Why is a giant moss garden found in the fast water of the French King Gorge?

Green plants use light energy to produce sugar and oxygen. Although the process of photosynthesis is very complex and still not completely understood by biologists; photosynthesis may be summarized by the following simple equation:

6 molecules of carbon dioxide + 6 molecules of water + light energy

GIVE RISE TO
1 sugar molecule + 6 molecules of gaseous oxygen

Green plants are, therefore, at the base of all of the food chains within the Connecticut River ecosystem. All animal life is dependent upon the products of photosynthesis.

The rate at which green plants produce sugars and oxygen is controlled by the amount and kind of light and the amount of carbon dioxide. Aquatic plants (plants growing submerged in water) must have photosynthetic adaptations to compensate for low light levels and low carbon dioxide.

It is easy to understand how light is diminished in the river (the sediment level during high water is probably the principal factor), but why carbon dioxide is limiting is not so straight forward.

Air consists of 0.03% of carbon dioxide; a body of water (river, lake, ocean) will be in equilibrium with the air and also have 0.03% carbon dioxide. But to plants living in water, carbon dioxide is much less available than it is in the terrestrial environment. Why? In air, carbon dioxide diffuses 10,000 faster than in water; thus as a submerged leaf depletes carbon dioxide in its immediate vicinity, it takes longer for this depleted zone to be refilled with carbon dioxide from the nearby waters by diffusion.

To compensate for the limited carbon dioxide available in aquatic environments, many different kinds of aquatic plants have evolved the capacity to utilize two sources for carbon dioxide for photosynthesis:

1. dissolved carbon dioxide in the water
2. dissolved bicarbonate ions.

Aquatic bryophytes, alas, are deficient in this regard; they can only utilize the dissolved carbon dioxide in the water. So in most aquatic environments, bryophytes are at a disadvantage.

Where do we find aquatic bryophytes? In fast moving waters (rapids, waterfalls); in such environments the carbon dioxide depleted zones around the plant are not refilled by diffusion but by water movement.

References:

Smith, A.J.E. 1982. Bryophyte Ecology. Chapman and Hall,

London and New York.

Lucas, W.J. and J.A. Berry (eds.) 1985. Inorganic Carbon,

Uptake by Aquatic Photosynthetic Organisms. American Society

of Plant Physiologists, Rockville, Md.

For inquiries contact Ed Klekowski