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I wanted to draw attention to this recently published study by Qin Lu and
others at the UF Indian River Research and Education Center. Although the
basic ideas being demonstrated in this study have been generally known for
at least 30 years, the application of highly productive floating plants
such as water lettuce is still very controversial because of (in my view,
somewhat misguided) concerns about invasiveness.

Homework assignment for the 4th of July weekend: Do a Google search of
"William Bartram," "Florida," and "Pistia stratiotes." Next, do a few
searches on Google Scholar about the bioenergy potential from water
lettuce. As a bonus, take note of what countries (and, indeed, U.S.
states) were doing the bulk of bioenergy research on aquatic plants in the
1970s, and then compare that to where the research has mostly been
conducted in the past 5-10 years. And if tubing down the Ichetucknee River
over the weekend, ask yourself why you don't see any water lettuce in that
river... and what roles that plant may have played in a highly
nitrogen-enriched stream.

I'll get off my soapbox now...

Phytoremediation to remove nutrients and improve eutrophic stormwaters
using water lettuce (Pistia stratiotes L.)

http://www.springerlink.com/content/c3648213t581h851/

Many aquatic plants have been used to remove nutrients from eutrophic
waters but water lettuce proved superior to most other plants in nutrient
removal efficiency, owing to its rapid growth and high biomass yield
potential. However, the growth and nutrient removal potential are affected
by many factors such as temperature, water salinity, and physiological
limitations of the plant. Low temperature, high concentration of salts,
and low concentration of nutrients may reduce the performance of this
plant in removing nutrients. The results from this study indicate that
water lettuce has a great potential in removing N and P from eutrophic
stormwaters and improving other water quality properties.

Jason