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A few days back Jeff Kropp asked about something I find puzzling, so I
thought I'd dig the letter up again and repond to what I could.
> 'dave' has also explained how growth rate is determined by a ratio
> of a plants respiration and photosynthesis. So... what I observe when
> watching my pH swings is simply my plants respiration and not necessarily a
> particular amount of photosynthesis resulting in growth.
I think Jeff might be confusing respiration and photosynthesis. What we
see when the pH swings is mostly from photosynthesis. CO2 decreases and
pH increases while the lights are on because the plants are consuming
CO2. CO2 increases and pH increases while the lights are off mostly
because we are adding CO2 and the plants aren't using it. Respiration by
the fish, snails, plants and microbes contributes to the CO2 supply as
Plants respire for the same reason we do - they combine oxygen from the
air with carbohydrates to get energy, and use the energy to grow and carry
on all those wonderful things we call life. Plant growth is the product
of *both* respiration and photosynthesis.
> Is observed respiration pertinent to knowing how well my tank is
> doing? (In my tank I notice a lot of respiration with very slow growth,
> pH6.75 to 7.25 dKH 5.)
> What sort of deficiency or inhibitor might retard growth and still
> permit a lot of respiration?
The only thing that's really pertinent to how well your tank is doing is
whether of not your plants are healthy and attractive. Technical details
are really only useful if things aren't going well and you're trying to
solve a problem, or if you (like me) just have some bizzare interest in
the technical stuff.
What I've seen before and what I think Jeff is talking about here is that
you can sometimes see evidence for a lot of photosynthesis (pH swing,
hoards of oxygen bubbles), but little corresponding growth.
I'd really like to know what causes that. I've seen it mostly under
bright light in tanks without iron or trace element supplements, but also
under flourescent lights in a tank fertilized with potassium and chelated
One possibility is that the plants aren't getting enough time with the
lights out. Aside from that, I can speculate about a few possibilities:
A -- The plants are missing something (K?) they need to get the
carbohydrates from the cells where they're made to the cells where they
need to be for new growth to occur.
B -- The plants are missing some key nutrient (N?, P?, trace metal?) they
need to turn simple carbohydrates into new tissue, so their carbohydrates
either build up or escape back into the water (that might promote algae
growth; there's evidence that algae can "feed" on carbohydrates in the
water as well as on the carbohydrates they make with photosynthesis).
C -- Something is damaging the plants in some inobvious fashion and
the plants are using all of the carbohydrates they can make just to stay
ahead of the damage, so there's very little new growth.
D -- The plants are growing, but the growth is in their roots where you
can't see it, not in their leaves and stems.
E -- (getting real tenuous here) At least some plants use changes in
the quality of light to tell them when to change from daytime to nightime
"behavior". Without that specific stimulus, maybe their respiration could
remain suppressed at night, so that growth might be very limited.
Any other ideas?