-~ 51 form by precipitation, co-precipitation with accompanying materials, or by adsorption to organic material or inorganic silts and clays. As in other solutions, precipitation will occur only when the solubility product of the least soluble compound of the element has been exceeded. Prediction of the physical state of fission products in sea water is difficult because most of them are elements that occur only rarely in the complex mixture of elements in the sea and little is known of their ionic activity. Although water movement is the principal method of horizontal transport of fallout materials in the ocean and the physical and chemical forms of fallout are important to vertical transport, the uptake of radioisotopes by the biota temporarily removes some of the fallout material from the forces of gravity and also may be an important factor in the vertical transport of materials in the ionic form across water boundaries such as the thermocline and the stratified layers below the thermocline. Plankton and the organisms of the “deep scattering layer" commonly make diurnal migrations from deep water to | near the surface. When fallout material first enters the water, plankton acquire radioisotopes in both the soluble and insoluble