"Pond" Cultures of Microscopic Invertebrates
Sources
The diversity of microscopic life in samples from
a well-illuminated, unpolluted source of fresh
water can be astounding. So too can be the contrast
between the quantity and diversity of life in such
a sample and one from a source that is polluted,
protected from sunlight, or lacking in key nutrients.
For both quantity and diversity, an unpolluted
heterotrophic pond or lake in full sunlight is
ideal. sunlight, of course, provides a source of
energy for autotrophs - both protists and plants
- and allows the establishment of a food chain.
Photosynthetic organisms, being pigmented, are
easily seen with a dedicated bright field microscope
or even with a hand lens. A freshly collected sample
should show a lot of action in any microscope field.
Sampling should be done the day the observations
are to be made, since a balanced system depends
on many factors in order to maintain diversity.
We have found that a day or two in a lab in artificial
light and constant temperature is sufficient to
reduce the diversity in a sample considerably.
The photosyhthetics are the first to go, followed
by many of the non-photosynthetic protist and animal
species, leaving just a few heterotrophs to feed
on the organic material they leave behind.
We use a plastic beaker attached to a long pole
for collecting. Plan to take multiple samples.
A scoopful from open water may contain very little
life, while a scoopful from an algae mat may be
teeming. Collect debris from the bottom, including
rotting leaves and branches. Samples collected
from a shady part of the source will contain different
species than those collected from a sunny location.
Try a number of sources, including permanent ponds,
lakes, slow streams, and even wet areas that dry
up in summer. Take notes on what you find at what
times of the year. A fine source in spring might
be hopelessly devoid of interest in fall.
Samples are best kept separately in large jug
or jars, with high surface-to-volume ratio. That
is, fill them part way so that gas exhchange can
take place. Avoid temperature extremes, and place
samples in wide bowls with strong illumination
(but watch the heat!) once they are in the laboratory.
Setting up a laboratory source
It is difficult to maintain a diverse population
of invertebrates in the laboratory, however a balanced
'miniature ecosystem' can be maintained in the short
term. The materials needed are inexpensive.
- 1 Glass aquarium, 10 gal. or more, placed in
a sunny location
- 1 Aquarium cover or homemade cover of glass
or plastic
- Clean blasting sand or other fine gravel for
the bottom of the tank
- A source of clean pond water
- Pond, lake, and/or river samples
- [optional] Air pump, tubing, and air stone;
shelf or opaque cover
Place the aquarium in a window where it will
receive sunlight for most of the day. Alternatively
or additionally you may place a grow light over
the tank. Add gravel to cover the bottom to a depth
of about one inch. Add water from a natural source
such as a lake or pond, if available. If not, you
can use bottled drinking water (not distilled or
carbonated water), or use tap water treated to
remove chlorine. If you do not treat the tap water
let it stand for a couple of days before adding
samples. If your water contains chloramines, the
use of tap water is not recommended. A tropical
fish store can test your water for chloramine.
If you have an air pump, anchor the air stone
at the bottom of the tank. Don't use an undergravel
filter. The idea is to have everything growing
naturally. Undergravel filters trap bacteria below
the gravel where they process ammonia and other
deadly chemicals.
To diversify the environment, you can cover the
top and sides of one end of the tank with aluminum
foil so that there is a gradient of light from
one end of the tank to the other. Light cycles
are important - put the light on a timer so that
the light cycle is close to the daily outdoor light
cycle for the time of year.
Add pond samples and other materials to your
tank.
You can get samples from almost any safe freshwater
source, since even rainwater collected in your
back yard will grow a variety of plants and animals
if you toss a bit of dirt in the bottom of the
container. The best source, however, is a shallow
pond in a sunny location. Collect samples from
the surface - pond scum teems with algae and animal
life. Collect some of the debris on the bottom
of the pond such as rotting leaves or twigs - living
things congregate around anchoring sites, not usually
in the open water. Collect some vegetation as well,
including vascular plants and mats of algae. Dirt
adhering to the plants should be kept, not washed
off.
Add samples to your tank. You only need a small
quantity of each type of sample. If the samples
make the tank cloudy don't worry, as it will clear
in a few hours or days. To add some interest to
your tank you might toss in a few large invertebrates
such as snails or a crayfish or two.
To observe the organisms, periodically take samples
from microenvironments within the tank. For example,
a dish containing some of the floating material
on the surface should be rich with green (photosynthetic)
organisms, since the light is strongest there.
At the bottom, scavengers live among the gravel
and in the clumps of rotting vegetation. Floating
mats of algae may have a different collection of
plants and animals. Look at the leaves and roots
of plants as well. You should be able to see a
succession of different species as each population
peaks then dies back.
A large outside or greenhouse tank with renewable
water source offers more promise in terms of maintaining
a balanced system with both quantity and diversity
of life.
Making observations
Direct observation of cultures
Changes in the color and texture of the surface or
liquid provide hints as to the presence of living
organisms. In freshwater samples, look for plant
growth or movement in "microenvironments" such as
on top of gravel, around rocks, sticks, or plants,
or floating on the surface of an aquarium. You might
find a difference in color or texture of the water
near a light source or air stone. Any differences
in an otherwise uniform environment are a hint that
there might be a microscopic "zoo" right there under
your nose.
A good way to shake things loose is to shake a plant
in a bucket of clean water, and see what comes out. Soil
or gravel can be washed in a bucket, releasing "beasties" into
the water. You can scoop some scum from the surface of
the water into a shallow dish and watch carefully for
movement. Decaying material at the bottom of a tank or
pond is usually a rich and diverse source of freshwater
invertebrates.
Using a magnifying lens - 2x to 4x
You would be surprised what you can see just by magnifying
something a few times. In water samples you may see creatures
swimming around that you couldn't detect without the magnifier.
It is best to use a shallow dish to examine water samples,
and keep in mind there are different levels. If you focus
on the surface, you may miss something that is on the bottom.
A drawback to looking at living things is that they often
move! If something moves too fast, you might try catching
it and confining it to a small space, such as one well
of a depression slide.
Using a stereomicroscope - 10x to 40x
A low power view of a shallow (petri) dish or depression
slide containing a pond water sample can reveal an incredible
microscopic "zoo." Transmitted light (through the bottom
of the dish) is best for viewing such a sample, since direct
light produces reflections. Move the dish around until
some material is in the field of view, focus, then watch
carefully without touching the dish. When the dish is still,
you will see that things in the water are still moving.
As you become more skilled in observing the sample, you
will be surprised at how many things are moving.
Using a compound microscope - 40x to 1000x
If you have access to a compound microscope, you might prepare
a wet mount (also called a vaseline mount) for long term (e.g.,
several hours) observation of cultures. Specialized lighting
(phase contrast or dark field) is best for viewing organisms
that require 100x magnification or better, since they generally
have very little pigmentation. You will find more and more diverse
forms of life as your observing skills improve. |
