Scientists at the AEC’s Oak Ridge National Laboratory, for example, have developed experiments in which an analog computer is programmed to keep a running balance of the net changes—simultaneous gains and losses—of radioactivity in the various compartments of a representative ecosystem. The computer becomes an electronic image of the biosphere, using known or assumed rates of energy transfer and photosynthesis to predict probable radiological results of tracer experiments of environmental contamination. ENVIRONMENTS—SINGULAR, YET PARTS OF A WHOLE Each environment presents its own sets of conditions and unknowns. It is important to appreciate those that are characteristic of water, land, and atmosphere. Aquatic Systems The oceans are the basins into which are poured all the nutrients or wastes transported from the land by rivers and winds. The difficulty of determining the fate of radionuclides in aquatic systems is complicated by chemical and biological differences within the system and by the variety and scope of the circulatory mechanisms. In oceans the sheer immensity of the water volume usually makes observation superficial or fragmentary. Rivers present great differences in flow, and lakes vary in internal dynamics. Above all, an ocean, a river, or a lake is an area of constant physical and biological motion and change. In the ocean the surface waters form a theater of kaleidoscopic, and frequently violent, action. The presence of man-made radioactivity in water has made it possible to follow the disposition of nutrients and wastes in the restless aquatic ecosystem. Biological Uptake In a water environment the minerals necessary to life are held in solution or lie in bottom sediments. They become available to animal life after being absorbed by plants, both large floating or rooted plants and tiny floating ones called phytoplankton; because the phytoplankton are found everywhere in the sea, they play a larger role. The phytoplankton concentrate minerals and become food for filter-feeding fish and other creatures, including the smaller zooplankton,[14] which, in turn, are food for other organisms. Thus the minerals enter extremely complex food chains. The cycles of nutrition are completed when fish and plants die and decomposition again makes the minerals available to the phytoplankton. ENVIRONMENTAL RESEARCH [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._]