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 Aquaponics Question - Algae Oxygenate Water? 
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Post Aquaponics Question - Algae Oxygenate Water?
jamvega (Mari) posted this video in the cultivation section (link)



Besides the fact that they should really add a BSF bin or two it's mentioned in the video that algae will oxygenate water for the fish. I knew that algae in the oceans produce oxygen which ends up in the atmosphere so I guess that it's logical that some oxygen would stay in the water but this is the first time I've seen it mentioned.

Have any of you that do aquaculture heard of this?

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Fri Nov 15, 2013 3:23 pm
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Location: Western Australia
Post Re: Aquaponics Question - Algae Oxygenate Water?
Algae is a plant with chlorophyll present in its cells, just as any land-borne plant has, so it is capable of photosynthesizing. Also, similar to other plants it will consume oxygen in the absence of light, but in the case of algae it is dissolved oxygen (DO). I don't know if algae's rate of consumption of DO is the same as its rate of production of DO but, if it is, then one would want more daylight hours than hours of darkness in order to achieve a net credit.
I don't mind having some algae present in my aquaponic and aquaculture systems but it is a concern if it develops into an algal bloom because, when it dies off, it continues to consume DO during its decomposition. Apparently during this die off stage some substance is produced by the algae as it breaks down which inhibits further algal growth - at least for a while, anyway, until the cycle begins anew.


Sun Mar 09, 2014 7:34 am
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Post Re: Aquaponics Question - Algae Oxygenate Water?
To add to what Rooster said, this is one of the problems with chemical fertilizer runoff. The fertilizer gets into streams, ponds and lakes and the algae population takes off. Consequently, the algae produce huge quantities of oxygen during the day and then consume it all at night. When the nightly drop goes below the threshold for the fish, you have a massive die off of the fish. Boom - 1 night and an entire fish population can disappear...


Sun Mar 09, 2014 12:34 pm
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Post Re: Aquaponics Question - Algae Oxygenate Water?
You're right, MRW, and the problem is further compounded by the additional oxygen requirement of the decomposing algae. Algae are short lived and while a bloom is happening there is a concurrent die off of algal cells which produces a significant proportion of decaying organic matter in the water. There used to be fairly regular mass fish kills resulting from algal blooms in a river near my place (the Swan River) until the fertiliser runoff issue was better controlled.

I meant to say in my previous post that I run an air pump on a timer during non-daylight hours in order to keep the oxygen level up at night for my Rainbow Trout, which are more oxygen hungry than most fish.


Sun Mar 09, 2014 11:50 pm
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Post Re: Aquaponics Question - Algae Oxygenate Water?
A few observations come to mind. First is that airstones (underwater aerators) don't really increase the amount of dissolved oxygen as much as one would think. What those bubbles do best is break the water surface tension & allow more of a fish tank's CO2 to gas out into the air. Temperature is another factor; as it rises the amount of O2 capable of being "held" by water diminishes
Plants/algae photosynthesize in light, which results in a swinging paradigm. During the day they use sunlight spectrum + 6 CO2 + 6 H2O to make carbohydrates (C6H12O6) + spare 6 O2. This daytime using up of CO2 in a relatively small volume can actually result in a daytime fish tank reading showing a 0.5 pH rise since sunrise pH.
Meanwhile, come dark of night, the same plants will take the C6H12O6 (carbohydrate) + 6 O2 & put out ATP (energy) + 6 H2O + 6 CO2. This can show a sunrise pH drop; basically elevating CO2 decreases tank water pH.
Fish, likewise, process carbohyradates in what they eat & making ATP means that after eating the fish then release relatively more CO2 into their immediate water environment. Airstone/aeration is again relevant for increasing the amount of CO2 gassing out from the water, which then relatively ups the water pH .
Once fish digestion of carbohydrates is over they are contributing relatively less to the total content of CO2 in their tank's water. And there can be a swing toward a relative drop in water's pH.
To intervene quickly to drop pH in fish tanks that have no algae/plants just pump in CO2. More often people want to quickly raise pH & can simply get rid of a lot of the CO2 by exchanging ~25% of a fish tank's water volume with fresh water.
If you have algae/plants in the fish tank they will keep the pH from rising during day when will scrounge CO2; which is good. Just be sure to have airstone/aeration at night (to foster CO2 gassing out) or pH could relatively decrease from daylight pH.
To complicate the picture somewhat; CO2 + H2O can combine to form carbonic acid (HCO3), which itself decreases the water pH.
One oversimplification is to add magnesium carbonate or bicarbonate as a "buffer", thinking that will also automatically regulate water pH.
However, without enough airstone/aeration too many buffer molecules can actually raise the amount of water held CO2 molecules. It is another chemical shuttling whereby the "buffer" molecules in their reactive function go into solution via the water yielding some HCO3. Then H ions + HCO3 can inter-convert into CO2 + H2O, & back again & back again as CO2 readily receives Hydrogen (as per next post).
This is why homemade freshwater aquarium salt formulations include a little bit of baking soda & not just buffer magnesium from epsom salts. The baking soda helps to progressively gas out CO2; therefore, if you've an algal bloom the pH won't be stabilized with buffers, but may be defeated with sensible use of dissolved baking soda.


Fri Apr 11, 2014 5:38 pm
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Post Re: Aquaponics Question - Algae Oxygenate Water?
Edit: next to last paragraph's last sentence final word is missing & should say "Hydrogen."

Corrected your previous post - BW


Fri Apr 11, 2014 7:23 pm
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