Earthworms in Environmental Research - Earthworm Ecology - page 311

biological aspects. Again, like pH, soil moisture (or desiccation) and other confounding factors like
temperature (e.g., frost) can affect earthworm behavior and hence metabolism in general. Bauer
and Rmbke (1997) reported negative relationships for the effects of soil moisture on pesticide
uptake by

for parathion. Holmstrup et al. (1998) studied the impact of frost combined
with desiccation on the toxicity of copper to earthworm cocoons.
The influence of pH is also important in the ecotoxicological assessment of different soil cleanup
techniques. One of the cleanup methods is to wash the contaminated soil with acids, thereby
extracting the heavy metals present in those soils. To test the ecological change in the cleaned soil,
Van Gestel et al. (1993b, 2002) used some earthworm bioassays. They showed that these extractive
treatments might even increase the uptake of metal residues by earthworms from remediated soils.
In this context, there was an interesting observation by Cheng and Wong (2002) that earthworms
(

E. fetida

species) had a decreasing effect on the pH of a red soil, thereby influencing the
availability of Zn diethylene triamine pentaacetic acid (DTPA)- and NH

Pheretima

OHÏHCl-extractable frac-

2

tions) to earthworms in that soil.
Biological factors also influence seasonal fluctuations in the uptake processes of chemicals by
earthworms. Bengtsson and Rundgren (1992) reported, in a field experiment with Lumbricus

terrestris , that the uptake of Pb was lower during wintertime than in summer. The steady state of

the lead burden of the earthworms during the cold winter period indicated that uptake is an active
process, probably related to feeding, in which soil temperature, pH, and moisture play important
roles. Morgan and Morgan (1993) observed that the epigeic species accumulated

a higher Zn concentration during winter and early spring when earthworm activity is high. The
endogeic earthworm species

Lumbricus rubellus

accumulated lower Cd and Zn concentrations
during diapause than when active, which may be explained by active elimination and a significantly
higher Pb content in the earthworm. The higher Pb content was explained by greater retainment
of Pb at the same time that biomass decreased.
For the toxicity of organic compounds to earthworms, Van Gestel (1992) and Van Gestel and Ma

Aporrectodea caliginosa

(1993) developed a soil pore-water partitioning approach ( Figure 17.3 ) and derived quantitative
structure-activity relationships (QSARs) for a number of chlorinated hydrocarbons ( Figure 17.4 ) . This

has been further modeled and validated by Jager et al. (1998, 2000, 2003a,b) for the toxicity of organic
chemicals in general and extended to hydrophobic chemicals and dioxins/furans by Belfroid (1994)
and Loonen (1994). In studies on the uptake of these chemicals by earthworms from water, moist
soil, and soil plus food, they showed that the uptake proceeds in a monophasic way. By contrast, the
elimination of chemicals by earthworms in soil is biphasic, with a slow second phase similar to the
elimination rate in water (Belfroid et al. 1994). The first stage of fast elimination of chemicals could
therefore be ascribed to emptying soil from the gut. Loonen (1994) observed that, in the presence of
sediments, the aquatic earthworm species accumulated additional chemicals

not accounted for by the soil-water partitioning model, suggesting also an active uptake from sediment
particles. Such uptake was measured for soil in laboratory experiments by Belfroid (1994).
Jager investigated further these two uptake and elimination pathways using ligatured earth-
worms (a tissue adhesive technique developed by Vijver et al. 2003) and observed that the gut route
of elimination became more important compared with the skin route of loss with increasing
hydrophobicity of the contaminant. It seems that, in organic rich soils, this gut route of loss of
chemicals can be of greater importance. Relating these data to field conditions is still not possible
(Belfroid et al. 1996). As a consequence, there is a strong argument for using the potential or critical
body burden (or better, the critical body concentration) as an index of actual chemical exposure
instead of using applied doses (as suggested by Lanno et al. 1997 and Fitzgerald et al. 1997).
Loonen (1994) also observed, when repeating an accumulation study of toxicants in earthworms

Lumbricus variegatus

after a contact period of more than 2 years, that there was clearly decreased bioavailability ( Figure
17.5 ) . This indicates that contaminants may become bound to organic matter, mineral particles,

and micropores and thereby become less available for uptake into earthworms or other biological
process. This aging process is now broadly accepted and provides one of the main arguments why