Chemical Mixtures and
Pulsed Exposure Tests

Results

Food Type and Maternal Age Experiment.  There were no trends or significant differences in survival or reproduction between the 7 and 14-day maternal age treatments for any of the copper or zinc tests (Table 3).  Therefore, USEPA maternal age standards should be sufficient to ensure that differences in maternal age up to 7 days are not a significant source of variability in C. dubia chronic toxicity tests.

For the copper tests, all LC50s and IC50s were consistently greater for high solid treatments than for low solid treatments within the same maternal age group (Table 4).  For survival, significant differences were found between LC50 values for high and low solids treatments for both the 7 and 14-day treatments of the Dec. 4 toxicity test, as well as for the Nov. 8 and Dec. 4 toxicity tests when maternal age treatments were combined. For reproduction, significant differences were found between IC50 values for high and low solids treatments for the 7-day treatments of the Oct. 18 and the Dec. 4 tests, as well as for both these dates when maternal age treatments were combined.

There were no significant differences in survival between high and low solids treatments for zinc (Table 4).  For zinc, with the exception of the March 8 test, the high solid treatment LC50s and IC50s were generally greater than low solid treatment LC50s and IC50s within the same maternal age group, although not significantly so.  For diazinon, there was no significant difference in survival or reproduction between high and low food solid treatments in either of the two toxicity tests.

For diazinon, C. dubia survival was reduced at concentrations that did not inhibit reproduction, making survival the more sensitive endpoint measurement.  Consistent with previous studies (Belanger and Cherry 1990), reproduction was reduced at concentrations of zinc and copper well below those that affected survival.  For these metals, inhibition of reproduction is a more sensitive measure of toxicity than mortality.  In natural waters, reproduction may be reduced even at sub-lethal concentrations, resulting in compromised community structure.  Differences in endpoint sensitivity between the metals and the organophosphate are probably a result of the very different modes of action that the two chemical groups have in exposed organisms (Hunter and Liss 1982).

The pattern of differing degrees of toxicity for each of the three toxicants can be explained by how available the toxicants are to the organism.  Studies of Daphnia magna have shown that mortality in the presence of zinc and cadmium contaminated solids was consistently lower in static tests than in flow-through designs (Weltens et al. 2000).  This indicates that particulate adsorbed contaminants that settle out of suspension are less available to filter-feeding organisms, changing the concentration of bioavailable toxicant to which the filter-feeding organism is exposed. Therefore, the degree of adsorption of toxicants is important in interpreting toxicity test results.

The expected adsorption of each of the toxicants was related to the degree of decreased toxicity exhibited by the toxicants in the presence of increased suspended solids. Copper and zinc, which exist in water as positively charged divalent ions, will likely adsorb to the negative surface charge of suspended aquatic particles (Hunter and Liss 1982, Loder and Liss 1985), resulting in less free dissolved copper or zinc in solution (Weltens et al. 2000, Pantani et al. 1995). Our results also showed that copper toxicity decreased to a greater degree than did zinc toxicity when the amount of solids was increased or when the solids were added prior to, as opposed to after, organism transfer.  This may be because the adsorption constants for Cu(II) are significantly greater than those for Zn(II) (Ashley 1996). Most research indicates that diazinon, with a water solubility of 40 mg per ml and an octanol-water partitioning coefficient of 3.30, is both sparingly soluble (Iglesias-Jimenez et al. 1996) and moderately environmentally mobile (USEPA 2000b), indicating that a high degree of adsorption does not occur.  This is supported by studies which indicate that only 15 to 20% of diazinon adsorbs to soil (USEPA 2000b, Arienzo et al. 1994) and reinforces the lack of significant differences between toxicity in the high and low food solid treatments.

Over the period from January 2001 to February 2002, samples of stormwater entering Burlington Bay have contained total suspended solids ranging from 13.17 to 355.5 mg/L  (avg = 96.62 mg per L, n = 12) (Watzin et al. 2002).  Urban stormwater runoff in Denmark typically contained between 30 and 100 mg per L of suspended sediment (Ellis and Hvitved-Jacobson 1996).  In traditional, non-effluent, concentration-response toxicity tests, organic solids are introduced only in the form of USEPA recommended food (Lewis et al. 1994).  Taking into account only YCT solids, our test chambers contained 11.6 mg per L and 17.0 mg per L of suspended solids in the low and high food treatments, respectively.  Even if the non-algal food solids were included in this calculation, the resulting suspended solid concentrations would still be much lower than those commonly found in the stormwater runoff.

Therefore, in aquatic systems containing high concentrations of suspended solids, it is likely that the particulates adsorb some of the contaminants in stormwater.  As these particles settle out of suspension, they decrease the concentration of toxicants to which the organisms are exposed, and generally lower the toxicity to suspended filter feeders.  However, it is also possible that in natural conditions, particulates that do not settle out of suspension are ingested by filter feeders and might actually increase toxicity to the exposed organism.  Such a scenario was demonstrated with D. magna exposed to zinc and cadmium in a flow-through system (Weltens et al. 2000).  Simultaneously, as these contaminant-laden particles accumulate on the bottom, organisms that live among and feed upon the settled particles may also have a greater exposure to contaminants on the lake floor. 

Arienzo, M., T. Crisanto, M.J. Sanchez-Martin and M. Sanchez-Camazano. 1994. Effect of soil characteristics on adsorption and mobility of 14C diazinon. J. Agric. Food Chem. 42(8):1803-1808.

Ashley, J.T.F. 1996. Adsorption of Cu(II) and Zn(II) by estuarine, riverine and terrestrial humic acids. Chemosphere. 33(11):2175-2187.

Belanger, S.E. and D.S. Cherry. 1990. Interacting effects of pH acclimation, pH, and heavy metals on acute and chronic toxicity to Ceriodaphnia dubia (Cladocera). J. Crustacean Biol. 10(2):225-235.

Ellis, J.B. and T. Hvitved-Jacobson. 1996.  Urban drainage impacts on receiving waters.  J. Hydraul Res. 34(6):771-783.

Hunter, A., and P.S. Liss. 1982. Organic matter and the surface charge of suspended particles in estuarine waters. Limnol. Oceanogr. 27:322-335.

Iglesias-Jimenez, E., M.J. Sanchez-Martin and M. Sanchez-Camazano. 1996. Pesticide adsorption in a soil-water system in the presence of surfactants. Chemosphere. 32(9):1771-1782.

Lewis, P.A., D.J. Klemm, J.M. Lazorchak, T.J. Norberg-King , W.H. Peltier and M.A. Heber (eds).  1994. Short-term methods for estimating the chronic toxicity of effluents and receiving waters to freshwater organisms, 3rd edition.  EPA/600/4-91/002. U.S. Environmental Protection Agency - Office of Research and Development. Cincinnati, Ohio.

Loder, J.C. and P.S. Liss. 1985. Control by organic coatings of the surface charge of estuarine suspended particles. Limnol. Oceanogr. 30:418-421.

Pantani, C., N. Spreti, A. Arizzi Novelli, A. Volpi Ghirardini and P.F. Ghetti. 1995. Effect of particulate matter on copper and surfactants’ acute toxicity to Echinogammarus tibaldii (Crustacea, Amphipoda). Environ. Technol. 16:263-270.

U.S. Environmental Protection Agency.  2000b. EFED RED Chapter for diazinon: Environmental Risk assessment for diazinon. Docket Control No. OPP-34225, 0011. PC Code No. 057801, Case No. 818962. Office of Prevention, Pesticides and Toxic substances.

Watzin, M., A. McIntosh, A. Shambaugh, A. Pitt, A. Mahar, E. Brines. The Burlington Bay Project: Water quality and ecosystem health along the shores of Lake Champlain. Progress report April 30, 2002. Unpublished report.

Weltens, R., R. Goossens, S. Van Puymbroeck. 2000. Ecotoxicity of contaminated suspended solids for filter feeders (Daphnia magna). Arch. Environ. Contam. Toxicol. 39:315-323.