From: "Thomas Barr" <tcbiii at earthlink_net>
A couple of problems with the allelopathic effect being a reason for
significant algae decline/presence:
One issue that is very tough to get around about allelopathy's potenital
role in aquariums: we see/observe a reduction in algae with _all plant
species_ present in the tank, there's close to 300species of aquatic plants
in this hobby, and no matter how you mix it it up, they are produce the
same effect when plants are healthy on algae presence.
Riccia to Hornwort, to Ammannia which doesn't live even close to water
normally, to crypts, to har-grass, to Micrantherum to Lugwigia, to Chara(an
algae) to Green water(an algae), Gloss or just about anything you want to
choose. CO2 high light methods, non CO2 etc, large frequent water changes,
no water changes, adding carbon/UV(which may cleave the chemicals bonds) or
not.
Now what are the odds that each and every one of the these 300 species
retards all the same algae in the same manner through allelopathic
chemicals?
Does each/all plant species produce some secret algae killing allelopathic
chemical?
_Perhaps_ a few might, but those effects are _subtle_ at best.
Review the research studies on these compounds, they involve grinding the
plant/algae up and dumping a concentrated pulp into an algal culture.This
is not the same as a live plants growing natrually in the tank or in
nature. Some active chemicals have been isolated, BUT, these are much
higher concentrations than would be found in natural systems.
But all 300species produce the same allelopathic conpounds or similar ones
that significantly reduce algae?
I'm skeptical. **You can have the same nutrient levels in each tank and add
nutrients to *match* relative uptake by the plants(fast or slow growers
etc) and get the same effect no matter what the plant species. All 300
plant species have the same allelopathic potential that is this effective?
What about the algae<=> algae interactions?
I think it's something much more general based on observations with all
these plant species and I think it's quite rare and subtle if it exist in
our tanks.
But you can always add activated carbon to see as it will remove these just
like it removes tannins and humic acids and many organic compounds.
I skeptical about the notion that the allelopathic chemical somehow are
"emmedded" on the plant's surface/cuticle. Again, all 300 species do this?
Also, why does algae not grow elsewhere in there tank besides the plant
surface which also leak out nutrients for algae? You will often find
different species growing on plant surfaces than rocks or wood in nature,
not less algae biomass but often more. Adding carbon will take out the
active chemicals(and a few plant nutrients, but these can be
resupplied/added to the substrate) from the water column but the surfaces
but it still seems unlikely that _all_ the plant species do this.
Another issue: How does a plant know how large of a water body does it live
in? If it released the chemical compounds, it would be diluted in large
lake but fair well in small pool. What about flowing water? Rivers/streams
are unidirectional so any chemicals released would wash down stream and you
should see a decrease in the algae down stream also. Many plants do
extremely well in flowing rivers/springs.
While it might be an unknown, perhaps an entertaining idea, I have grave
doubts about its effect and the significance of it's role in planted
aquariums.
Regards,
Tom Barr