Zooplankton Fact Sheet
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.
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.
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.
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.
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
DID YOU KNOW?
- The total weight of zooplankton produced annually
in Rainbow Bay, a Carolina bay on the Savannah River Site, is about
twice the weight of the thousands of salamanders found there.
- If environmental factors did not regulate the
population growth of zooplankton, a variety called Filinia could
cover the entire world to a depth of 1 meter in 130 days.
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
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.