Surface scum has its own
community of bacteria,
algae, fungi and protozoa.
It even has its
own ecological niche label,
the "neuston."
We don't encourage surface
scum in the aquarium,
to put it mildly. I regularly
drag a sheet
of paper towel across water
surfaces, lifting
the front edge as I go,
to take up the scum,
neuston or not!
In fairly still ("lentic")
waters,
surface tension creates
a stable environment
for organisms and offers
the best access
to carbon dioxide and oxygen
in the air.
The density of living cells
on the surface
may be ten to a hundred
times what it is
in the water layers just
beneath the surface
slick. Many bacterial strains
in this biologically-enriched
surface layer carry photosynthetic
pigments,
though you might not notice
cyanobacteria,
for example, until they
stain your paper
towel blue-green. There
are also true algae,
diatoms, fungi and single-celled
protozoans
in these organically-enriched
microlayers.
In still waters, the upper
face of the neuston
is home to many mites and
some springtails.
Occasionally you may see
an Otocinclus flip
on its back and make some
passes through
the nourishing oily bacterial
and algal slick
of the water surface. If
you doubt the nutritional
rewards, consider what
a dangerous feeding
strategy this is for the
Oto in the wild.
Indoor pollution makes
a less positive contribution
to surface scum in the
aquarium. Many household
cleaners and air "fresheners"
that
mask odors impart a pine
or lemon scent,
which diffuses in aerosol
droplets that contribute
to the surface slick. Terpenes
in those aerosols
can even react with household
ozone to lay
down a thin coating of
particulates. (Forest
haze is formed in a comparable
way, when
natural terpenes react
with ozone.) Oil droplets
and smoke particles from
cooking and heating
make their own contribution,
even in a house
where no one smokes tobacco.
