Microcosm and mesocosm studies: revisiting their representativeness, and usefulness, in pesticide risk assessments

An article has recently been published that questioned the representativity of mesocosm/microcosm (M/M) studies for macroinvertebrate communities. Whilst the publication highlights some uncertainties associated with risk assessments for natural waterbodies, the limitations in the design and analysis on a highly constrained dataset have led, in our opinion, to a statement title that is misleading. Many of the restrictions imposed on the data used reduce the context of the individual study results and their relevance to the risk assessments for which they were designed. Little or no consideration is given to organism seasonality, community interactions or organism abundance prior to applications, all of which form part of the expert judgement used in the assessment of higher tier study data.

Here we offer a counter opinion to the publication with the assertion that, when treated correctly, the results of M/M studies are eminently relevant and appropriate for use in risk assessment practices.

In contrast to the finding of the recent publication, a previous research project conducted by CEA compared the taxa found in various mesocosm facilities (including our own) with natural edge of field waterbodies, determining that a greater number of sensitive taxa were included within the mesocosm studies (full report available here).

We have previously reported on the use of M/M studies in relation to species sensitivity, vulnerable taxa, experimental design improvements and their protectiveness in comparison to lower tier endpoints. Mesocosm studies are inherently variable due to their complex nature and thus are statistically less powerful than the more controlled microcosm studies. This means that analysis of such data needs to include data for taxa that could be considered less reliable in more constrained study designs , but should be used in conjunction with expert knowledge of the organisms within the system, including their seasonality and their capability to migrate into, and out of, such experimental systems. Mesocosm studies also enable us to be protective of a wider range of species, within a more relevant exposure system, with the most sensitive taxa in these studies often being a non-standard test species. There is also the ability to tailor experimental designs to incorporate known vulnerable species, such as EPT taxa, as well as optimizing experimental designs to ensure collection of robust and reliable data for endpoint derivation.

All of which, we feel, indicate the need for, and relevance of, higher tier M/M studies in environmental risk assessment practices.