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Ecological Effects 91 '"'. administration of AgNO. for intestinal disorders, 5 X 10-1 g ml-1 of silver has been measured in the blood with no adverse effects [Lawrence and Block, 1968]. Continued intake of silver salts by mammals causes an irremediable discoloration of the skin and mucous membranes called argyria. This apparently results from deposition of organo- silver compounds, which are reduced to a dark colored form by sunlight. A patient with 'the most generalized argyria ever reported in the literature' had the following concentrations of metallic silver: kidIley 0.24%, bone 0.21%, muscle 0.16%, heart 0.15%, liver 0.07%, and brain 0.01%. There was no evidence that the silver had produced any untoward effects except discoloration [Lawrence and Block, 1968]. Silver deposits apparently do not cause adverse reac- tions in adjacent cells. Ag+ injected in relatively small quantities into the blood stream of experimental animals combines almost quantitatively with plasma protein. It is rapidly removed from circulation by the liver, and eliminated in the bile through the gastrointestinal tract [Durbin, 1960]. The silver which does not follow this pathway is sequestered in the skin and mucous tissues. The so-called blood-brain barrier excludes Ag al- most completely from brain tissue [Scott, 1967]. Unlike lead and mercury, silver is not a cumula- tive poison. There are apparently no reports dealing with effects of silver on birds or reptiles, either with respect to adult metabolism or reproduc- tion. The Chemistry Section of the Patuxent Wildlife Research Center, U. S. Fish and Wild- life ~ervice, conducts routine assays of pesticide residues and heavy metals in birds and mam- mals from all over the United States, but they have not considered Ag to be a sufficient prob- lem to justify analyses for this element so far. Fish and aquatic invertebrates. Silver com- pounds are much more toxic to fish than to warm-blooded vertebrates. In one set of ex- periments, sticklebacks were able to with- stand indefinitely no more than 3 X 10-0 g ml-1 Ag in water at 15-180 C [McKee and Wolf, 1963]. The fish survived one week at 4 X 10-0 g ml-" four days at 10-8 g ml-" and only one day at 10-1 g ml-1. Lethalities were not stated in terms of LD.., so it is not clear what portion of the population expired at various ~ <; i~;' "~ I., times. Other investigators consider Ag to be among the most toxic of metals to fish. Solutions containing 2 to 4 X 10-8 g ml-1 of the metal have proved harmful to fresh water fish under various conditions [DoudorofJ and Katz, 1953]. There seems little doubt that silver interferes with gas exchange by the gills, but the precise mechanism is not clear. Some investigators believe that Ag in dilute solution precipitates mucous secretions in the gills, filling the inter- lammelar spaces to the point that normal move- ment of the gill filaments is impossible. This pre- vents the intimate contact between water and giII tissues necessary for oxygen uptake, and the fish die of suffocation. Others consider the mode of action to be swelling and breakdown of gill epithelium, perhaps through blocking of enzymes [Pringle et al., 1968]. This seems more likely in view of the low concentrations that are sometimes lethal. Virtually all laboratory tests of effects of silver on fish have been carried out with AgNO., which is presumably fully ionized at the con- centrations employed. The Ag in precipitation from seeded storms will be in the form of nearly insoluble AgI and perhaps in complex inorganic ions whose properties are only partly known. Additional research is needed on the chemical form and behavior of Ag compounds after nucleation. Silver in lakes and streams will reflect average concentration in precipitation from all storms, seeded and unseeded, and adsorption on vegeta- tion and bottom sediments will further reduce concentrations in water. For this reason silver concentrations in fresh water wiII generally decrease with distance from the source. Threshold concentrations of Ag toward aquatic invertebrates were 3 X 10-8 g ml-1 for Daphnia and Microregma and 15 X 10-8 for the flatworm Polycelis nigra in one set of investigations in- volving exposure for four days at 23-270 C [McKee and Wolf, 1963]. Terrestrial plants. Silver levels required to induce direct damage to higher plants are many times greater than wiII occur in precipitation from seeded storms. In one of the few known studies of the subject, Clark [1899J reported that Ag in a concentration of about 9.8 X 10-6 g ml-1 was fatal to maize, and 4.9 X 10-8 g ml-1 was fatal to lupines. Microorganisms. Silver is highly toxic to