactor offline (new build actually) and switched to the Balling method / dosing 3 part (BRS Calcium Chloride, BRS Sodium Carbonate, Tropic Marin part C).

<Q1>What is the likely primary biological/chemical cause of Alkalinity 'cycles' like this over a 24 hour day/night period?
<Q2>Is flattening the curve of Alkalinity a goal worth pursuing? Just dose evenly over a 24 hour period and be done with it?

In the below chart you can see my system's:

1. PH swing from 8 - 8.3 with the peak at around 5pm and the minimum at around 7am.
2. Alk swing from 8.25 - 8.65 with the peak between 6am and 12pm, and the minimum usually around midnight (although not consistently).

Thanks in advance for any knowledge!

-Justin

 

[homer1475]

That curve looks about right according to the PH, and lighting.

As your lights ramp up for the day, photosynthesis ramps up, PH goes up, using up ALK, as your lights go down, PH goes down because photosynthesis is ramping down, there for your ALK usage goes down, but your dosing the same so your testing the excess.

If you look at the curves, where your PH is at the peak, your ALK is at it lowest, and vice VS.

Not sure on the second question. To me is seems to be just chasing numbers, as it seems pretty stable right now.

 

[blasterman]

When I read alk in my heavy SPS tanks over 90% of it is consumed during the light cycle. Virtually none is consumed at night.

If I dosed over a 24hour period I would see a spike at night when in fact its not a spike.

 

[arking_mark]

That curve looks about right according to the PH, and lighting.

As your lights ramp up for the day, photosynthesis ramps up, PH goes up, using up ALK, as your lights go down, PH goes down because photosynthesis is ramping down, there for your ALK usage goes down, but your dosing the same so your testing the excess.

If you look at the curves, where your PH is at the peak, your ALK is at it lowest, and vice VS.

Not sure on the second question. To me is seems to be just chasing numbers, as it seems pretty stable right now.

These are my observations as well. I have achieved very stable pH and Alk by:
1. Kalk dosing plus a controlled CO2 Scrubber w/ Skimmer.
2. Refugium on reverse light cycle.

I'm able to maintain pH within a .05 range and there are no day/night swings for pH or Alk. Now my tank is only 5 months old so we will have to see what happens as the tank matures and coral bioload goes up.

 

[arking_mark]

When I read alk in my heavy SPS tanks over 90% of it is consumed during the light cycle. Virtually none is consumed at night.

If I dosed over a 24hour period I would see a spike at night when in fact its not a spike.

I'd be interested to see if this correlated with your pH. In my tank, Alk consumption is directly correlated with pH. As pH goes up Alk goes down and as pH goes down Alk goes up. In many tanks, pH has a day/night cycle and many wonder if this pH swing is why Alk consumption goes up during the day and down at night.

 

[Randy Holmes-Farley]

I agree. It is an unanswered question, IMO, whether the high demand for alk during the day (compared to night) is due to the elevated pH during the day, to effects of the light directly on coral calcification, or a combination of the two.

The answer may vary tank to tank.

 

[SDJustin]

Digging in more... it seems well established that corals calcify faster in light than dark. So, given an even/constant addition of Alkalinity over the 24 hour cycle, it makes sense that Alk is 'consumed' over the daylight hours.

Study of calcification during a daily cycle of the coral Stylophora pistillata: implications for `light-enhanced calcification'

SUMMARY. This work, performed on the scleractinian coral Stylophora pistillata, aims at providing new information on the `light-enhanced calcification' process. In a first step, in controlled conditions of culture and constant light supply, we studied the diurnal cycle of calcification. We...

journals.biologists.com

What's interesting is that we aren't sure WHY this happens (which is what I believe @Randy Holmes-Farley is referring to above.)

It looks like there are 4 leading hypothesis, and it's unknown if it's caused by external environmental variables (pH, light, etc) or by an endogenous (internally caused) circadian rhythm.

For my purposes, knowing that the corals calcify / uptake more during the daylight is as deep as I need to go.

The next question is... if I play around with different dosing rates during the day/night cycles, will it help flatten out the curve? And/or will it just cause ionic imbalances unless I follow suite with Calcium and 'part C' (mostly magnesium)?

 

[Randy Holmes-Farley]

The next question is... if I play around with different dosing rates during the day/night cycles, will it help flatten out the curve? And/or will it just cause ionic imbalances unless I follow suite with Calcium and 'part C' (mostly magnesium)?

Certainly. Many folks with alkalinity controllers keep it very stable, and see that most is dosed during the day.

If you manually mess around with dosing most alk in the late AM to late afternoon, you will likely have much more stable alk than if you dose evenly 24/7.