After receiving a gentle scolding from one BEST member who was somewhat confused by my posting from last Friday, I wanted to go ahead and clarify what I was getting at. In short, the study at the Indian River Research Center demonstrated the superior nutrient remediation capabilities of water lettuce (Pistia stratioes) in stormwater. Many other studies have found similar results in studies of sewage, agricultural runoff, and other nutrient streams. Essentially, water lettuce is among those aquatic plants that are hyper-efficient in their use of solar energy to sequester dissolved nutrients and, at least to some degree, atmospheric carbon.
However, water lettuce is classified as an invasive exotic species in Florida. This classification means that it is the official policy of the State to maintain the plant at the lowest feasible levels, generally through fossil fuel-based herbicides. The water lettuce at Ichetucknee is an exception in that it was pretty much eradicated through a sustained hand removal program. My long-term observation is that the removed water lettuce at Ichetucknee has largely been replaced by thick mats of filamentous cyanobacteria, stands of highly poisonous water hemlock, and, to a lesser degree, emergent meadows of wild rice.
Although there are a number of reasons that we may want to control water lettuce (it can form thick floating mats that are hard to get a boat through, and some species of mosquitoes really like it), the exotic rap is pretty questionable. In particular, famed 18th century explorer William Bartram reported water lettuce throughout his travels through Florida and the lower SE in the late 1700s. The few studies of water lettuce ecological communities have noted that it is preferred habitat for at least one snail endemic to Florida spring runs, a number of endemic insects (albeit, ones that do like to bite humans), a few rare fish, and a relatively rich invertebrate fauna.
Considering that the plant has been in the southeast U.S. for that long, my somewhat rhetorical question is this: Is it reasonable to not use water lettuce as a tool for remediating nutrients in stormwater ponds, treatment wetlands, or even natural water bodies based solely on fears that it may be exotic? I personally don't think so, but the reality is that the invasive exotic label has long been, and still remains, a huge barrier to larger-scale utilization of water lettuce in Florida.
There is also a good amount of research into the bienergy potential of harvested water lettuce (and other aquatic plants) from nutrient remediation sites. The U.S. was the leader of this research throughout the 1970s. Over the past decade the research and application of aquatic plant systems has continued to move forward in developing countries, particularly India and China, with comparatively little being done in the U.S. This is yet another sustainability area where I think we can do better in this country.
Again, my apologies for the soapbox... If anyone has further interest in any of this, feel free to send me an individual e-mail or give a call.
Jason M. Evans, Ph.D.
Environmental Sustainability Analyst
Environmental Policy Program
Carl Vinson Institute of Government
University of Georgia
Lucy Cobb 318
Office phone: 706-542-2808
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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
I'll get off my soapbox now...
Phytoremediation to remove nutrients and improve eutrophic stormwaters
using water lettuce (Pistia stratiotes L.)
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.