[APD] RE: competition with algae

["Thomas Barr" <tcbiii at earthlink_net>]

> Tom Barr wrote, in a question about what causes algae:
> "Poor plant growth/health/density. If you have a superior competitor, the
> algae will only grow at low levels, you can never comletely remove them,
> but you make a tank appear not to have any consistently."

Well algae are better competitors at _lower_ nutrients.
Nutrients are never quite absent.
Theory and reality are different when dealing with competition models. 
Few things go to complete exclusion with algae. 

> Tom, can you elaborate as to what exactly the "superior competitor" is
> competing for? 

Nutrients, through enzyme kinetic values of uptake transportors.
These follow Michaelis-Menten dynamics. 
At some point, the smaller algae can no longer take up or store any more
and max their growth rate, make more reserves etc no matter how much
nutrients are present, the plants on the other hand, can take up more.  

You can see this is you keep two systems, say one an algae scrubber with
small algae vs a planted tank and watch the N rates with dosing.
The plants will remove a lot more.
I did this 15 years ago in my quest for the perfect filter set up.

My understanding of competition is based on some sort of
> scarce resource. But we're talking here about well-fertilized tanks with
> enough nutrients to feed the algae as well as the plants.

Well if you consider competition between two algae, one large, one small,
you'll see the uptake rates will dominate for the larger alga at higher
nutrients and the 
smaller alga will dominate at lower nutrients. At intermediate levels where
these uptake rates are similar, both will grow.
Again, you can see this in a tank. Plant and algae both growing well.
This is very likely not the only mechanism for plants dominating, but it is
one of the larger players.
Light is another, plants are able to better use it, block algae from it etc
in most ecosystems. 

> In other words, what difference does it make if the plants are sucking up
> N, P, and K like there's no tomorrow, if you keep adding enough fertilizer
> so that the water constantly remains eutrophic?

You are keeping them well supplied with nutrients and growing well. New
growth, relative to light, is able to outpace the epiphytic algae, this
allows less resistence to gas exchange/nutrient uptake/light for the plant
leaf surfaces.

You can see this in lakes with a high % of aqautic macrophytes(Chara beds
are aslo similar), if you add say more PO4, you'll only get more plant
growth with 30-50% coverage.This does not just occur in planted aquariums.  

> Not understanding the answer to this question is why I'm repeated drawn to
> allelopathy as an explanation for algae control. Since allelopathy is an
> unpopular theory on this list, I'd really like to understand your
> explanation. :-)

It's not unpopular on this or other list, it is unpopular with aquatic
plant researchers working on aquatic macrophytes.
If you ask the leading experts on aquatic plants they will generally say
light.George Bowes, Limnologist, the folks at IFAS etc. 

Regards, 
Tom Barr

> - Jim Seidman



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