[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Nutrient levels and limiting

Tom wrote:

> A plant needs a lot more of a nutrient ( say NO3) than the algae does. The
> plant gets stunted at 1ppm or less for more than 3-4 days(a general range
> for most plants). The algae just slows down but can still look bad.


> Some glass algae can persist in absence of P. Plants certainly grow better
> than algae can due to obtaining P etc from the soil(bacterial lysis etc &
> PO4 complexes etc) but the algae can live on virtually nothing. Algae can
> concentrate P at very low levels. Fish food is enough of a source for algae
> to respond and grow to nuisance levels... but not the plants. Riccia
> certainly does quite well with enough PO4. A number of other "difficult
> plants" do very well in higher PO4 levels.

This makes it sounds like the now venerable idea that aquarium plants
will out-compete algae for low nutrients is actually wrong.  If algae do
better at low nutrient supplies, then that competition will always be
won by the algae.

Under natural conditions plants usually dominate over algae when the
dissolved nutrients are low.  That's the opposite of what you seem to be
saying here.  Of course, under natural conditions the plants are rooted
in a rich substrate and get most of their nutrients through their
roots.  It's only the algae that suffer poor growth because of low
dissolved nutrient levels.

Under conditions with high dissolved nutrients algae usually dominate
over plants because plants lose the competition for light, not the
competition for nutrients.

Most of use don't use a rich substrate and our aquarium plants have to
compete head-to-head with the algae for dissolved nutrients.  There must
be something in our aquariums that tilts the advantage to the plants,
because with the same nutrient levels under natural conditions the algae
will usually squeeze out the plants.

> Most of the algae we hate has very efficient size ratios compared to the
> higher plants. They can grow faster since there's less issue of transport,
> most of the algae is a photosynthetic machine, no roots/xylem/flowers etc
> that don't contribute to the production of plant mass. So it's ALL algae
> production. We don't see the plants roots, xylem and other parts of the
> plant. We see algae.

Not all advantages go to the algae.

The other thing that we don't see is that algae have relatively high
energy and material demand compared to plants.  Phytoplankton don't just
float around all day; they swim, and that takes energy.  Diatoms and
other algae that build biofilms produce fairly large amounts of slime
that they exude around themselves; that takes both energy and carbon. 
Even the alga that don't do these things still have to build up and
maintain all of the metabolic "machinery" necessary to maintain life
within a single cell; specialization is limited and I imagine that
metabolic efficiency is limited along with it.

Plants, on the other hand, are sedentary.  They don't swim. 
Non-pathologic plants exude some material (mostly through their roots)
but nothing that compares volumetrically to the slime produced in a
biofilm.  And of course, plants have highly specialized tissues and I
suppose they should have higher metabolic efficiency as a result.

> But since algae can live in low and in high levels of nutrients and are
> seeming more efficient, why do the plants continue to dominate? Algae is
> still there but it's not dominating. There's plenty of nutrients for the
> algae as well as the plants. Why then is this happening? It seems like a
> contradiction.

I agree, but I wonder if the advantage has much at all to do with
nutrients.  Most algae (except the low-light specialists) may require at
least brief periods of high-intensity light in order to get the energy
they need to function; long periods or relatively low light may not be
enough.  The result would be that our tank conditions lead to the
selection of only a few, specialized algae.  You don't have to get very
much direct sunlight on an aquarium before you see drastic changes in
the type of nuisance algae that grow in the tank.

Algae are also subject to far higher grazing pressures than plants. 
Especially in our tanks, where the grazing species are selected so that
plant grazing is discouraged and algae grazing is encouraged.  Algae
grazing isn't limited to the macroscopic grazing we see and promote.  It
also occurs on a microscopic level.  Some protozoans, for instance, feed
on algae.

> Perhaps the tide is only tilted when there's a nutrient that too low rather
> than this popular "excess nutrient" idea.

I don't assign much importance to excess nutrients.  You may recall the
fairly long discussion that Steve Dixon and I had on that topic.  The
fact is that a lot of essential nutrients are usually in excess;
calcium, magnesium, chloride and sulfate all come to mind.  There aren't
many problems associated with excesses of those nutrients.
> You can't eliminate
> algae from limiting a nutrient down super low (there is not any real
> absence, there's almost always a trace amount that's all the algae need to
> show themselves in tanks often, were as the plants need far more to show
> good growth, basically you can see the algae easier in response to nutrient
> problems). They will be the first to use any nutrient when you add more back
> into your tank. Plants can get stunted and weak. Algae? Stunted algae? Yes
> you can hurt it some but it's still waiting for more nutrients to be added
> back. Then when you do and add enough so the plants are not stunted anymore
> you start to see a reduction in algae once again. Why?

That's my experience.  I remember that Paul Krombholz came to the same
conclusion regarding a problem hair algae.

I suspect that algae are disadvantaged by their relatively high energy
demands and their susceptability to grazing.
> Roger had pointed to the plants "leaking" idea of perhaps sugars etc
> leaching out and cause perhaps some inhibition of algae.

Er... I don't remember saying that.  I do remember saying recently that
I thought nutrient-limited plants would leak sugars that would *feed*
(not inhibit) specialized algae.  That is my current best explanation
for nuisance algae growths that appear when plants are limited by

> But I've done
> serious water changes........in effort to see if removal will encourage
> algae growth of high nutrient levels without any effects( or at least
> minimal effects) of any plant by products. I got even less algae after I re
> added all the nutrients then did the same water change plus nutrient rebuild
> each time. Plants grew like mad. What's up with all that?

If I understand what you're saying then I think that would be consistent
with sugars leaked by the nutrient-limited plants actually feeding the
algae.  A big water change removes the products already leaked from the
plants and takes away the suppliment that the sugars gave to the algae. 
Bringing the nutrient levels back immediately after the water change
would help keep the plants from leaking more sugars into the water. 
Rebuilding the nutrient levels doesn't do much to help the algae since
the algae probably never was nutrient-limited.

> Oh well it's all just a guess anyway:)

Hopefully our guesses are getting more educated as time goes by.

Roger Miller