Agriculture Reference
In-Depth Information
at least a known, dependence on physiological and physicochemical conditions and a
good baseline data set. The induction time and the persistence of a potential biomarker
response should be known to estimate the likelihood and significance of detecting a true
response (avoiding false positives) in field samples. The possibility of adaptation to
stressful conditions could also influence the reliability (S.A. Reinecke et al. 1999; S.A.
Reinecke and Reinecke 2003). The earthworm ecotoxicology workshop in 2001 recom-
mended that confounding factors, such as drought, temperature, and a linkage between
the biomarker and the physiological responses, should be clearly established. This
includes species differences and the influence of both endogenous (reproductive cycle,
life stage) and exogenous cycles (diurnal, annual). It is clear from the more recent
literature that research on biomarker responses in earthworms has a strong bias toward
the effects of metals on them, and biomarkers for assessing exposure or effects of organic
and estrogenic compounds have received much less attention.
ESTIMATING ENVIRONMENTAL EXPOSURE OF
EARTHWORMS TO TOXICANTS
To obtain some idea of an exposure to the environmentally available chemical (in contrast to its
bioavailability), the application rate (of the active ingredient) per unit area should be taken into
account to determine the degree of exposure experienced by the earthworms in the soil. The actual
exposure will also depend on the behavior of the earthworms, soil conditions, and characteristics
of the particular chemical. Only the bioavailable fraction of the chemical is relevant, and only
biological responses or chemical analysis of the earthworms will provide an indication whether the
earthworms were effectively exposed to the chemical. By assuming that the soil is evenly packed
and the chemical is evenly distributed, the critical depth limit can be estimated and will vary
according to the soil type and soil cover. All the factors affecting exposure should be considered,
but some are difficult to measure. Various authors have made different assumptions in determining
the estimated environmental concentration (EEC). The following assumptions advanced by Kokta
and Rothert (1992) and others are still valid for purposes of estimating exposure following liquid
toxicant applications:
1.There is an even distribution of the chemical in the upper 2.5 cm of soil.
2.There is a bulk density of soil of 1.5g
.
3.The full amount of toxicant is used if applied to bare soil.
4.50% of the total toxicant is used if plant cover exists.
5.For pesticides with fewer than three applications per season, the total amount applied is
summed to allow for persistence.
cm
−
1
3
EARTHWORMS IN BIOASSAYS, MICROCOSMS, AND
MODEL ECOSYSTEMS
There is currently considerable renewed interest in studies of the mesocosm, semifield, model
ecosystem types (Rmbke personal communication; Burrows and Edwards 2002; Edwards 2002);
these were very often used in pesticide degradation studies in the 1970s. Model ecosystems (slightly
more complex than microcosms), and which approximate real ecosystems, have been used by
various authors. Design features and pros and cons were discussed by Van Straalen and Van Gestel
(1993) and Edwards (2002).
The effects of toxicants are assessed in systems of various complexities, simulating field
conditions as closely as possible. Two or more interacting organisms are usually exposed simulta-
neously. The development of microcosms and model ecosystems as ecotoxicological tools has
