Re:NO3 uptake rates mass balance

[Thomas Barr <tcbiii at earthlink_net>]

> I'm new here.  I'm a chemical engineer and have worked with a lot of water
> treatment projects.  I'm a bit skeptical of some of what I read on the
> internet of aquariums, but enjoy reading about the biology of a planted tank.
> The problem with biology is the lack of quantitative data.

Problem?
Biology and certainly plant science, is far from merely "descriptive" in
approach. Pick up a journal of Plant Physiologist, Plant Science etc. Plenty
of graphs and analysis with quantitative data.

It's not all theoretical physics, although....... the development of
fluorescence for measuring Chl a is from an physic background, but it's
pretty much core quantitative data.  Statistical analysis in biology can get
down right ugly. Hydrological modeling and ecosystems modeling are not far
apart. The study of fluid dynamics/motion (Kinematics) for instance is
useful in studying meristematic growth in plants.

Let's get to the meat on the bone here:

> I uptake of 
> Nitrate by plants is over estimated.
> Nitrate contains about 22.6% nitrogen.  A 100 gallon tank with 30 mg/L nitrate
> contains  2.7 grams of nitrogen.

N= 2.567 grams, cool so far.
 
> Now a dry plant contains 10% N and a wet plant contains 98.5% water.

Potamogeton has about 8% dry weight to wet weight ratio(APIS). Assuming that
8% dry weight is 1.5% N, this means that for every kg of Potamogeton
produced wet weight(the amount of water should be fairly close since they
all have the same access in the area of water relations being that they do
live in water), we would get about 8% of that in dry weight?
1000grams x 8%= 80 grams of total mass.

80 grams of which 1.5% is N => 1.2 grams of N in plant tissue for each Kg
wet weight of Potamgeton.

In one week I could easily fill a couple of buckets of this weed that would
weigh more than a kilo in a 110-120 gallon tank.
You come out at 4 pounds. I get about 4.7 pounds or 0.377 pounds dry.

> So for 
> the plants to consume 2.7 grams Nitrogen they must grow 10 times that or 27
> grams (dry).  With the water this is 27/1.5%= 1800 grams.  Remember Nitrogen
> is an element it can't be broken down (short of nuclear fission!) so what
> comes out= what goes in  minus what stays in.

Basic 2 box model.
 
> In other words for plants to clean a tank with 30ppm NO3 to 0 ppm NO3 the
> plants would need to grow 1800 grams or about 4 POUNDS WET.  If I'm wrong on
> these calculations it would be the estimates for % H2O in an aquatic plant and
> the % N in an aquatic plant but I shouldn't be off by much.

You are not. You sound right to me.

Something like "this is impossible and the uptake rates of 3-8ppm are
impossible?"

The observations in a healthy tank with CO2, good lighting, good nutrients
and fast growing "Weeds" as most aquatic plants are considered to be with
good reason show some rather high numbers. My own tank's have uptakes in the
3-5ppm range. A few reports by some very credible folks using high grade
testing equipment at Monterey Bay Aquarium had 8ppm drop in one day. I kind
of doubt that would occur over an extended period of say 2-3 weeks but if
you had the right plants and perhaps a large tank which he did, a single day
drop like that does not seem to unreasonable. I was a bit aghast at first
but then considered it and looked for an explanation for the observation.

I suggest folks use Lamott test kits for NO3, always have. Reason being
_better reliable quantitative_ measurements:-) Unlike some biologist:-)

I think your sediment was similar to my own at first. Many still feel this
way about PO4. I routinely sold 25 bags of weeds at the local SF aquarium
society for about 4 years every month and had a huge excess after 10 days
later. The middle of the month I could have easily taken the same amount out
again for sell. That was for a 90 gallon, and a couple of 20's. The other
tank never has anything removed for sale.
The weight was well over 5lbs since I shipped plants to folks via mail
during the middle of the month many times. 6-7 lbs I recall on a few.

Aquatic plants are weeds if given the nutrients, CO2 and lighting.

Regards, 
Tom Barr
> 
> Dave Kysar