[APD] RE: Comp/limitation

["Thomas Barr" <tcbiii at earthlink_net>]

"Roger, Tom, when you say that algae are not limited by phosphates until
the concentrations are in the PPB, you are not saying that if you
pollute a tank with excess nutrients, you won't get an algae explosion
of some type?"

Well, you need to be specific when you ask this question. With good CO2,
substrate, plant density/biomass/health, other nutrients is relatively good
ranges, etc yes.
Everything is in excess in terms of algae's growth needs and you don't see
explosions of algae.     

"If you don't dump in more nutrients than the system can properly
metabolize at a certain rate, then you would not experience the
explosion; can't we then say we are controlling or managing the algae?"

Properly manage?What does that mean? 
I would say we are selecting the better environment for the macrophytes by
giving them enough and the right type of nutrients to grow well.
The control/management is indirect with the algae.

"When we have a well tuned system where algae is not rampant, don't the
plants absorb the free phosphates down to the point where algae have a
"tough" time competing for it?"

No, the plants cannot remove enough PO4 to do this even in the best case
senario, 20-50ppb at best. This has been looked at in research. Algae,
periphyton(not just the tiny little phytoplankton) have been looked at and
even at 3ppb they were not limited. Plants also cannot remove the DOP
fraction, algae can.
You might see decline in algae biomass initially, but it persist and often
comes back worse later after the plants growth has been slowed even further
which is why you got the algae in the first place. Some spcies of algae
like richer conditions like the plants, but many do not. 

One thing when comparing natural cases to aquariums: our light is FAR less
than the light outside, likely by close to 10X the level. Most aquatic
plants are essentially shade plants, the algae have ways of dealing with
this but light competition seems to really play a larger riole in FW tanks
than many may think or believe. 
More light is not better, you can see this trend clearly in plant tanks
with respect to algae presence over time.  
So if the algae are at the lower limits of light, CO2, etc, same for plants
for that matter, then co limiting something like PO4 may have a much
greater effect.

If you do not add CO2 and have very low light, the plants will not grow
nearly as well or effiiciently as with adding it. At high light, limiting
PO4 will cause more issues than at low light also. Often the co limiting
factors will hurt plant growth and this occurs quite often in natural
systems. 

"Of course, phosphates are constantly being liberated into solution by
bacterial/biological decay processes; even though the concentration in
the water is very low, the supply is still abundant because of the
continuous flux of phosphate."

Yes, same can be said for most nutrients.

"At some point, the concentration of a given nutrient becomes the rate
governing factor for growth/reproduction. Growth doesn't cease; it
changes rate. That's the Leibig principle.
<http://www.soils.wisc.edu/~barak/soilscience326/lawofmin.htm>"

Well you have to show that this occurs first with the algae in question. No
ones done this with PO4 to less than 3ppb from what I've seen and removing
down to this level is impractical for many and there's no way for any
hobbyist to test this low anyway. 

Better to consider something you can measure, CO2, NO3 etc. Or at high
luight/CO2, plant or algae biomass and high PO4 levels and see how much is
removed, this has the problem of luxury uptake but at least you can get
some idea.

"When you talk about limitation do you mean a threshold where net growth
cannot occur even though all other non-competitive factors (nutrients,
light) are in abundance? I agree, it is not limitation in this sense
that governs (controls) the algae population."

Where RGR is reduced when the level of PO4 is dropped below a certain
level. Where growth is limited. 

"So maybe we are not controlling algae by limiting them to the point
where they can grow (in isolation) but we are slowing down the growth
and/or reproduction rates to where algae consumers can keep up. Each
type of algae has a different set of consumers."

Well, I'm not sure about that. We have examples of non CO2 tanks that don't
need cleaning for long periods and low light tanks w/CO2 that also have the
same result.
We are herbivores in a sense, we remove excess waste/growth etc and keep
the uptake stable, the light, the nutrients etc. We also remove old leaves,
lower older portions that have algae, scrub glass, equipment etc. I do not
have a lot of herbivores in some tanks, a few snails perhaps are all. But
the tank is algae free.  

 "When the system is
modelled with consumers & competitors, you have a kind of dynamic system
control. The laboratory experiments designed to measure the growth
limits of a species, must do so in isolation from competitors. In
isolation, the algae population would increase under a given set of
nutrient concentrations whereas in a dynamic & competitive environment
with the same concentrations, the net population of any one species may
decline."

I started with gas levels with CO2 and O2. I got relatively the same amount
of algae in controlled conditions/different species, but similar amounts in
terms of attached algae per unit area. CO2 ambient or 25ppm, O2 ambient or
15ppm at 23C temp for 3 weeks under non nutrient/light limiting conditions.
I used Chl a levels to gauge algae growth on glass substrates   

"Don't certain types of algae compete with other algae species using
"chemical warfare"?"

Yes. More so than plants ever do. 

" I think there is something to consider that some
types of bacteria would inhibit the growth of other types. When you
consider bio-films, there are complex factors that might govern just who
is able to grow over another neighbouring colony. If you are a biofilm,
once you cover the competition, you have eliminated its access to the
environment. Kill or be killed. Similar factors governing bacterial
bio-film growth upon neighbours might apply to epiphytic algae."

Typically morphology plays a large role. Tall hairly forms, often the types
that bother us, are very good at rising above the film and highly visible
to our eyes.

Regards, 
Tom Barr


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