[Illustration: _RAIN FOREST. A giant fan pulls air through a
plastic-enclosed portion of a Puerto Rico rain forest to study the
metabolism rate of trees._]
[Illustration: _HARDWOOD FOREST. Technicians preparing to tag Tennessee
trees with a solution containing a radioactive cesium isotope in the
start of a 10-year project. Scientists will study movement of the
radioactivity into insects and their predators._]
[Illustration: _FRESHWATER. Aquatic biologists emptying plankton traps
to study concentrations of radioactivity in microscopic organisms in the
Columbia River downstream from the Hanford atomic plant in Washington
State._]
[Illustration: _MOUNTAINS. Weather station in a deer-forage area of the
Rocky Mountains in Colorado provides environmental data and fallout
samples that are correlated with levels of radionuclides found in the
deer._]
[Illustration: _TUNDRA. This caribou was examined in detail as part of a
study of transfer of fallout nuclides in food chains from plants to
animals to man. Caribou is the principal meat animal of some Alaska
Eskimos._]
[Illustration: _DESERT. Zoologist examines an animal trap as part of a
field ecological study of a Nevada nuclear test site._]
Some radionuclides that are introduced into an aquatic environment enter
the food chains exactly as do the stable minerals essential to life,
because the radionuclides are merely radioactive forms of the nutrients.
Elements such as copper, zinc, and iron are less plentiful in the water
environment than hydrogen, carbon, or oxygen, for example, but are
concentrated by phytoplankton because they are necessary for life. Such
elements are in short supply but in constant demand; thus, when their
radioactive forms are deposited in water, they are immediately taken up
by aquatic plants and begin to move through the food chains. Fission
products such as strontium-90, for which there is little or no metabolic
demand, are taken up by aquatic food chains to only a minor extent.
The precise paths of radioelements through aquatic ecosystems are almost
unknown. In addition to their movement in food chains, radioelements
also may be moved physically from place to place in the tissues of fish
or other creatures. Some radionuclides for which there is no biological
demand may sink into bottom sediments and remain there until they have
lost their radioactivity. Or radioactivity actually may be transported
“uphill”, from water to land, as when birds that feed on fish containing
radioactivity leave their excretions at nesting areas. The routes and
modes of transport seem numberless.
[Illustration: _Movement of radioactive elements in a forest-lake
ecological system. Most nutrient-flow is “downhill”, but birds,
migrating fish, and the evaporation-rainfall cycle may move them
“uphill”._]