484
be quite limited. In some cases, because of these limitations, researchers have
instead used more appropriate test organisms while still applying methods or modified methods based on these standard EPA toxicity test protocols.
Acute toxicity tests are often the most commonly employed assessments, as the
measured endpoint is easily determined (mortality), but the levels of contaminant
concentrations needed to produce mortality at the statistically required response
rates are rarely found in the environment. But, even though high concentrations of
contaminants may be unrealistic in the environment, the LC50 (lethal concentration
that results in 50% mortality of the population) generated using standard toxicity
protocols confirms the likely potential tipping point at which mortality will occur
under standard toxicity testing conditions. This standard metric allows for assessments regarding the comparison of different oil types and/or dispersants and mixtures and can provide a ranking of species sensitivities to a range of toxicants, useful
for many reasons. When an acute and highly concentrated contaminant spill occurs,
and surrogate organism responses have already been documented, a prediction of
mortality for the affected environment can be completed rapidly. Chronic and sublethal exposure studies use LC50 values to bracket the range of concentrations to
target for test solution creation. These data are also useful in defining environmental
risk, the combination of known exposure concentrations (i.e., measured concentrations in the field) and standard toxicological assessments.
In traditional acute toxicity studies, early life stages are generally preferred as
they are often more sensitive to toxicant exposure, whereas either early life stages
or mature adult organisms are used in chronic testing. The exposure regimes for
both acute or chronic are either (1) static nonrenewal where the test organisms are
exposed to the same test solution for the duration of the test (less than 24 hrs); (2)
static renewal where solutions are replaced with fresh solutions at specified time
intervals (i.e., every 48 hrs); or (3) flow-through, where there is a continuous passage of test solution or dilution water through a test chamber with no recycling.
Flow-through regimes may employ either a single “spiked” exposure or a continuous concentration. The typical range of exposure duration for traditional toxicity
testing is from 24 hrs to 21 days or more depending on the organism, life stage, and
endpoint.
28.2.2 Preparation of Toxicity Test Exposure Media
Many of the test condition protocols for test solution makeup, environmental conditions (e.g., water quality, lighting regime, temperature), species, experimental
design, endpoints, and analyses are detailed in the standard methods (EPA 2002,
2003). Normally, there are five test solution concentrations and a control required
for single contaminant/mixture toxicity testing. Under ideal test designs, a response
increases proportionally as a function of concentration to a maximum point (i.e.,
monotonic dose response) allowing the estimation of specific endpoint metrics (ECs
and LCs) along the curve. The minimum control survival required to obtain a valid
test is 80% or 90% but is test protocol specific. Statistical results for acute and
C. L. Mitchelmore et al.
be quite limited. In some cases, because of these limitations, researchers have
instead used more appropriate test organisms while still applying methods or modified methods based on these standard EPA toxicity test protocols.
Acute toxicity tests are often the most commonly employed assessments, as the
measured endpoint is easily determined (mortality), but the levels of contaminant
concentrations needed to produce mortality at the statistically required response
rates are rarely found in the environment. But, even though high concentrations of
contaminants may be unrealistic in the environment, the LC50 (lethal concentration
that results in 50% mortality of the population) generated using standard toxicity
protocols confirms the likely potential tipping point at which mortality will occur
under standard toxicity testing conditions. This standard metric allows for assessments regarding the comparison of different oil types and/or dispersants and mixtures and can provide a ranking of species sensitivities to a range of toxicants, useful
for many reasons. When an acute and highly concentrated contaminant spill occurs,
and surrogate organism responses have already been documented, a prediction of
mortality for the affected environment can be completed rapidly. Chronic and sublethal exposure studies use LC50 values to bracket the range of concentrations to
target for test solution creation. These data are also useful in defining environmental
risk, the combination of known exposure concentrations (i.e., measured concentrations in the field) and standard toxicological assessments.
In traditional acute toxicity studies, early life stages are generally preferred as
they are often more sensitive to toxicant exposure, whereas either early life stages
or mature adult organisms are used in chronic testing. The exposure regimes for
both acute or chronic are either (1) static nonrenewal where the test organisms are
exposed to the same test solution for the duration of the test (less than 24 hrs); (2)
static renewal where solutions are replaced with fresh solutions at specified time
intervals (i.e., every 48 hrs); or (3) flow-through, where there is a continuous passage of test solution or dilution water through a test chamber with no recycling.
Flow-through regimes may employ either a single “spiked” exposure or a continuous concentration. The typical range of exposure duration for traditional toxicity
testing is from 24 hrs to 21 days or more depending on the organism, life stage, and
endpoint.
28.2.2 Preparation of Toxicity Test Exposure Media
Many of the test condition protocols for test solution makeup, environmental conditions (e.g., water quality, lighting regime, temperature), species, experimental
design, endpoints, and analyses are detailed in the standard methods (EPA 2002,
2003). Normally, there are five test solution concentrations and a control required
for single contaminant/mixture toxicity testing. Under ideal test designs, a response
increases proportionally as a function of concentration to a maximum point (i.e.,
monotonic dose response) allowing the estimation of specific endpoint metrics (ECs
and LCs) along the curve. The minimum control survival required to obtain a valid
test is 80% or 90% but is test protocol specific. Statistical results for acute and
C. L. Mitchelmore et al.