Abstract
Although benthic diatoms are widely used in ecological studies of aquatic systems, there is still a dearth of data concerning species sensitivities towards several contaminants. Within the same community, different species may respond differently depending on their physiological and ecological characteristics. This lack of knowledge makes specific appropriate risk assessment impossible. To find out whether species sensitivity distribution (SSD) could be used to estimate the risk of herbicide toxicity for diatoms, we need to know whether their sensitivity depends on their physiological and ecological characteristics. We carried out single-species bioassays on 11 diatom species exposed to 8 herbicides. Dose-responses relationships were used to extrapolate the Effective Concentration 5 (EC5) and the Effective Concentration 50 (EC50) for each exposure. These data were used to fit a SSD curve for each herbicide, and to determine the Hazardous concentration 5 (HC5) and 50 (HC50). Our results revealed a high level of variability of the sensitivity in the set of species tested. For photosystem-II inhibitor (PSII) herbicides, diatoms species displayed a typical grouping of sensitivity levels consistent with their trophic mode and their ecological guild. N-heterotroph and “motile” guild species were more tolerant of PSII inhibitors, while N-autotroph and “low profile” guild species were more sensitive. Comprehensive SSD curves were obtained for 5 herbicides, but not for sulfonylurea herbicides or for dimetachlor, which had toxicity levels that were below the range of concentration tested. The SSD curves provided the following ranking of toxicity: diuron> terbutryn> isoproturon> atrazine> metolachlor. The HC that affected 5% of the species revealed that, even at the usual environmental concentrations of herbicides, diatom assemblages could be affected, especially by isoproturon, terbutryn, and diuron.
Highlights
Rivers and lakes are often compromised by contaminants such as pesticides derived from watershed runoff or urban discharges [1,2,3]
In this study we develop a data base of Effective Concentration 5 (EC5) and Effective Concentration 50 (EC50) thresholds based on single-species bioassays for 11 benthic diatoms exposed to 8 agricultural herbicides
The diatoms selected for this study were: Fragilaria capucina var. vaucheriae (FCVA), Fragilaria rumpens (FRUM), Fragilaria ulna (FULN), Craticula accomoda (CRAC), Mayamaea fossalis (MAFO), Eolimna minima (EOMI), Nitzschia palea (NPAL), Achnanthidium minutissimum (ADMI), Cyclotella meneghiniana (CMEN), Encyonema silesiacum (ESLE) and Gomphonema parvulum (GPAR)
Summary
Rivers and lakes are often compromised by contaminants such as pesticides derived from watershed runoff or urban discharges [1,2,3]. Among the various herbicides found in water bodies, atrazine (s-triazines), diuron and isoproturon (phenylureas) are some of the main substances monitored in France [5]. Some of these herbicides are banned or restricted, but they are still found in European inland water ecosystems, and sometimes reach concentrations above the Annual Average- Environmental Quality Standard (AA-EQS) defined in the Water Framework Directive [2,5,6,7]. Ponds, and coastal zones of lakes, microalgae and especially diatoms are crucial primary producers and account for a considerable proportion of the fixed biomass as well as carrying out some of the main biochemical processes. Benthic diatoms are known to be good indicators of ecosystem water quality, especially of the trophic level and physical disturbances [8,9]
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