[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”._] The Oceans