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Re: Musings


On Fri, 31 Mar 2000, Karen Randall wrote:

> My problem, and the reason I've chosen until now to sit back and just see
> what you guys had to say is that from what I can see, basically _all_ our
> aquariums would qualify as eutrophic.  But UNLESS we keep nutrient levels
> up there, our plants done' grow particularly well.  I don't know why this
> is, and I don't know why we are able to maintain tanks in this condition
> without excessive algae problems, but I'm sure it is so.

I've mused about this myself quite a bit and I conjecture that the reasons
are mostly ecological.  In nature aquatic plants grow in settings as
different as murky slime-bottomed estuaries and fresh mountain spring
floored with fractured bedrock.  But the best growth seems to be in fairly
clean water over silty organic sediments.  The clean water allows for
bright light penetrating into deep water and the silty sediments provide a
great nutrient-holding capacity and an enormous level of biological
activity (worms, chironomids, molluscs etc. and their whole supporting
casts of creepy-crawlies) that constantly cycle nutrients through reactive
and readily plant-available forms.

The plants growing naturally thus have a constant and easily available
supply of nutrients from the substrate and enough energy (from the
sunlight) that they can easily apply that energy to actively import
nutrients that impedes their growth.

In contrast, our substrates are typically coarse-grained with little
nutrient-holding capacity and we either fail to introduce or actively
discourage all but a very few of the critters that might otherwise live
there.  We provide out tanks with the minimum amount of light necessary to
grow a variety of plants, so many of them are left with little capacity to
actively import nutrients that are in short supply.  We offset those
limits by giving our plants high concentrations of nutrients and since
most of us are also maintaining fish in our tanks we use nutrients that
are in chemically stable, less readily-available forms.

So our tanks typically have dissolved nutrient concentrations that are sky
high compared to natural waters; eutrophic would be an understatement.
Some of us have ways to keep algae from taking over in their tanks
like it would in nature and many of us read this list to find out how they
do it.

I can think of several factors that in contrast to natural conditions may
discourage algae growth in our tanks.  Generally these are:

1) limited light -- this prevents any algae species that requires periods
of intense light from growing in our tanks and limits the amount of energy
the algae can apply to its needs.

2) grazing -- all natural systems support grazing but in our tanks we can
select for specific grazers (including microscopic grazers) and we can
eliminate predatory controls on their population.

3) disease -- there are bacterial and (as someone else pointed out
recently) also viral controls on algae; perhaps in the closed confines of
our tanks these controls are very important.

4) unreactive nutrients -- while our nutrient levels are very high, we
tend to use nutrients in forms (like nitrate, and chelated metals) that
aren't readily available.  This is compounded by fairly low light.

5) unbalanced nutrients -- our tanks may contain very high levels of most
nutrients but low levels of one or two specific nutrients (e.g. phosphorus
and/or iron).  Alternatively, those nutrients migh be restricted to the
substrate where the algae can't get it them.  I think this is a distinctly
unnatural conditions and one that has to be actively maintained in our
tanks; it isn't likely to happen by itself.

I'm sure there's other reasons, but those are the ones that come to mind
right now.


Roger Miller