Sublethal toxicity of metals to copepods. Sharon Hook. Hook@skio.peachnet.edu

Toxicity tests used for water quality criteria only examine exposure to metals via the dissolved phase, express results relative to ambient concentrations only, and do not consider the mechanism of toxicity. These tests may not mimic environmental conditions closely enough to accurately predict the impact of anthroprogenic contamination. My objectives were: 1) to compare the response of copepods to sublethal levels of metals (Ag, Hg, Cd, Zn, Mn and Se) accumulated from food and from water, 2) to relate any detected effects as a function of ambient concentration and to metal concentration within sensitive tissues of the organism, and 3) to characterize the mechanisms by which any toxic effects occur.

I conducted a series of experiments, in which copepods were exposed to sublethal concentrations of stable metals to determine toxicity via the dissolved phase, while another set was exposed to radiotracers of the same metals to determine metal body burden. In the next set of experiments, algal food was exposed to the same metal concentrations (either of radiotracers or of stable metals) and then fed to copepods. Toxic impact and body burdens were compared. I found that while exposure to dissolved metals did elevate metal body burdens, there were no discernible toxic impacts at sublethal concentrations (nM range), except following exposure to dissolved Hg, where a toxic impact occurred at concentrations elevated 50 fold above background. However, when copepods were exposed to metals via food, egg production decreased by roughly 50% at 1 nM Ag and Hg, or 5 nM Cd. Hatching rate and egg viability also decreased by 50%. As a consequence, the total reproductive output of the animal decreased by 75%.

This toxic impact occurred at body burdens elevated roughly 4-fold for Ag, 6-fold for Hg and 2-fold for Cd. These body burdens were well within the range observed following dissolved phase exposure. I hypothesized that the difference in toxic effect arose because of a difference in metal deposition following different uptake routes. Metals accumulated from food were primarily deposited in internal tissues, where they appear to have interfered with the organisms' metabolic activity, whereas those accumulated from water were primarily deposited in the exoskeleton. Zn behaved differently from the other metals in that Zn exerted toxicity at concentrations that are less than the copepod's normal Zn body burden. However, when copepods are allowed to accumulate equivalent levels of Zn over a week's time (instead of during a four hour pulse feed), no toxic effect was observed. This result implies that when metals are accumulated too quickly, they can overwhelm the cell's ability to detoxify. Thus, influx rate may be more important than body burden in determining toxic effect. When metal toxicity is expressed relative to body burden, there is a linear relationship between the affinity of the metal for sulfur and the amount of metal that needs to be accumulated in internal tissues to generate a toxic effect. Since the relationship between affinity for sulfur and toxicity is not as tight for either ambient concentrations or concentrations in phytoplankton, metal toxicity may be best expressed relative to internal body burden.