The organisms that make
up the freshwater
plankton are all those
microscopic creatures
living free in the open
water, including
photosynthesizers, whose
careers depend more
on the currents of water
movement than on
any swimming ability of
their own. Planktonic
animals may be able to
rise and fall in the
water column, depending
on whether they are
attracted to sunlight or
flee it, but their
powers are so slight that
they are at the
mercy of water movements.
Wind and wave,
upwelling currents and
downdrafts, are equally
in control. In fast-moving
streams, planktonic
organisms tend to be swept
away. In a swift
upland stream, there may
be many insect larvae,
but there are few planktonic
creatures. Freshwater
plankton more characteristically
develops
in the slower-moving waters
of lakes and
swamps, and of course our
aquaria provide
comparably hospitable lentic
habitats.
Freshwater plankton are most abundant in
still waters that are rich in algae
or organic matter
that supports bacteria. No matter how powerful
your filtration or how strong the outflow
current from your filters, aquarium systems
are "lentic" rather than "lotic,"
characteristic of still waters rather than
flowing ones. Algae, organic debris or solutes,
and bacteria: these form the three common
bases of the planktonic food web.
