Pesticide-induced Alterations to Phytoplankton Abundance and Community Structure Alter Ecosystem Respiration: Implications for the Carbon Cycle?

AO_SCPLOWBSTRACTC_SCPLOWCurrent predictions of the effects of synthetic chemicals on freshwater ecosystems are hampered by the sheer number of chemical contaminants entering aquatic systems, the diversity of organisms inhabiting these systems, and uncertainties about how contaminants alter ecosystem metabolism. We conducted a mesocosm experiment that elucidated the responses of ponds composed of phytoplankton and zooplankton to standardized concentrations of 12 pesticides, nested within four pesticide classes and two pesticide types. We show that the effects of the pesticides on algae were consistent within herbicides and insecticides and responses of over 70 phytoplankton species and genera were consistent within broad taxonomic groups. Insecticides generated top-down effects on phytoplankton community composition and abundance, which were associated with persistent increases in ecosystem respiration. Herbicides reduced phytoplankton abundance, which was associated with decreases in primary productivity and ecosystem respiration. These results suggest that widespread pesticide use could have underexplored implications for the global carbon cycle. While these effects on ecosystem respiration were mediated through complex effects on communities, taxonomic groups of organisms responded similarly to pesticide types, suggesting opportunities to simplify ecological risk assessment.