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Re: Cause of cyanobacteria

A few months ago I stated in this digest my belief that
blue-green algae did poorly in highly oxygenated water.
However, I couldn't explain the seeming contradiction
of these same bacteria growing profusely in the return
water of power filters such as the AquaClears.

Well, there is no contradiction. The return flow, although
moving quickly and with a large surface area exposed to
the air, is depleted of oxygen due to the activity of the
nitrosomos and nitrobacter and the biological filtration
which has just occurred. In this environment, in addition
to water low in oxygen, the cyanobacteria also enjoy a
high level of nutrients and carbon dioxide (from the
bacteria), and unobstructed constant exposure to light.
Oxygenation occurs not in the return flow, but only as
the surface waters of the tank are churned and replaced.

This theory would explain several common observations.
One is that cyanobacteria first show up in crevices,
between the gravel and the glass, on dead or slow-growing
leaves, or in the bottom layers of the tank. What all
of these places have in common are lower than average
oxygen levels. Fast growing plants not only consume
phosphate and other nutrients, they also produce lots
of oxygen to be diffused at the leaf surface. They are
the last to be covered by the algae.

If the  carbon dioxide in my tank falls causing slower
plant growth, the cyanobacteria start to creep back.
If one of my power heads stops, the result is the same.
I've also noticed that outbreaks of blue-green algae
are more frequent in summer when temperatures are apt
to rise reducing the amount of dissolved oxygen.

It's fairly easy to elicit blue-green algae simply by
creating a dead, lighted area in a corner of an other-
wise healthy, heavily planted tank. I recently did this
by placing a glass jar against one far corner of my
aquarium. Within a week or so the algae had started
to migrate to the surfaces which encompassed the dead
space between the jar and the aquarium glass.

Anyway, I don't believe the "cyanobacteria grows on
phosphate-laden detritus at the bottom of the tank"
theory. I have yet to hear a satisfactory explanation
of their appearance and sometimes persistence in

Maybe someone would like to experiment with hydrogen
peroxide over a five day period. I bet that it would
work in the short term.

Dave Whittaker
ac554 at FreeNet_Carleton.ca