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On May 4, Ric Cooney wrote:

OK dave you got me, I was able to look up oligotrophic but DIC thoroughly
evaded me. Could you explain.

It means dissolved inorganic carbon, or basically, free, dissolved CO2.
The plants that Dr. dave listed live in soft water lakes with very little
bicarbonate, nutrients or organic matter in the water.  The amount of CO2
that dissolves in soft water isn't very much, and, also taking into account
the fact that CO2 diffuses through water about 100,000 times more slowly
than it does through air, it means that aquatic plants are severely limited
by low CO2 in soft water.  G. E. Hutchinson, in Fundamentals of Limnology,
Vol. 3, cites research and calculations showing that they get CO2 at only
1/10 the rate they could in air.  The plants that grow in these lakes are
small, low-growing and have remarkably large, air-filled roots.  Way back
in the '30's Birge and Juday, the founders of the science of limnology, did
some experiments on the plants in a soft water lake in Northern Wisconsin.
They dumped in all the known mineral nutrients, but found no increase in
the growth of the plants.  Finally, they dumped in a bunch of soybean meal,
and that did cause a big increase in growth rate.  Birge and Juday were
able to rule out mineral nutrients released from the soybean meal, and the
only thing left was the extra CO2 generated by its decomposition.  I can't
recall for sure, but I think they went back and partitioned off a part of
the lake and bubbled in CO2 from tanks and got the same big growth rate

Birge and Juday showed that the plants were CO2 limited, and later others,
with whom Dr. Dave is familiar (and I am not),  showed that the plants were
getting a good part of their CO2 by way of their roots which extended into
an organic substrate and picked up CO2 produced by bacteria oxidizing the
organic matter.  Not only do the air channels of the roots transport CO2
from the substrate to the leaves, but they also transport oxygen downwards,
which diffuses out and is used by the bacteria, enabling them to make more
CO2 than they could without the roots.  In addition, these aquatic plants
have biochemical pathways (C-4 metabolism---seen also in cacti and other
succulents) for capturing and storing any CO2 they are able to pick up
during the night.  They don't miss a trick.

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174