Higher Tier Laboratory Aquatic Toxicity Testing
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Higher Tier Laboratory Aquatic Toxicity Testing
Registration schemes for plant protection products require registrants to assess the potential ecological risk of their products using a tiered approach. Standard tests are used at the lower tiers and clearly defined methodologies are available for assessing the potential environmental risks (eg toxicity exposure ratios or risk quotients). The lower tier single species studies rely upon a number of assumptions.
- that the response of selected organisms in single-species laboratory tests will correspond to those of a larger array of organisms in natural systems;
- that the chosen endpoints in single-species tests are more sensitive than any other at any level of organisation;
- that a species shown to be sensitive to a few toxicants will be equally sensitive to a much wider range of substances.
As none of these assumptions will always hold true, uncertainty factors are incorporated into the risk assessment process. In order to refine the assessments, it will be appropriate to critically evaluate the base dataset. However, if these assessments indicate that a compound is likely to pose a risk to the environment, then impacts can be determined using more environmentally realistic conditions.
A number of approaches have been used in the past to address concerns arising from the preliminary assessments, including the use of pond mesocosms, artificial streams, field monitoring studies and experimental ditches. These approaches have a number of advantages over single species investigations, including the ability to assess: a) endpoints at higher levels of biological organisation; b) species interactions; and c) indirect ecological effects. The approaches also allow the assessment of population and community recovery. Furthermore, model ecosystems can be used to simultaneously integrate fate processes with exposure and consequently include the effects of transformation products. Model ecosystems have been successfully applied to the assessment of pesticides and surfactants.
There are however a number of limitations associated with model ecosystem approaches, most notably that the results are difficult to interpret and the extrapolation to other situations can be problematic. Reasons for this include the fact that no-effect concentrations are dependent on the choice of test concentrations; studies are performed at different times and necessarily different starting conditions; studies vary in duration; sensitivities may vary across experimental systems; and measured endpoints are not always consistent or comparable. The high background variation associated with the studies means that the discriminatory power of the tests can be low. Moreover, in order to simulate natural conditions and to ensure the maintenance of basic ecological characteristics and functions (ie different trophic levels, possibilities for organisms to find new habitats, energy input and nutrient cycles), the test systems need to be of a sufficient size and complexity. This means that the studies can be costly.
It has been recognised that other approaches that are intermediate between standard aquatic toxicity tests and field and microcosm studies might provide valuable data for use in the risk assessment of plant protection products. The majority of approaches that have been investigated have involved further analysis of core data or have been performed in the laboratory using single species. A number of simple multi-species studies have also been used. The objectives of these laboratory-based higher tier studies include to:
- account for the effects of non-continuous exposure;
- better assess the sensitivity of species to contaminants;
- assess the effects of contaminants on sensitive life stages and populations;
- determine the potential for organisms, populations and systems to recover after exposure to a pesticide;
- determine indirect effects of compounds on biological communities.
Pesticide regulators and registrants have identified the need to evaluate these different approaches in terms of what methods are available and how each approach could be used in the risk assessment process. This study was therefore performed to review current research and practice with respect to higher tier laboratory studies, to recommend methodological approaches within a tiered assessment and to identify any further work required to produce guideline protocols. The report draws upon the results of studies documented in the literature along with a survey of regulators and laboratories involved in the testing and environmental risk assessment of chemicals.
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