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Re: Why plants grow and algae don't

> I have been following the algae discussion and wondering exactly why
> high-nutrient/CO2 injected tanks are algae free (in the hands of people
> who know what they are doing). The results speak for themselves to some
> extent of course - but it isn't clear to me why the algae should die
> back just because the plants are happy. I find the low-nutrient argument
> much more intuitive - something like "plants have roots and can store
> more nutrients so they tolerate low nutrients in the water column better
> than algae" - that makes sense. But if there are plenty of macros and
> micros and light and CO2 etc etc, what holds the algae back?

I might entertain this one and it's not something that folks have addressed
often. This question has bugged a few folks over the years but I think
there's a few easy reasonable causes for this.

Larger algae out compete smaller algae in nature. This is because they have
higher uptake rates at higher constant nutrients levvel. Why? Because they
have more of the enzyme that brings the nutrient into the cell.
This is graphically represented by the Michaelis- Menten Equation V=
Vmax[S/S+Ks] (this is for enzymatic uptake and the nutrient is the
substrate) and a few of you may be more familar with u= umax[S/Ks+S].
umax is the max growth rate possible, S is the nutrient substrate
concentration, Ks is the 1/2 saturation constant for that enzyme.

I know math and jargon.... but it means that the smaller algae are much
better adapted to low nutrient poor environments(like picoplankton-dominate
oligotrophic waters) and high nutrient environments (like large diatoms in
eutrophic waters) are better adapted for the larger algae.
A smaller plant needs less nutrients to live. They also have enzymes that
are more sensitive to low nutrient levels even at very concentrations.
Take this one step further and add a third group, vascular plants in there.
They need even more nutrients. But there's a breaking point as you increase
all this nutrient concentration and biomass/decompsition rates and this
causes instability. The higher plants have roots and a better transport
system also. 

I have a graph of this if someone wishes to post it that I did from an old
test. You can see the midpoint where both species can dominate.

2) Disturbance.
Every water change algae scrubbing, pruning of the old leaves(with algae
whether you can see it or not), vacuuming, light variation, we selectively
remove algae at every turn.

3) Herbivores eat algae and turn it into plant food. More disturbance.

> I like the 
> allelochemicals argument - but then (as others - meaning Mr. Tom Barr if
> I remember right - have argued) big water changes should allow a spurt
> in algae production by lowering allelochemical concentrations - which
> doesn't seem to happen.

But if you practice a non CO2 method, this is very likely to be the
case(very few water changes-like once every few moths). Depends on how your
approaching things.

 It would also make sense to me to argue that by
> keeping background algea populations low with manual removal and algae
> eaters there isn't the potential there for a major population explosion.
> No one would dispute that this is true - would they? - but it doesn't
> explain the cases where simply adding nutrients makes problem algae go away.

Well plants need more nutrients to live than the algae. But your question
ask why even if they are available to both the plants and algae(eg no
competition for resources). Algae at high levels of CO2 photo respire.
Normally this is not the case. This has something to do with dissolved O2
content also or may be indirect (say internal O2 vs external O2 levels).
> I guess you could ask similar questions about regular roadside weeds -
> do weeds take over in poor soil because they really _like_ poor soil, or
> are they just filling in where other plants can't make it?

Both in some cases. It's different because that's nature, this is not. Some
plants such as many of those adapted to serpentine soils can grow on non
serpentine soil but cannot compete with the other non serpentine soil
plants. Some have to grow on nutrient poor serpentine soil.

So you 
> fertilize the roadside and a bigger variety of plants will grow and the
> weeds will go away (or seem like less of a problem)?

Well time is another factor. They invaded a disturbed area fast. The Climax
species may be shrub perennials or a tree etc but for the first 5 years,
it's all weeds. If you continually disturb a place there will be nothing but
weeds there. 

> Hmm. Probably true.
> But do algea and plants compete directly for the same resources in the
> same way that different varieties of plants do?

I'd say not as much.

> I've been enjoying the discussions on this list very much - I've been
> following silently for a year or so. I have a particular interest in
> algae because I have a green water problem that I've knocked back from
> pea soup to a persistent but more transparent haze over the past few
> months - I'll spare everyone the details of the things I have tried -
> all of them suggestions from this list. I'm sure they are all good
> suggestions but I have a hard time following them consistently. When I
> try to starve my algae (you laugh?)

HaHaHa! Of course, at this one especially.

 I'm defeated by the junky substrate
> I put together when I didn't know any better. I have a hard time
> following the high-nutrient regime because I'm nervous that I'll be back
> to pea soup if I overdo it.

You very well may induce it back by doing this.

> I would also like to have a better
> understanding of why it is supposed to work....

That may take a while. Search "Green water" and find a few of my rants from
the last year or maybe a little longer.
Once inoculated, GW is hard to get rid of unless you use fine
filtration(diatom/Micron), UV's, blackouts.

To prevent it from coming back: Don't add things like Jobes sticks to you
substrate and lift them back up not the water column when you replant/prune.
Add more biofilter material. Keep the NO3 up. Reduce the fish load if very
high. NH4 is a trigger that inoculates GW. Avoid things that allow NH4 in
your tank's water and if you do a pruning etc do a water change right after.
Don't harm your biofilter etc

Tom Barr