Carbon is one of the most needed nutrients for a plant. Sure plenty of plants will grow WELL without adding more CO2, but you throw in a ton of light, then the plants will consume the Carbon faster then it can be replenished without injecting it.
It has also been shown that injecting CO2 to a lower light tank will also improve growth as well.
Of course this is just a very small extract on the whole process. There are plenty of resources on the net that would better explain it.
EDIT: This is from BarrReport October 2006:
"In general, many appear to be limited due to low CO2 at the midday to afternoon time
frames. All the CO2 is used up in submersed aquatic macrophytes, in terrestrial systems,
low water potentials can also do this as well as photonhibition (Barko et al, 1991; Corlett
et al, 1994; Copertino, et al 2006). Poor underpowered CO2 delivery can produce such
curves; it adds enough to take advantage of part of the light, but not all. This is quite common
and plants appear not as healthy, they do not thrive to the same degree as other tanks
with non limiting CO2 levels. Some species may have smaller miniature growth towards
the apical tips as well as stunting. With lower light intensities, such curves begin to follow
the radiation levels."
This quote was in reference to changes in plant growth. There is a whole report devoted to just Carbon. If you are one of those people who would like to gain valuable knowledge in the planted realm, I highly suggest getting a subscription to the Barr Report news letter. It's something like 12 dollars a year and you can download ALL the previous issues.
BarrReport April 2005:
"High excess CO2 in water can have deleterious effect on locomotion
and metabolic activity in many animals so measurement of total inorganic carbon
is an important variable in water management. While excess CO2 is often
considered a negative parameter for aquatic ecosystems, when SAM’s are present
in high density, it allows rapid growth and high levels of the by product
O2. This high O2 level mitigates the effect of the elevated CO2 levels (within limits) within the respiration cycle of other organisms. High densities of fish
and animals are found in naturally occurring waters where elevated CO2 levels
are present (Florida and Brazilian freshwater springs) and this has a pronounced
effect on the plant production."
I wish I could post some more info contained in these reports, but I am sure Tom would not like that at all. If you want the in-depth reasons behind CO2, I would look into getting the Issues yourself.
Randy
PS. You also have to take into account the amount of algae growth at higher light levels. If you have an aquarium that is Carbon limited and proved a lot of light, the algae can live in almost any condition. So now you went from growing plants to growing algae.
EDIT 2:
This is from Tropica:
"CO2 is beyond comparison the most important of all plant nutrients. Without sufficient CO2, plants cannot photosynthesize and convert inorganic carbon into energy-rich sugars, starch and all the other carbon containing molecules that constitute a plant. It may seem odd to look at CO2 as a plant nutrient. When growing terrestrial plants, we are used to provide light, water and nutrients but never CO2. This article explains why we need additional CO2 in the planted aquarium, how CO2 is utilized by aquatic plants, how we can supply CO2 and how much CO2 they need."
It can be found here:
http://www.tropica.com/article.asp?type=aquaristic&id=835
BTW, bookmark that site as it contains a ton of good information!
