Zooplankton are microscopic invertebrate animals that
swim or drift in water. They are at the base of the food chain, feeding on microscopic
plants and being fed upon by aquatic insects, fish and salamanders. Their sizes usually
range from one-tenth of a millimeter to four millimeters, which is smaller than
the head of a pin.
HABITAT
At the Savannah River Site, zooplankton are found in freshwater reservoirs,
ponds and streams. They are abundant in Carolina
bays, wetlands
that often dry in the summer and fill with water in the fall.
Because the animals are wholly aquatic but often live in habitats that dry temporarily,
they are faced with problems related to the unpredictability of their watery
existence. Different types of zooplankton respond to these challenges in various
ways. Adult females may lay different types of eggs, depending on the season
and whether the pond is likely to dry soon.
Some eggs are resistant and do not hatch until the pond fills. Some emerge within
24 hours of the pond filling; others may take days or weeks to emerge. Some
juveniles go down in the mud and rest in a protective case when the bay dries
up. Some species produce resting stages when the pond dries, food supply declines
or the temperature changes.
Because zooplankton have low migration rates, zooplankton found in Carolina
bays provide a good system to study for population and community processes.
The community is fairly well contained, and researchers know a direct link exists
between what is found in the habitat from year to year.
FEEDING
Most zooplankton are filter feeders, using their appendages to strain bacteria
and algae and other fine particles in the water. Others are predators, feeding
on smaller zooplankton.
BREEDING
Zooplankton can reproduce rapidly, and populations can increase by about 30
percent a day under favorable conditions. Zooplankton reach maturity quickly
and live short, but productive lives. For example, adult females of a zooplankter
called Daphnia can produce their body mass in eggs every two to three
days. Daphnia live an average of one month.
RESEARCH
Researchers at the Savannah River Ecology Laboratory are studying how zooplankton
respond to the extreme variability of water levels in Carolina bays. This information
is being used to develop mathematical models that could predict the effects
of climatic change, or global warming, on biological communities. Also, scientists
are developing a computer-based model to examine how population variation is
related to environmental variation and what adaptations zooplankton have that
enable them to survive and leave "the best" number of offspring in
different kinds of environments.
Results from the models can be used to evaluate ecological consequences of decisions
that affect the water levels of Carolina bays and similar wetlands, thus supporting
the U.S. Department of Energy's land management
and conservation activities on the Savannah River Site.
DID YOU KNOW?
COMMON VARIETIES
Although food, temperature and water chemistry all are important in determining
what kinds of zooplankton can live in a particular lake or pond, the most important
factor is predators, particularly fish. Fish prefer to eat the larger and more
visible kinds of zooplankton. Thus, zooplankton that coexist with fish are typically
small (less than 1 to 1.5 millimeters) and transparent.
In contrast, zooplankton that live in ponds without fish, such as temporary
ponds, often are much larger (up to 3 to 4 millimeters). Some kinds of zooplankton
in fishless ponds are quite colorful: a variety called copepods can be bright
orange, blue or blue with red antennae. The very largest zooplankton, called
fairy and clam shrimps, live only in fishless ponds.
On the Savannah River Site and throughout the Upper Coastal Plain, three varieties
of freshwater zooplankton are most common. They are rotifers, cladocerans and
copepods. Also, fairy and clam shrimps live in ponds that dry out seasonally.
Rotifers are called such because some species -- scientists have identified
more than 1,700 species -- have a disc-shaped anterior end, covered with hairlike
protrusions, that resembles a pair of revolving wheels. The movement is actually
the synchronized beating of the hairlike protrusions, called cilia.
Rotifers are small animals with simple body forms. They have no legs, although
some have a single foot at the end of the body. Some species are protected by
a shield or shell-like structure called a lorica; some have spines or paddles
for protection; others live in colonies encased in a jelly-like substance.
An organ called the mastax is unique to the digestive system of rotifers, and
no comparable device is known elsewhere in the animal kingdom. It consists of
a complicated arrangement of muscles that activate a set of translucent jaws
used to seize, tear and grind food.
Cladocerans, commonly called water fleas, have been favorite objects of observation
by both amateur and professional biologists since the invention of the microscope.
Their bodies are not clearly segmented, and many species are covered by a shell-like
material. The head is a compact structure and bends downward. The most conspicuous
internal structure of the head is the cladocerans' large compound eye. Cladocerans
have two antennae, which help them move, and five or six pairs of lobed, leaf-like
legs. Cladocerans live in nearly all types of freshwater habitats; they are
most abundant in the spring.
Adult copepods have long, cylindrical or torpedo-shaped bodies with a single
eye and a pair of long antenna at the front, five pairs of legs along the middle
and a paddle-like tail at the end. They use their legs for swimming. Copepods
use small appendages near their mouths to capture food. Copepods lay eggs that
hatch into tiny nauplii that have only three pairs of appendages. Nauplii gain
legs and change form as they grow. Unlike other kinds of freshwater zooplankton,
copepods also are very abundant and important in the plankton communities of
the oceans.
Fairy and clam shrimps move along the bottom or swim about and glide about gracefully
in temporary ponds. They are distinctly segmented and have 10 to 71 pairs of
delicate, flat, swimming and respiratory appendages. These zooplankton are rarely
found in lakes; temporary ponds, such as Carolina bays, are one of their main
habitats.
This fact sheet was produced by the Outreach Program of Savannah River Ecology Laboratory.
Last review: October 12, 2007