Redfield ratio

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Hello

We are four students from the University of Copenhagen in Denmark who are having trouble figuring out the Redfield Ratio. We are doing an assignment about iron fertilization in the Oceans. We would like to convert the ratio, from an atomic ratio (106:16:1) to a mass based ratio. This is because we have values for NO3 and PO4. So far we reached these results for a mass based ratio.
C:N = 41,1:7,2:1. Is this correct?

Best regards
Sami

 

[Tom Barr]

Well, you are already doing well to realize that the ratio is atomic(atoms), not based on mass.
You got at least 10 points right there avoiding that mistake.

Note, RR is atoms of C: N : P.

So you simply take their mass molar equivalents to convert to mass.

106 x 12.01 for C and 16 x 14.0067 g/mol for N and 1 x 30.9737 for P.

1273.06: 224.1072: 30.97 for mass.
Divide by the lowest no# in this case P(30.9737)

Mass ratio:
41.1 : 7.235 : 1

If you have NO3 and PO4, you'll need to do this for those, not N and P. So include the O3/O4 in the molar calculations.

To go back to atomic ratios, then do the reverse.

 

[Greg Watson]

Here is an old post that Mr. Barr wrote that also might be relevant (note the links are dead so I've made them non-clickable). I also remember Tom writing something awhile back about the ratios varying in the dry weight analysis from different aquatic plant species.

Best wishes,
Greg

Redfield ratio and aquarist mistakes
I've had a dozen or more discussions over this silly mistake.
Hopefully more folks will not make the same mistake in the process.

Redfield ratio is just that, it is not a mass based ratio, nothing is weighed, there are no ppm's, no mg/l involved with the RR. I have no idea why folks have done this, but it keeps getting promoted and the myth only gets worse.

Redfield came up with a general ratio, it's an atomic ratio, meaning number of atoms, not mass. 16:1 Nitrogen : Phosphorus ATOMS

To convert to mass equivalents, you need to multiply the weight of N by 30.97 g/mole P divided by 14.01 g/mol of N= ~ 2.2
So what was once 16:1, is now 7.2:1 N to P by mass.

If we look at research on dry weight tissue analysis by mass, we see ranges from 4:1 to 10:1 with an average about 5-7:1 N.

Not these extremely low limiting P ranges relative to N.
If you want to convert to NO3 and PO4 (note: this does not take into account any NH4 sources, thus this can skew your interpretation and increase the N ratio when you think it's actually much lower-especially true with high fish load/feeding, lower light etc) you need to add the molar weights of NO3 vs N and P vs PO4.

Note: you rarely measure or can measure NH4 waste, so factor that into the ratio as well. Generally, we can use more PO4 than we think based on the ratio alone due to other non tested Nitrogen sources.

Some error examples on the web:

hxxp://www.xs4all.nl/%7Ebuddendo/aqu...dfield_eng.htm

hxxp://www.aquaticplantcentral.com/f...4-ratio-2.html

Seems even telling them about it, trying to make them aware... they still make the same mistake. I'm not sure why.

The ratio in terms of mass, is not 16:1 N
That is atomic, yet they keep making the same mistakes and do nothing to correct their mistakes.

It's one thing to make a mistake(we all do, I sure do), but when you do, try and correct it when it's pointed out of something is not clear. that way you learn and prevent others from making similar mistakes.

I made no mention of the atomic vs mass: but I did look up aquatic plant and algae (FW) atomic ratios:

hxxp://fins.actwin.com/aquatic-plant.../msg00026.html

Still, not being clear, I have since noticed some assumptions that made this issue worse. The folks arguing that lower PO4 is better, a long held belief that I've been a strong opponent of for a decade or more, have often cited this, but cited and misinterpreted it incorrectly.

But let us be more specific with respect to algae:
hxxp://www.blackwell-synergy.com/doi...7.2007.01916.x

Here you can see the massive differences within algae species.
You also see another atom molar reference to RR, not mass.

Such details can throw you off, and can be common mistakes, however, learn from them and correct them when you see them. Aquatic plants, are a group of nearly 400 species we keep, their ratios will differ. Main thing is to prevent strong limitations.

You may do this by reducing the light, which drives CO2 and nutrient uptake.
Light is far easier to change, reduce and control than CO2 or any nutrient.
So light is the most practical variable to change to slow or increase rates of growth, which is not an argument some like to make to support their nutrient limited methods. If it does not support our contention, we should ignore it?
No, we should go back and figure it out and where we could have gone wrong. Then understand it better and fix it and see where the new information leads us. I've made many mistakes but learn from them, so that other hobbyist do not repeat them.

Regards,
Tom Barr