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Toxicity to aquatic algae and cyanobacteria

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Three algal chronic NOECs were taken forward for use in the derivation of the freshwater PNEC by statistical extrapolation. These were an EC10 of 0.41 ug Ag/L for Pseudokirchneriella subcapitata and an EC10 of 0.54 ug Ag/L for Chlamydomonas reinhardtii (Hiriart-Baer et al., 2006) and an EC10 of 0.41 ug Ag/L for Nostoc muscorum (geometric mean from Rai and Raizada (1985),  Rai and Raizada (1987), Rai et al. (1990)). A NOEC of 1.2 ug Ag/L  for Champia parvula (Steele et al., 1983) is taken forward for the derivation of the marine PNEC.
Three aquatic plant chronic NOECs were taken forward for use in the derivation of the freshwater PNEC by statistical extrapolation. These were an EC10 of 14.8 ug Ag/L for Salvinia natans (Hutchinson and Czyrska, 1975), an EC10 of 6.0ug Ag/L for Lemna minor (Naumann et al., 2007), and an EC10 of 10.33 ug Ag/L for Lemna paucicostata (geometric mean from two EC10 values reported in Nasu and Kugimoto (1981)).

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Key long-term data for three algal species were available. Data for Chlamydomonas reinhardtii and Pseudokirchneriella subcapitata were selected from Hiriart-Baer et al. (2006). Exposures to silver were conducted in the presence and absence of thiosulphate (an inorganic ligand) in “turbidostat” culture vessels, which maintain populations of algae at optimum conditions for growth and allow the measurement of the rate of population increase. Only the exposures conducted in the absence of thiosulphate were considered sufficiently reliable for PNEC derivation. Three replicate turbidostats per algal species were used to determine the growth rate of algae after successive increases in the concentration of silver in the turbidostat nutrient solution influent. The population growth rate was allowed to stabilise between successive increases (usually for between 18-24 hours). Results were reported on the basis of measured dissolved silver concentrations in the turbidostat effluent. EC10 values for population growth rate were calculated as 0.54 and 0.41 ug/L dissolved silver (measured) for C. reinhardtii and P. subcapitata, respectively.

Data were also available for Nostoc muscorm, a species of blue-green algae, from a series of three papers published between 1985 and 1990 by Rai et al. (Rai and Raizada (1985),  Rai and Raizada (1987), Rai et al. (1990)). In all of the three studies algae were cultured under static conditions for 15 days in 150 ml Erlenmeyer flasks at 25 ± 1°C containing 50 ml of buffered (pH 7.5) modified CHU10 nutrient media. Light intensity was 2500 lux with a 14 hour light and 10 hour dark photoperiod. Optical density at 663 nm was measured every three days in a spectrophotometer as a surrogate measurement for growth (although only 15-day data are reported in the papers). Silver exposure was based on nominal concentrations. EC10s for algal yield obtained from four independent tests ranged from 0.16 to 0.66 μg/L dissolved silver. The geometric mean of the EC10s was calculated as 0.41 ug/L dissolved silver, which is consistent with the data for other algal species.

Key long-term data for three species of macrophyte were available. The first is a seven-day EC10 of 6.0μg/L total silver (nominal concentration) for Lemna minor growth rate inhibition generated from a static test conducted according to ISO 20079 (Naumann et al., 2007). The second macrophyte species with data available was Lemna paucicostata with data from a further seven-day growth inhibition test, although this was not conducted according to a standard method (Nasu and Kugimoto, 1981). As this test was conducted under two pH regimes (6.1 and 7.1) the geometric mean of the two EC10 values (6.4 and 16.67μg/L total silver (nominal)) was used in the SSD (10.33 μg/L). The third macrophyte species with data available is Salvinia natans, a floating fern (Hutchinson and Czyrska, 1975). A 21-day EC10 of 14.8 ug/L total silver (nominal) was calculated from growth data presented in the paper.

The lowest reliable long-term result for marine algae is a 14-day reproductive NOEC of 1.2µg/L total silver for Champia parvula (Steele and Thursby, 1983). Day 0 analytical values were used to derive the toxicity values, as the lowest three concentrations were below the reported limit of detection (1.0µg/L). The Day 0 data for undetectable concentrations were calculated based on the dilution of the volume of standard added to the flasks. EDTA was present at 100µg/L in the exposure medium so that some of the silver would have complexed and potentially have been unavailable to the test organisms. The loss of silver over the 7-day period in the concentrations was approximately 50% (most likely as a result of sorption to algal cells and the test vessel).