Eutrophic status influences the impact of pesticide mixtures and predation on Daphnia pulex populations

Pesticides, nutrients, and ecological stressors such as competition or
predation co-occur in freshwater ecosystems impacted by agricultural
pollution. The extent to which combinations of these stressors affect
aquatic populations and the role of nutrients availability in modulating
these responses requires further understanding. In this study, we assessed
how pesticides affecting different taxonomic groups and predation
influence the response of Daphnia pulex populations under different
trophic conditions. An outdoor experiment was designed following a
factorial design, with the insecticide chlorpyrifos, the herbicide diuron,
and predation by Notonecta sp. individuals as key stressors. The single
impact of each of these stressors, and their binary and tertiary
combinations were evaluated on D. pulex abundance and population structure
under mesotrophic and eutrophic conditions for 21 days. Data were analysed
using generalized linear mixed models estimated by means of a novel
Bayesian shrinkage technique. Our study shows a significant influence of
each of the evaluated stressors on D. pulex abundance, however, the
impacts of the herbicide and predation were lower under eutrophic
conditions as compared to the mesotrophic ones. We found that binary
stressor interactions were generally additive in the mesotrophic scenario,
except for the herbicide-predation combination, which resulted in
synergistic effects. The impacts of the binary stressor combinations in
the eutrophic scenario were classified as antagonistic, except for the
insecticide-herbicide combination, which was additive. The tertiary
interaction resulted in significant effects on several sampling dates,
however, those were rather antagonistic and resembled the most important
binary stressor combination in each trophic scenario. Our study shows that
the impact of pesticides on freshwater populations depends on the
predation pressure, and demonstrates that the combined effect of
pesticides and ecological stressors is influenced by the food availability
and organism fitness related to the trophic status of freshwater
ecosystems.