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Re: Light level / nutrient uptake

> Date: Fri, 04 Dec 1998 09:18:27 +0000
> From: Morten Karlsen <markii at online_no>
> Yesterday I asked myself; why does some advocate high light levels, so
> that plants can "use up" nutrients in the water column. Like when
> running in a new tank.
> Are plants "better" than algae at burning nutrients at higher light
> levels?

This may be a topic up for debate, but here's a stab at it that should
approach good enough.

Plants are not better than algae at absorbing waterborne nutrients
because monofilament and unicellular algae simply produce more surface
area in the water column than an equivalent mass of plants.  Algae can
colonize on plants, directly shading their leaves and 'stealing' light. 
Plants also photosynthesize in only a fraction of their cells, while
algae photosynthesize in all cells.  Plants may however, be able to
utilize more light in some wavelengths that algae cannot.

In a situation where biomass is being removed or recycled in a tank,
plants are better at storing nutrients than algae.  

If the algae is eaten, it is converted back to nutrients, some of which
are stored in the consumer, some of which become insoluble waste and
sink to the substrate, and some of which are released back to the water
as nutrients.  At this point, the nutrients become available again to
both the plants and the algae.  If the plant/algae nutrient consumption
in the tank exceeds the nutrient input through feeding and/or
fertilization, the plants will eventually sequester all the nutrients
unless they die or are consumed.

When plants are harvested, nutrients are directly removed from the
aquarium in the form of the harvested biomass.

The only algae I know of that are easily harvested are severe
cyanobacteria and spirogyra infestations.  Some people are allergic to
cyanobacteria (me) and may need to take precautions before harvesting it
(antihistamines and/or gloves).  Algae harvesting is also a way of
removing nutrients from the tank (and algae at the same time).  Of
course, algae scrapers don't harvest algae unless you have something to
scoop up the recently scraped algae and remove it from the tank.

The answer to your original question is: No.  Plants are not better at
burning nutrients at higher light levels. 
Plants are capable of sequestering available nutrients roughly as a
function of their biomass.  Algae also are capable of sequestering
available nutrients roughly as a function of their biomass.  My
speculation is that algae can sequester and consume more nutrients per
gram of biomass due to the surface area and photosynthesis
considerations mentioned above.  Evidence presented to this list over
the years leads me to believe that light wavelength is not an effective
place to combat algae, since plants and algae seem both able to utilize
light across the spectrum, although perhaps with different spectra where
their respective efficiency is greatest (also, you probably want your
tanks to look good, which means providing a wide range of spectra).

If the biomass of the plants is enough to quickly sequester all the
nutrients in the water, and the biomass of the algae is a negligible
fraction of a percentage of the plants' biomass, the plants will
sequester the available nutrients in the water before the algae has an
opportunity to sequester enough nutrients to significantly increase its
biomass.  If an algae consumer is in the tank at the same time, the
algae may then be eaten, releasing its nutrients again to the water,
where the plants (in a heavily planted tank) will sequester most of
them.  If the nutrient addition to a tank does not exceed the plants
ability to consume nutrients, and there is no imbalance of limiting
factors, the plants will continue to outcompete the algae.

If the biomass of the algae is a significant fraction of the biomass of
the plants, or if some limiting factor provides an excess of nutrients
in the water that the plants cannot consume, the algae may be able to
outcompete the plants for the available nutrients.  Common limiting
factors in an aquarium are: light, CO2, Nitrogen, Potassium, Phosphorus,
Calcium, Magnesium, and Iron.  Other nutrients may also become limiting
factors, depending on your water, substrate, and fertilization.  A lack
or abundance of any limiting factor in relation to the levels normally
needed by plants will often allow the algae to outcompete the plants.

This means that bright lighting alone will not necessarily provide the
plants with a competitive advantage over algae.  This also means that
CO2 alone will not necessarily favor plants, nor will any other limiting
factor applied non-judiciously.

David W. Webb
Live-Foods list administrator