7.3.1 marine water
Maximum and 30-day average criteria have been set despite the paucity of data for temperate marine fish since they are generally not sensitive to silver below levels of 70 µg/L; a factor of 10 greater than the sensitivity of most invertebrate and algal species. Criteria are set to protect the most sensitive lifestage of the most sensitive species. The literature indicates the most sensitive organisms are phytoplankton, followed by the embryonic and larval stages of invertebrates. Since the sensitivity of invertebrates and phytoplankton to silver is much greater than that of fish, and because a great deal of literature is available concerning their sensitivies, the requirements for marine fish datawere considered to be superfluous and were waived.
The criteria set are in agreement with the literature value of 0.1 µg/L of total silver, in an unfiltered sample, set to protect aquatic life (IJC 1982, Taylor et al. 1980, Ontario 1979 and 1984, CCME 1987).
Phytoplankton are at the base of the marine food chain, thus an affect on them has the potential to produce an effect on almost all marine primary and secondary consumers. A fifty percent reduction in growth or biomass, a standard chronic response or EC50 measure, represents a substantial drop in primary production. The open coast generally does not experience dramatic fluctuations in either salinity or temperature throughout the year and and it is relatively easy to set defensible and operationally functional criteria. Estuaries are a much more difficult situation. Silver toxicity has been shown to be inversely proportional to salinity (higher salinity results in lower toxicity) and in the estuarine environment both salinity and hardness fluctuate seasonally and with tidal changes. A criterion should reflect environmental fluctuations that can occur in the estuarine setting but it must also be operationally practical. We have chosen to leave the estuarine criteria the same as the marine and hard freshwater criteria recognizing that site specific situations may dictate that the low hardness criteria are more applicable in certain situations. The data support setting marine criteria the same as freshwater criteria in hard water, which is a reflection of the high hardness and high capacity of seawater to complex silver.
Site-specific criteria should be set for sewage outfall areas. Although a tolerance to elevated levels of silver apparently occurs, there is debate within the literature regarding whether accumulated levels of silver are toxic to the organisms. Detoxifying mechanisms, acclimation, and biomagnification potential are not well understood. Studies have shown that silver is tightly bound in sewage outfall areas, and that bacterial activity associated with sewage sludge may enhance the bioavailability of silver. Further research is needed in these areas to elucidate these processes. Complexation and speciation of silver in marine waters is not well understood. Silver may act synergisticly with copper, and cadmium may reduce silver toxicity. Field attempts to attribute an effect to the action of a single metal are often difficult, if not impossible. Experiments to determine the fate of silver, as well as that of other metals and their synergistic effects, are required. More good data are also needed on the acute and chronic effects of silver on marine fish species, especially larval and embryo life stages.
7.3.2 fresh waters
Water hardness, organism size and lifestage, and length of exposure affect the toxicity of silver to freshwater organsisms. In 1980a, the USEPA determined criteria from an equation which included the effect of water hardness; however, the results did not protect the most sensitive species in water with hardness >100. From the literature, Daphnia magna appears to be the most sensitive organism, followed by embryos and larvae of fish species. Therefore, we propose both a maximum value, to protect D. magna, and a 30-day average value, to protect the embryos and larvae of fish species. Juvenile and adult fish, aquatic insects, and aquatic plants all seem to have a resistance to silver 10 to 1000 times greater than the organisms and lifestages upon which the criterion is based.
Further research is needed in order to determine the mechanism of toxicity of silver, its synergistic effects, and its environmental fate. No data were found concerning effects on anandromous salmonids, particularly to the fry in soft freshwater habitats where the toxicity of silver is expected to be much higher (Davies et al. 1978). Research should be undertaken in this area in order to determine if the criteria will protect both hatchery fry and wild fry.
See Table 7.11 for the safety factors used in deriving criteria from the lowest reliable effects levels found in the literature.