Our study highlights trends in the risk assessment of herbicides
for benthic diatoms. Exposure of diatoms seems to be modified by
the biofilm matrix which can act either as a protective layer against
hydrophilic herbicides or, on the contrary as a layer in which hydrophobic herbicides accumulate, thus increasing benthic diatom exposure. To prevent any underestimation of the risk of hydrophobic
substance (in this study, log KowN3.4), deriving thresholds from sensitivity data obtained under benthic conditions would seem to offer a
potentially useful alternative. However, these trends must be confirmed by analytical data and further work is in progress to identify