Re: How deep will plants grow

[David Marshburn <marshbur at cs_unc.edu>]

On Mon, 3 Feb 2003, Aquatic Plants Digest wrote:

> Date: Sun, 2 Feb 2003 13:35:01 EST
> From: Moontanman at aol_com
> Subject: Re: Aquatic Plants Digest V6 #40 How deep will plants grow
> 
> I have to strongly disagree, while pressure at extreme depths might change 
> the biochemistry of living creatures the point at which this occurs is far 
> beyond the penetration of light. Water pressure doesn't have anything to do 
> with plant growth under water since the inside pressure of a water filled 
> plant is the same as the outside. I would expect find moss at a more extreme 
> depth due to the capacity of moss to grow in low light levels. Also 
> freshwater usually isn't very transparent due to organic loads in the water. 
> (yes I know there are very clear freshwaters but they are usually found in 
> very cold places with very little life) I'm betting that live vascular plants 
> have been found in water as deep as 100 feet or more depending on the clarity 
> of the water. I scuba dive and I have never seen any cut off of plants due to 
> anything but light levels.

The change at depths is not biochemical; it's a physical problem
(specifically pressure).  Plants move air internally through a series of
capillary-diameter chambers called lacunae, and, at greater pressures
cannot vent the necessary gas to drive their air-circulation systems. The
issue is not specifically the water pressure, either, but the partial gas
pressure at depths.  The limit seems to be about 1 atm of hydrostatic
pressure [1] (by the way, that ends up being a total pressure of 2atm at
sea level); however, this can be modulated by elevated light levels or
elevated temperature.  This can also be modulated by elevation, e.g., in
Lake Titicaca -- it turns out that the total pressure at 13m depth
in Titicaca is the same as at about 10m at sea level.  The two incorrect
assumptions above are that 1) it's a biochemical phenomenon, and 2) the
plants are water-filled (as much as 60-70% of the root tissue of aquatic
plants is gas storage, and gas storage and movement structures are present
throughout aquatic plants).  Also, the drop-off in light levels as one
descends in fresh water is accompanied by a decrease in temperature as
well as an increase in pressure (so the factors are confounding in making
any inference)

Mosses and algaes don't have the internal air-transport system
of vascular plants, and so can better deal with the greater depth.  

Also, marine angiosperms are adapted to depths of 30m.  Were you diving in
salt water, Moon, when you saw plants at that depth? 

[1] this is from course notes from "Ecology of Aquatic Plants and Wetland
Ecosystems" R. Wetzel, UNC, Spring, 2003.  I haven't dug up references to
the original works cited, but can try if anyone's particularly interested.

Tom Barr:  maximum depth for freshwater angiosperms is 10-12m, or 2 atm of
total pressure, or 1 atm of hydrostatic pressure.

cheers,
-david