De-gassing of CO2 - observation

[tubamanandy]

Pressurised CO2 running for months although moved over to a Tunze diffuser a week or so ago.

With the overhead (wet/dry) filter on my Aqua-one AR510 tank, CO2 readings varied from 7.09-7.59 almost every day.

Changed the filter a few days ago from the standard overhead wet/dry to an internal filter (NO OTHER CHANGES) and CO2 now varies from 7.09-7.29 (a daily change of 0.3).

Clearly the old filter was causing some CO2 de-gassing - I'm guessing this would be quite normal ?

 

[dw1305]

Hi all,

Clearly the old filter was causing some CO2 de-gassing - I'm guessing this would be quite normal ?

Yes, that is what you would expect. Wet and dry trickle filters are so effective as biological filters because the film of flowing water gives you a large gas exchange surface. Usually this is to your advantage as gases (O2, CO2) equilibriate with atmospheric levels.

The natural levels of dissolved CO2 are very low, mainly because C02 is only about 350ppm in the atmosphere, compared to O2 at ~ 20%, and water is fully saturated with ~ 1- 2ppm CO2. When you add CO2 to 20 - 30ppm you create a steep gradient between the level of CO2 in the water and the atmosphere, and the water de-gasses. The larger gas exchange surface you have the quicker this will happen.

cheers Darrel

 

[AverageWhiteBloke]

Those ar trickle filter are notorious for degassing co2

 

[tubamanandy]

I actually changed the overhead trickle filter to an internal filter a few weeks back for that very reason.

 

[niru]

Hi all,

Yes, that is what you would expect. Wet and dry trickle filters are so effective as biological filters because the film of flowing water gives you a large gas exchange surface. Usually this is to your advantage as gases (O2, CO2) equilibriate with atmospheric levels.

The natural levels of dissolved CO2 are very low, mainly because C02 is only about 350ppm in the atmosphere, compared to O2 at ~ 20%, and water is fully saturated with ~ 1- 2ppm CO2. When you add CO2 to 20 - 30ppm you create a steep gradient between the level of CO2 in the water and the atmosphere, and the water de-gasses. The larger gas exchange surface you have the quicker this will happen.

cheers Darrel

Hi Darrel

as always, thanks for your insight! I have a question: how do plants with high CO2 needs grow in natural systems? Where do they get their CO2 from? We already know that sunlight almost always delivers much higher PAR values than even our highest lighted tanks, so the CO2 needs for such "difficult" species is indeed high. With all the splashing and flowing water in streams etc, doesnt the gas equilibrium tend towards 1-2 ppm CO2 as you mention above? Whats the trick nature has up its sleaves to provide CO2 to the plants before it degasses, and how does it get accumulated to high values in the first place?

Sorry for the thread hijack...

cheers

 

[dw1305]

Hi all,

how do plants with high CO2 needs grow in natural systems?

Emersed, either the whole plant for plants like Hemianthus, as emergents in Staurogyne, Hygrophila, with floating leaves (Limnobium, Nymphaea) or plants that would develop floating leaves if we allowed them to, like Heternathera zosterifloia & Cambomba spp. Also most true aquatic plants won't naturally have stem leaves all the way to the bottom of the stem, they will concentrate their leaves in the surface zone where both PAR and CO2 are more available.

Another complicating factor is carbonate hardness, plants from hard water can utilise bicarbonate (HCO3-) ions, this doesn't mean that they can't use CO2, but this gives them a source of carbon in situations which are naturally low in CO2. You can see this happening in water which combines being reasonably nutrient rich with have high carbonate hardness.

In the UK a "chalk stream" will fulfil these criteria and have a large vegetation biomass. This is the Hampshire Avon (in S. Wiltshire), with Hemlock Water-Dropwort, Water Cress and Water Crowfoot. <Restoration Open Day on the Avon | Cain Bio-Engineering>

With all the splashing and flowing water in streams etc, doesnt the gas equilibrium tend towards 1-2 ppm CO2 as you mention above?

Yes, pretty much. Also all heavily vegetated water will become CO2 depleted during the photo-period, almost irrespective of their flow.

Whats the trick nature has up its sleaves to provide CO2 to the plants before it degasses, and how does it get accumulated to high values in the first place?

As well as the ability to use HCO3-, a few aquatic plants like Isoetes are CAM plants who store carbon in the cells as malate, and a few more C4 plants (Elodea, Hydrilla) which concentrate CO2 (as HCO3-). Most "standard" C3 plants, don't have methods for concentrating CO2 and just combine available CO2 with ribulose diphosphate to form 2 x 3-phosphoglycerate ("3-PG") molecules, (this is where the "C3" bit comes from).

This is why C3 plants show a strong growth response to added CO2, more CO2 diffusing in through the stomata leads to higher CO2 levels in the mesophyll and more carbon fixation, assuming that other parameters aren't limiting. In this non-limiting situation photosynthetic rate is governed purely by incident energy (PAR), (and this was the idea behind EI).

cheers Darrel

 

[AverageWhiteBloke]

Interesting read! Always been a confusing issue to me water flow turbulence and natural water courses. I think it all stemmed from a Dennerle book I read when first starting out. It said and I'm roughly quoting here from memory "If you look at a natural stream plants grow in abundance where it meanders and the flow slows down, the only thing that grows in fast moving torrents is algae" Going on to recommend no more than 1.5x turnover. Now we aim for 10x that we understand that more turnover equates to make sure nutrients and co2 are available to all the plants at all levels. Then on the other hand I have a low tech shrimp setup with very slow growth and little turnover which has plants as healthy as my main high energy tank just not growing as fast. It would seem the plants adapt to either style of tank with the amount of lighting being the catalyst for everything else.

I also saw one of them annoying tanks you come across yesterday Was picking up some furniture for a friend and the person had an AR850 same as mine. They were still using the original trickle filter, normal gravel, no co2 and using a commercial trace mix from the lfs. The lighting hood is exact same as mine but was left on for 2 hours longer. Plants busting the lid off and if you were to take out the skull and the red leds out was on a par with a few jungle scapes I've seen in here. Most annoying

 

[dw1305]

Hi all,

They were still using the original trickle filter, normal gravel, no co2 and using a commercial trace mix from the lfs. The lighting hood is exact same as mine but was left on for 2 hours longer. Plants busting the lid off and if you were to take out the skull and the red leds out was on a par with a few jungle scapes I've seen in here. Most annoying

We used to use trickle filters in the lab for the waste water work with landfill leachate. You can think of the leachate as uber polluted tank water. As long as you can get enough oxygen in and keep the pH above pH7 you get phenomenal plant growth. We only used emergents as in the planted system (to make use of aerial CO2 and transport O2 to the substrate via their aerenchyma), but other systems use submerged algae or macrophytes.

cheers Darrel

 

[niru]

Hi Darrel

some of that was a total new info to me.. Many many thanks for sharing.

Our tanks are typically only half a meter or so deep, and we are told that its the CO2 and not the light that matters the most in keeping the stems as good leafed stems all the way down to the bottom. The plants are much shallower than the tank itself, and are seldom shaded in high tech tanks. I can recall some discussion wherein Clive (or some another mastermind; dont want to take blame on this!) mentioned that the plants, when provided with ample CO2, adjust so that their light requirements can decrease without any degradation of lower stems (bareness, lack of leaves, etc). But in nature (or at least photos of biotopes uploaded on the net) we see native stems having lush & healthy leaves for much lower depths. And these arent Elodea-like carbonate hardness using plants (e.g. L. aromatica, or Rotalas in submerged form). From what you mention, I can understand that the upper regions of these stems are more "lively" than lower regions due to good PAR and CO2 near surface. But the lower halfs are still much better than what we see in our tanks, even the best ones.

So if CO2 in natural systems is indeed much lower than what we inject into the tank, how these systems flourish way better than tanks? Add the high light and its even a bigger mystery (to me). Not sure how much part the flow plays, since its always the flow & never circulation (aka same water reflowing past the plant) that occurs in natural systems.. For the sake of simplicity, I am assuming that nutrients are unlimited for that particular native plant..

Thanks once again.

 

[AverageWhiteBloke]

Perhaps in nature a lot of co2 comes up from decomposing sediment. In our tanks there is very little going on in the gravel even when enriched gravel is being used. Just a thought.

 

[dw1305]

Hi all,

Perhaps in nature a lot of co2 comes up from decomposing sediment.

That was one of the reasons that Diana Walstad gave for adding decomposable matter to her substrate. On pg. 60 of "Ecology of the Planted Aquarium" it says:

The decomposition of sediment organic matter by heterotrophic bacteria releases CO2 and methane into the water. Almost all lakes have more CO2 than what would result solely from their equilibrium with atmospheric CO2. Much of this surplus comes from decomposition in the substrate.

The references given are:
Cole, C. et al. (1994) "CO2 supersaturation in the surface water of lakes" Science 265. pp 1568:1570.
Barko, J. & Smart, R. (1983) "Effects of organic matter addition to sediment in the growth of aquatic plants" J. Ecol. 71. pp161:1725.

cheers Darrel

 

[AverageWhiteBloke]

I've often thought about keeping my plant cuttings and drying them out. Putting them in a blender and making a powder to go under the substrate of new set ups for that reason. Not sure if it would keep though and what problems I could cause by having decomposing matter in the tank. Breaks my heart throwing plants in the bin. Goes in the recycled garden waste bin so not a total loss I suppose.

That was one of the reasons that Diana Walstad gave for adding decomposable matter to her substrate. On pg. 60 of "Ecology of the Planted Aquarium" it says:

I will read that book one day, so many reference's in here come from there.