7.5.1 general
The following listed tables provide some data on the effects of silver, under various conditions, on freshwater aquatic life.
Table 7.4-Acute effects of silver on freshwater organisms;
Table 7.5-Chronic effects of silver on freshwater organisms;
Table 7.6-Effects of acclimatization on silver toxicity in water of hardness 141 to Pimephales promelas, the fathead minnow;
Table 7.7-Comparison of the acute and chronic toxicity (to embryo/larvae) of different silver salts to the fathead minnow, Pimephales promelas;
Table 7.8-Effects of water hardness on the 96h LC50 of silver nitrate to larval rainbow trout, Oncorhynchus mykiss and juvenile fathead minnows, Pimephales promelas, in static and flow through bioassays;
Table 7.9-Effects of hardness on the 48h EC50 of silver nitrate to Daphnia magna in static bioassays and
Table 7.10-Mortality of fathead minnows exposed to measured concentrations of silver.
Silver is one of the most toxic of the heavy metals to freshwater micro-organisms, in several multi-metal toxicity tests it was placed first, second or third in the rank order of toxicity. Water hardness, as mg/L calcium, length of exposure, size of the organism and life stage of the organism all appear to affect the toxicity values. Variable reports of the validity of static versus flow-through tests within the literature exist; however static with renewal tests appear to be as accurate as flow-through bioassays. Invertebrates and embryos of fish are generally much more sensitive than juvenile and adult fish.
7.5.2 fish
Juvenile rainbow trout, Oncorhynchus mykiss (51 to 76 mm) were subjected to metal mining wastes (Hale 1977). In a continuous flow experiment, the trout were subjected to 5 to 80 µg/L of silver in water of hardness 100. The 96-hour TL50 was determined to be 6.6 µg/L silver. The order of metal toxicity was cadmium > silver > lead.
In 1974 Coleman and Cearley studied silver accumulation and toxicity in the largemouth bass, Micropterus salmoides, and the bluegill, Lepomis macrochirus, in a 6 month static assay in water of hardness 180. Silver, as AgNO3, was tested at 0.9 µg/L, 7 µg/L and 70 µg/L silver exposure. Only the 70 µg/L silver exposure level had any significant effect. It was toxic to the bass within the first 24 hours; however, the bluegill tolerated this concentration for six months with no significant mortality. No effects on growth or weight gain were noted for either species at any treatment level. The water was hard and had a chloride concentration of 193 mg/L; much higher than most natural fresh waters. Davies et al. (1978) recommended that the results of this study be used with caution since the silver would be mostly complexed by the chloride, and would likely precipitate due to the low solubility of AgCl, (1.5 mg/L, Jones 1939) rather than be available as the free Ag+ ion.
Davies et al. (1978) also reported the toxicity of silver to rainbow trout in hard water, hardness 350, and soft water, hardness 26. The LC50 values for hard and soft water were 13.0 µg/L and 6.5 µg/L silver, respectively. An 18-month experiment gave a NOEL of 0.09 to 0.17 µg/L. At >017 µg/L silver, premature hatching, high mortality of larvae, and reduced growth rate were observed.
In the study with O. mykiss, Davies et al. (1978) examined the effect of silver iodide in soft water, hardness 30, in a six-week study with fry, a 13-month study initiated with eyed eggs, and a 10-month experiment started with green eggs. In the six-week study with fry, exposure was set at 0.88 µg/L silver, resulting in 94 percent mortality and retardation in development and growth in the surviving fish. The 13-month NOEL using eyed-eggs was 0.03 to 0.06 mg/L silver. No other effects on growth or development were evident. The 10-month NOEL using green eggs was 0.18 to 0.40 µg/L silver, a value much higher than the NOEL for the eyed-eggs. This may be due to embryonic acclimatization to silver in the three months exposure prior to swim up; it may also be due to genetic differences. No additional effects on growth or development were noted.
The toxicity of AgCl to the threespine stickleback, Gasterosteus aculeatus, was determined by Jones in 1939. The lethal limit was 3 µg/L silver, which was fatal in approximately 25 minutes. Silver was found to be the most toxic metal of a suite of metals tested. The apparent mode of action of silver was determined to be precipitation of gill secretions causing asphyxiation. Jones (1939) was apparently careful to determine that the levels of silver in solution were below the solubility limits for AgCl, but no water quality data were given.
The flagfish, Jordanella floridae, and the fathead minnow, Pimephales promelas, had 96-hour LC50 values determined in a flow-through bioassay in water of hardness 44 and temperature of 24.7C (Lima et al. 1982). The values were 9.2 µg/L and 10.7 µg/L silver for the flagfish and the fathead minnow, respectively. Most fish died shortly after showing signs of stress, and fathead minnows had bright red gills at death. Lemke (1981) gives a 96-hour LC50 value of 3.9 to 12 µg/L silver in water of hardness 40 to 49.
A static renewal assay in water of hardness 250 and temperature of 30C was used to determine the acute toxicity of silver to the fish Puntius sophore, Channa punctatus and Lebistes reticulatus (Khangarot et al. 1988b). Ninety-six hour LC50 values were 7.55 µg/L, 18.89 µg/L and 6.44 µg/L silver, respectively. LC50 values determined at 12, 24, 48 and 72 hours also indicate that silver toxicity increased with the time of exposure. Behavioural and pathological observations noted in early stages of exposure were erratic opercular movement, difficulty in respiration, convulsions and surfacing. In later stages of exposure, behaviour included loss of equilibrium, erratic body movement, upside-down swimming, irregular opercular movement and absence of shoaling. At death, observations made included copious mucus on skin and gills, light and dull color of gill filaments and shrinkage of gill lamellae and hemorrhaging of the mouth and caudal regions. Silver toxicity was inversely proportional to fish size, with greater toxicity in smaller fish. Cause of death was believed to be gill damage resulting in hypoxia, but was possibly also poisoning of enzymes related to gas exchange.
In 1983, Holcombe et al. reported on the acute toxicity of silver in both flow-through and static tests in water of hardness 40 to 45 and temperature of 23 to 25C, to the fathead minnow, Pimephales promelas, and the channel catfish, Ictaluraus punctatus. Chronic tests were also conducted on the larvae of the fathead minnow.
Ninety-six hour LC50 values for the fathead minnow were 6.7 µg/L silver for the flow-through test, and 14.0 µg/L silver for the static test, indicating that flow-through tests were more sensitive. Lemke (1981) reported 96-hour LC50 values for the fathead minnow in soft water ranging from 3.9 to 30 µg/L silver, and in hard water as ranging from 110 to 270 µg/L silver. The 96-hour LC50 value for the channel catfish (under flow-through conditions) was 17.3 µg/L silver, which is much greater than the value for the fathead minnow. However, the catfish average weight was approximately 100 times that of the minnow. Silver toxicity increased with increasing period of exposure. The 28-day chronic assay with the fathead minnow larvae gave a NOEL of 0.37 µg/L silver and a maximum acceptable concentration (MATC) of 0.65 µg/L silver.
The effect of speciation on the acute and chronic toxicity of silver was compared using the fathead minnow as the test organism (LeBlanc et al. 1984). Silver sulfide, silver thiosulfate and silver chloride were compared to Ag+, added as silver nitrate. The tests were flow-through in water of hardness 38 and temperature 25C. The 96-hour LC50 value for Ag+ (as silver nitrate) was 16 µg/L silver. Silver chloride was found to be 300 times less toxic, silver sulfide was 15,000 times less toxic, and silver thiosulfate was 17,500 times less toxic than Ag+. The 30 day MATC's for silver sulfide and silver thiosulfate were determined to be >11,000 µg/L silver and 16,000 to 35,000 µg/L silver, respectively.
Nebeker et al. (1983) reported on a series of side by side (within the same compartmentalized container), flow-through and static acute tests with both steelhead and rainbow trout (Oncorhynchus mykiss) in water of hardness 26 to 42 and temperature of 9 to 12C; the fathead minnow, Pimephales promelas , in water of hardness 38 to 46 and temperature of 20 to 22C; and a 60-day chronic assay with embryo/larvae of the steelhead trout.
Four static tests were conducted with rainbow trout, two aerated and two non-aerated. The 96-hour LC50 values were reported as 72.9 and 84.4 µg/L silver in the aerated tests and 10.9 and 8.5 µg/L silver in the non-aerated tests. A significant difference between the two was the much larger fish size in the aerated tests. The two flow-through tests conducted with the rainbow trout gave LC50 values of 8.6 and 9.7 µg/L silver, and for the steelhead trout 9.2 µg/L silver. The results of the flow-through and the static tests were not significantly different, nor were the results of the steelhead and the rainbow flow-through tests. The static fathead minnow assays gave LC50 values of 9.4 and 9.7 µg/L silver, and the flow-through assays gave LC50 values of 5.6 and 7.4 µg/L silver, an average factor of 0.7 lower; however, the authors did not specify if this was significant. The result of the chronic study was a 60-day MATC of <0.1 µg/L silver, and an LC100 value of 1.3 µg/L silver. The authors noted a significant decrease in percent survival at 0.5 µg/L silver and a significant reduction in growth at 0.1 µg/L silver.
A technique for simultaneous multi-species testing in freshwater of hardness 44 and temperature of 17C was developed by Holcombe et al. (1987). The authors believed the technique provided a more accurate direct comparison of species sensitivity and behavioural responses. Species tested for silver toxicity included the fathead minnow-Pimephales promelas, rainbow trout-Oncorhynchus mykiss and bluegill-Lepomis macrochirus. The 96-hour LC50 values obtained were 9 µg/L of silver, 6 µg/L of silver and 13 mg/L of silver, respectively. An overall species sensitivity factor, defined as {[LC50 (most resistant species)] ÷ [LC50 (least resistant species)]}, of 622 was calculated. Though not as sensitive as Daphnia magna to silver, the rainbow trout was considered the most all-round sensitive species for toxicity testing. Fish, except for the rainbow trout, were less sensitive than D. magna by a factor of 10. Silver was the only metal tested, the others were organics, and was the most toxic of the test reagents to all species, on average.
In 1970 Jackim, et al. reported on the activity of five liver enzymes from the estuarine killifish, Fundulus heteroclitus, exposed to the toxic metals copper, lead, mercury, cadmium, silver and beryllium at concentrations approximating the 96-h LC50. Silver did not affect all the enzymes to the same extent. Liver acid phosphatase was not affected in vivo although there was 50% inhibition by 10.8 mg/L in vitro. Mercury and silver were the most potent inhibitors of catalase in vitro and fish exposed to 40 µg/L had decreased liver catalase activity. One should use caution in extrapolating in vitro experiments to natural ecosystems since different chemical mechanisms may act under the two conditions and the chemicals may not reach the active sites in the whole animal.
Water hardness is an important factor for the toxicity of some heavy metals such as copper and zinc but appears to be less important for silver. The 96-h LC50 for rainbow trout, Oncorhynchus mykiss, was 8.1 µg silver/L in water of hardness 26 and 13 µg silver /L in water of hardness 350 (Goettl et al. 1976). Developing eggs were exposed to silver nitrate and the no effect level lay between 90 and 170 ng silver /L. Silver concentrations over 1 µg/L caused the eyed-eggs to hatch before complete development and the fry soon died. Silver iodide was more toxic than siver nitrate and the no effect level was between 30 and 70 ng silver /L (Goettl et al. 1976).
In 1976, Davies recommended a concentration between 70 and 130 ng/L as being safe. After a one-year exposure to silver, the mortality of rainbow trout fry after swim-up decreased from 38% at 500 ng silver/L to 18% at 130 ng silver/L and 3% at 70 ng silver/L. There was no mortality in control fish. For the carp, Cyprinus carpio, the 96-h LC50 was 3.8 µg AgNO3/L in water of hardness 118 (Rao et al. 1975).
7.5.3 amphipods
Crangonyx pseudogracilis, a freshwater amphipod, was subjected to acute exposures of silver and other metals in a static assay in water of hardness 45 to 55 and temperature of 13C for 96 hours (Martin and Holdich 1986). Forty-eight and 96-hour LC50 values were 6 µg/L of silver and 5 µg/L of silver, respectively. After 48-hours the increasing order of metal toxicity was silver > mercury > manganese (VII), but at 96 hours the order had changed slightly to mercury > silver >chromium (VI)> manganese (VII). The values were expressed as ppm in the paper, but the authors suggest that only molar equivalents are comparable.
7.5.4 rotifers
Rotifers are a major part of some fish diets and the effects of silver were reported by Buikema et al. in 1974. The 96-hour EC50 to Philodina acuticornis in water of hardness 17 was 1.4 to 1.7 mg of silver per litre. The silver salt used was AgNO3.
7.5.5 amphibians
Tadpoles of the toad, Bufo melanostictus, were tested for sensitivity to silver and other metals in water of hardness 185 and temperature of 31C (Khangarot and Ray 1987b). LC50 values were calculated at 12, 24, 48, 72 and 96 hours. The 96-hour LC50 value was 4.1 µg/L silver, an order of magnitude lower than the 12-hour LC50 value. The rank order of toxicity of the metals to the toad was: silver > mercury > copper > cadmium > zinc > nickel > chromium.
7.5.6 cladocerans
A D. magna renewal life cycle test method was evaluated in a series of inter-laboratory tests involving different water quality parameters (Nebeker 1982). The assays were to determine the 48-hour EC50, the 21-day EC50, the NOEL (as number of young/female/day) and the MATC. The 48-hour EC50 values ranged from 0.65 µg/L silver at a water hardness of 46, to 51.5 µg/L silver at a water hardness 255. The EC50 values from water of hardness 60 were generally <1.0 µg/L silver, while the EC50 values from water of hardness >60 were generally >10 µg/L silver. The 21-day EC50 values ranged from 2.9 to 3.9 µg/L silver. The NOEL ranged from 1.6 to 8.8 µg/L silver in water of hardness 60, and from 10.5 to 20.0 µg/L silver in water of hardness >60. The MATC ranged from 3.1 to 19.4 µg/L silver in water of hardness of 60, and from 21.2 to 41 µg/L silver in water of hardness of >60. The chronic concentrations are greater than the acute concentrations, due to the addition of food which would complex the silver and reduce its bioavailability.
Anderson (1948) studied the effects of AgNO3 and other chlorides of metals on Daphnia magna. The test took place over 64 hours to ensure that ecdysis would occur. The toxic threshold was determined to be 5.1 µg/L silver and ecdysis was deemed to exert little effect on the toxicity of silver. Silver appeared to be one million times more toxic than the other metals, including mercury. In 1959, Bringmann and Kuhn reported the lethal level of silver nitrate to D. magna to be 30 µg/L.
Khangarot and Ray (1987a) determined the acute toxicity of silver to D. magna and attempted to find a correlation between D. magna EC50 values and LC50 values for the rainbow trout, Oncorhynchus mykiss. The 48- and 96-hour D. magna EC50 values were 23 µg/L silver and 10 µg/L silver, respectively. The difference between the two values was significant.
A strong correlation between D. magna and rainbow trout sensitivity was found (straight line regression, r2 = 0.814) and therefore D. magna is presented as a pollution monitor species. The rank toxicity of metals to D. magna was: mercury > silver > copper > zinc > chromium = cadmium = lead > nickel.
A 48-hour toxicity test was conducted using three cladoceran species, Daphnia pulex, Ceriodaphnia reticulata and Simocephalus vetulus (Mount and Norberg 1984). The tests were static assays with food in the solution (50,000 to 100,000 bacterial cells per culture). It is not clear from the text whether the test used renewal techniques. The 48-hour LC50 values for silver were: 14 µg/L silver for D. pulex, 11 µg/L silver for C. reticulata and 15 µg/L for S. vetulus. It is likely that these values are higher than they would be if there were no food in the solution.
In a study using several species and methods, Nebeker et al. (1983) determined the toxicity of silver to D. magna with and without food. The mean result of three acute 96-hour assays without food, at a hardness of 33 to 40 and temperature of 20C, was an EC50 of 0.9 µg/L silver; one assay with food gave an EC50 of 12.5 µg/L silver. To determine how much of the silver was lost to the food, water with 4 µg/L silver was added. When the concentration of silver in the water was measured, the average level was 4.2 µg/L silver. When the water was filtered before the silver content was measured, 59% of the silver was lost. When food was added to the water, but the water was not filtered, 42% was lost. When food was added to the water and then the solution was filtered, 89% of the silver was lost.
7.5.7 worms
The tubificid worm, Tubifex tubifex, was exposed to several metals in a series of 96-hour acute assays at a hardness of 245 and temperature of 30C (Khangarot 1991). EC50 values were calculated at 24, 48 and 96 hours. The 96-hour EC50 value was 31 µg/L silver, an order of magnitude lower than the 24-hour EC50 value. The rank order of toxicity of the metals to Tubifex tubifex was: mercury > silver > copper > zinc > nickel > cadmium > lead.
7.5.8 molluscs
A 96-hour assay determined the toxicity of silver and other heavy metals to the pulmonate snail, Lymnaea luteola, at a water hardness of 195 and temperature of 32C (Khangarot and Ray 1988a). LC50 values were determined at 24, 48, 72 and 96 hours. The 96-hour LC50 value was 4.2 µg/L silver, 10 times lower than the 24-hour LC50 value. The highest mortality was observed to occur in the first 48 hours, after which it decreased. The rank order of metal toxicity to the snail was: silver > mercury > copper > nickel = cadmium = zinc > chromium.
A static-renewal test at a hardness of 50.4 and temperature of 25.5C was carried out using a number of aquatic organisms including the the snail, Aplexa hypnorum (Holcombe et al. 1983). In a 96-hour assay, with renewal taking place every 24 hours, the LC50 value was 241 µg/L silver. The snails were generally the most resistant species for all the chemicals tested.
7.5.9 insects
Nehring (1976) investigated the sensitivity to silver of two aquatic insects, the mayfly, Ephemerella grandis, and the stonefly, Pteronarcys californica, in a series of flow-through assays at a hardness of 30 to 70 and temperature of 3 to 9C. The mayfly was more sensitive than the stonefly with a TL50 value of <1 µg/L silver, compared to 4 to 9 µg/L silver for the stonefly. Accumulation of silver was also studied in both species of insect at several levels of exposure. The results indicated that both accumulated silver, with a concentration factor of 113 for the stonefly and 195 for the mayfly. Also noted was a strong correlation to accumulation with respect to exposure concentrations, indicating that the insects accumulated silver by some constant amount, which in turn suggests that they might be useful as a pollution monitor species. The 7-day LC50 for the immature stoneflies was below 4 µg/L but 9 µg/L killed 96% of the mayflies in 10 days (Goettl et al. 1976).
7.5.10 miscellaneous and mixed invertebrates
The 96-hour LC50 for the scud, Gammarus pseudolimnaeus, and the 48-hour LC50 for the midge, Tanytarsus dissimilis, were determined in water of hardness 44 in a test conducted under flowing conditions (Lima et al. 1982). The scud was much more sensitive than the midge, with a 48-hour LC50 value of 4.7 µg/L silver and a 96-hour LC50 value of 4.5 µg/L silver, as opposed to a 48-hour LC50 value of 3160 µg/L for the midge. After 22 hours of exposure, the scuds exposed to the two highest concentrations of silver, 15.3 µg/L and 35.6 µg/L, were dead. Lemke (1981) is cited, giving a 96-hour LC50 value of 0.39 to 2.9 µg/L silver at a hardness of 40 to 49 for Daphnia magna, indicating that Daphnia is more sensitive to silver than the scud.
Holcombe et al. (1987) conducted a series of simultaneous tests with a large number of species using a flowing system and compartmentalized tanks at a water hardness of 44.7 and temperature of 17C. Invertebrates tested were: the cladoceran, D. magna, the leech, Nephelopsis obscura, the snail, Aplexa hypnorum, the midge, Tanytarsus dissimilis and the crayfish, Orconectes immunis.. The results are an EC50 of 0.9 µg/L silver for D. magna , an LC50 of 420 µg/L silver for the midge and an LC50 of 560 µg/L silver for the crayfish. The snail had an LC50 value of 83 µg/L silver, and the leech an LC50 value of 29 µg/L silver. D. magna was the most sensitive species tested to silver.
7.5.11 plants
Brown and Rattigan (1979) carried out a 24-hour study to determine the effects of silver and other metals on photosynthesis, and a 28-day chronic study on phytotoxicity with the aquatic macrophyte Elodea canadensis. The test water was described as soft. Results of the acute study were given as the I50 or the I90 (inhibiting photosynthesis by 50% or 90%, respectively). The I50 and the I90 had values of 100 µg/L silver and 180 µg/L silver, respectively. The results of the chronic study were given as the concentration which caused 50% plant damage with respect to the control. For E. canadensis the value was 7500 µg/L silver, and for another aquatic macrophyte, Lemna minor, the value was 270 µg/L silver. It was noted that silver caused a significant increase in oxygen uptake, similar in magnitude to copper, just before complete cessation of photosynthesis.
Nasu and Kugimoto (1981) investigated the sensitivity of the duckweed, Lemna paucicostata, to silver. Using a static assay at 25C and two different growth media, the authors also attempted to determine if pH would have an effect. No pH effect was noted; however, in one medium at pH 4.1 and low free silver ion concentrations, 100 to 1000 µg/L, Lemna flowered.
Flowering was not observed in the second media. Growth of new fronds was reduced 50% with respect to the controls for plants grown in 10 mg/L silver. Hutchinson and Czyrska in (1975) reported that silver was less toxic to the duckweed, Lemna valdiviana, and the aquatic fern, Salvinia natans, than to several species of algae. There was no growth inhibition at 50 µg/L but there was inhibition at 500 µg/L.
7.5.12 algae
Chlorella vulgaris growth was first slowed at 10 µg silver/L and growth was completely inhibited at 60 µg/L (Hutchinson and Stokes 1975). Bringmann and Kuhn (1959) reported the lethal level of silver nitrate to Scenedesmus quadricauda to be 50 µg/L. The toxicity thresholds for the onset of cell multiplication inhibition, for the blue-green alga, Microcystis aeruginosa, and the green alga, Scenedesmus quadricauda, were 0.7 and 9.5 µg silver/L, respectively. They were grown in stock nutrient solutions and distilled water (Bringmann and Kuhn 1978). Lab strains of Scenedesmus and Chlorella were inhibited by 100 µg silver/L and 30 µg silver/L, respectively. The Scenedesmus grew at 50 µg silver/L (Stokes et al. 1973).
7.5.13 bacteria
Silver thiosulphate and insoluble silver sulphide had no effect on activated sludge at 100 mg/L, but 10 mg/L silver nitrate or silver chloride caused 84% and 43%, respectively, inhibition of oxygen uptake (Bard et al. 1976). Freshwater bacteria community numbers were not affected but their heterotrophic activity was affected by 10 and 100 ng silver/L (Albright and Wilson 1974). Klein and Giangiordano (1976) showed that silver at 5, 10 and 100 µg/L caused an increase in growth delays at low temperatures in populations of Escherichia coli and Hyphomicrobium. Transferring the cells to higher temperatures, 9 to 16C, neutralized the effect of the silver. In Pseudomonas aeruginosa, silver sulfadiazine affected the DNA. Silver ion binding to the DNA was both chemically and biologically reversible. Inhibition of growth started at 0.32 µg silver/L and was complete at 5.4 µg/L. Cell death occurred at 21.6 µg/L (Modak and Fox 1973). Bringmann and Kuhn (1959) reported the lethal level of silver nitrate to Escherichia coli to be 40 µg/L. Bacterial and fungal spores are more resistant to silver than the vegetative stages and yeasts are more resistant than bacteria (Klein 1978).
Research Needs
Most existing silver criteria, objectives or regulated amounts are not based on the free ionic monovalent ion, which is acutely toxic to aquatic life; they are based, instead, on total silver which includes the metal, complexes and precipitates all of which are very much less toxic than the monovalent ion. Thus, these existing regulations and criteria are often overprotective.
Regulations should reflect the appropriate risk but the problem is that there is no monovalent ion specific measurement. Therefore, total silver is measured to provide a margin of safety. In addition, some non biologically-available silver may be in forms that are in equilibrium with monovalent silver and thus much of the silver pool becomes ultimately available as the monovalent silver is taken up. Benthic organisms will take up some insoluble forms of silver as they graze, and thus more than just the monovalent form is available to them.
A method of measuring the biologically-available forms of silver is needed so that the criteria and the risk are well correlated.