Consultant's Report - Minnesota State Legislature

Results for LCso and LC99 data are summarized in Table 4. LC values are shown
in bold type, and 95 % confidence limits in parentheses. All data are mg CulL.
Table 4. Summary ofadult LCso and LC99 values
Cutrine®-Ultra Adult LCso Adult LC99
48 h 1.2 (0.9 - 2.1) 8.8 (4.1- 53.5)
72h 0.6 (0.4 - 0.9) 2.6 (1.3 - 38.2)
96 h 0.3 (0.2 - 0.5) 1.8 (l.0 -7.7)
The results from this study suggest that 99 % mortality of adult D. polymOlpha in
the size range 15 - 25 mm would be expected following 96-h exposure to 1 mg CulL.
However, our 24-h data (not shown) suggest that 24-h exposures to significantly higher
concentrations might not elicit a significant effect, due to reduced exposure following
valve closure (Mersch et al. 1996). Following an additional 24-h exposure, however,
mussels were observed to die at high rates at the aforementioned concentrations. Several
previous studies have derived Cu-relevant TRVs in various water characteristics for adult
D. polymOlpha. Rao and Khan (2000) exposed mussels (15 - 20 mm) to solutions at 20
°C using reagent grade CuCb and determined 48-h and 96-h LCso values of 0.755 and
0.487 mg CulL, respectively. They observed increased sensitivity at 25°C. Kraak et al.
(1992) observed approximately 28 % mortality for mussels (16 - 20 mm) at 15°C after 9week
exposures to 0.104 mg CulL (CuCb) but observed no adverse effect at 0.013 mg
CulL. These data compare well with results from the CUlTent study. Waller et al. (1993),
however, observed much higher tolerance to Cu for mussels (20 - 25 mm), resulting in a
48-h LCso of5.38 (3.65 -7.93) mg CulL.
Adult mussels exposed to 2:: 1.0 mg CulL remained closed for the duration of the
96-h toxicity test. Substantial avoidancelvalve closure was also observed in
approximately 90 % of the individuals in the 0.1 and 0.5 mg CulL treatments; adults
allowing valves to open did not extend their siphons (as in the control and 0.01
treatments) and were lethargic when gently probed. We did not observe a significant
difference in filtering activity in the 0.01 mg CulL treatment. Kraak et al. (1992) derived
a 48-h NOEC (0.016 mg CulL), and ECso (0.041) for filtering activity, which is supported
by our observations. Monitoring the frequency of valve closure in D. polym01pha is a
good indicator of overall water quality (Stuijfzand et al. 1998). According to previous
studies (e.g., Morton 1958, Kraak 1992), the filtering apparatus of adult D. polymorpha
may possess chemoreceptors that can detect Cu at elevated concentrations, resulting in
significantly reduced filtration rates. This valve closurelavoidance ability may have
contributed to the relatively low mortality observed after 24 h in the 5.0 mg CulL (27 %)
and 1.0 mg CulL (1 %) treatments.
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