Hydrogen as the limiting nutrient?

["Roger S. Miller" <rgrmill at rt66_com>]

Folks,

In one of John Raven's books he pointed out that when aquatic plants and
algae use nitrate as their nitrogen source they must also consume hydrogen
ions.  (When plants use ammonia/um as their nitrogen source, the necessary
amount of hydrogen is imported with the nitrogen.)  Hydrogen ions are
usually present at rather low concentrations -- 0.0001 milligrams per
liter at pH 7 -- which gives rise to the theoretical possibility that
hydrogen ion availability could be growth limiting.

Last night while playing with my aquarium computer model I found that when
ammonium concentrations dropped because of bacterial nitrification
hydrogen ion became limiting to growth.  This concept was brought home
rather solidly because I didn't design hydrogen limited growth into the
model and when hydrogen became growth-limiting the model crashed.  I fixed
that.

Then I started thinking, when plants are growing with nitrate as their
nitrogen source, where does the hydrogen ion come from?  It seems that it
must come from the carbonate buffer:

CO2+H2O -> H+ + HCO3-

with the H+ pulled off by the plants.

Over a period of time this mechanism would a) make the aquarium consume
more CO2 than it would otherwise - requiring the aquarist to use higher
bubble counts and b) increase the alkalinity over time.

I don't know how important either of these effects would be.  I didn't
notice these effects in my model aquarium.  My model aquarium is fairly
heavily buffered (8 degrees) and gets weekly 15% water changes so some
other changes in alkalinity may go unnoticed.  It also has pretty good
circulation so the CO2 concentration is controlled more by loss rates than
by internal consumption of CO2.

The possible symptoms of hydrogen ion use sound a little bit like what
Michael Moncur described recently.  I wonder if others might have specific
observations that could indicate hydrogen limitation.  Specifically, I'd
look for:

1) A tank with nitrate is the primary nitrogen source -- probably a
biologically filtered tank and/or a tank dosed with nitrate.

2) CO2 consumption that seems high compared to other aquarists'
experience or your experience on other tanks

3) Alkalinity increasing over time -- more observable if alkalinity is low
and water changes are small or infrequent.

The size of the effect will depend on the planting density and growth
rate, so will be more observable in a heavily planted tank with rapid
growth.

Any thoughts?


Roger Miller

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