Mesocosm studies aren’t worth doing anymore…right?

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Mesocosm studies aren’t worth doing anymore…right?

For the aquatic risk assessment of plant protection products (PPPs) a tiered approach is used to estimate effects on non-target taxa in the natural environment; where Tier 1 uses simplistic systems of single species with constant exposure, Tier 2 uses additional species data or modified exposures, and Tier 3 uses more realistic multi-species systems (EFSA Aquatic Guidance 2013). To extrapolate from the results of regulatory studies (any Tier) to the ‘real world’, an assessment factor (AF) is applied to the endpoint of a study to generate a regulatory acceptable concentration (RAC). The tiered system and AFs are designed so that Tier 1 is protective of Tier 3 and the value of the AF reflects the realism of a study, i.e. lower AFs are used for Tier 3 studies compared to Tier 1 (EFSA Aquatic Guidance 2013). Aquatic mesocosms, used in Tier 3 studies, are the most representative testing systems of natural aquatic communities and therefore act as a proxy for surface water bodies. As such, they allow for testing of PPPs under realistic conditions at the population and community levels of taxa of e.g. algae, zooplankton, macroinvertebrates and macrophytes.

Aquatic mesocosm studies provide information on both effects and recovery. Therefore, there are two options for generating RACs from mesocosm studies: ecological threshold option (ETO) and ecological recovery option (ERO).  The ETO-RAC is generally the endpoint taken into account by the regulators and is a very useful tool in aquatic risk assessment. Due to the realistic conditions of mesocosm studies, it offers significant improvement to the Tier I RAC. Look out for our article on how the ERO-RAC may be used in regulatory risk assessments (here).

To demonstrate this, we have taken in-house Tier 3 data for 10 PPPs (including insecticides, fungicides and herbicides), and directly compared the ETO-RACs to the Tier 1 RACs (sourced from EFSA conclusions) to determine the factor by which the critical RAC improved by having a mesocosm study (Table 1). 

Table 1: Comparison of regulatory acceptable concentrations (RACs) from mesocosm conducted at CEA and Tier I studies (data sourced from EFSA conclusions)

PPP type

Factor by which mesocosm ETO RAC is higher than Tier I RAC

Insecticide 1


Insecticide 2


Insecticide 3


Insecticide 4


Fungicide 1


Fungicide 2


Herbicide 1


Herbicide 2


Herbicide 3


Herbicide 4


* = decrease; ** = increase

The comparison of the RACs shows that those generated from the laboratory studies are generally more conservative than those from the more realistic exposure conditions and community assemblage of the mesocosm studies. In 9 out of 10 cases, the ETO-RAC from the mesocosm study was significantly higher (4.6-3409 times higher) than the Tier I RAC. In fact, for 6 PPPs, the ETO-RAC was >10x higher than the Tier I RAC, which clearly has significant advantages in terms of achieving the target use profile for the product.

A further benefit of using mesocosm studies is the incorporation of species that are not often used in the Tier 1 tests. This is highlighted by the study on herbicide 2, which is the only study with a more conservative ETO-RAC than the Tier 1 RAC. On closer inspection, the Lemna gibba endpoint at Tier I for this herbicide was not the lowest Tier I endpoint (algae were more sensitive, and there was no further Tier 1 macrophyte data) and therefore not found protective of other aquatic macrophytes present in the mesocosm study. Thus the data gap was then addressed at Tier 3. It would be expected that if Tier I studies were undertaken on other aquatic macrophytes than L. gibba, the subsequent Tier I RAC would be lower than the ETO-RAC, and thus an improvement at Tier 3 still achieved.

So why are mesocosm studies able to achieve much higher RACs than Tier I studies? Tier I studies are absolutely worst-case, with individual organisms exposed to the test item in an inert test vessel; there is minimal potential for degradation of the test substance, or for effects of various age classes of organisms to provide the natural mitigation that would occur in a real edge of field water body. Mesocosm studies, on the other hand, are complex systems, containing multiple interacting species and various biotic and abiotic processes, which together with a more natural exposure profile, result in less severe effects of the test item. This in turn helps the target GAP to be maximised and can help reduce the level of risk mitigation required for PPPs, whilst ensuring the protection goal is preserved.

So if you’re having issues with your aquatic risk assessment, and you’re not sure if a mesocosm study might help, why not get in touch and we’d be happy to discuss your needs. Please contact to find out more.

This work will be presented as a poster at SETAC Helsinki on Monday 27th May 2019. Find out more about our activities at SETAC Helsinki here: 

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