Freshwater ecosystems are among the most imperiled worldwide and supply humans with a multitude of goods and services, most of which are provided directly or indirectly by its biodiversity. In the US, pollution is regarded as the second greatest threat to aquatic biodiversity, but it is one of the most understudied stressors in conservation science. Despite recent emphases on the links between biodiversity and ecosystem functions, research on the indirect effects of contaminants on ecosystem properties mediated by changes in biodiversity is virtually nonexistent for freshwater or any ecosystem. We postulate that broad spectrum pesticides will affect an array of taxa and thus have a high probability of affecting ecosystem functions and services. We treated freshwater mesocosms with solvent (control) or 1x or 2x the expected environmental concentration (EEC) of the broad-spectrum fungicide chlorothalonil, the most commonly-used synthetic fungicide in the US. We quantified 34 community-level and 11 ecosystem-level responses and then used structural equation modeling (SEM) to provide support for indirect effects of this chemical on ecosystem functions mediated by changes in biodiversity.
Chlorothalonil caused significant multivariate declines in the amphibian, gastropod, zooplankton, algal, and macrophyte communities, but did not significantly affect the macroarthropod community. The EEC and 2x the EEC caused significant reductions in taxonomic richness. By the end of the experiment, both the zooplankton and algal communities showed evidence of recovery from the chlorothalonil treatments. Ecosystem properties were also affected by chlorothalonil. Chlorothalonil was negatively associated with decomposition rates, but its effects on other ecosystem properties, such as net primary productivity, water clarity, pH, and dissolved oxygen, were generally dependent on time during the experiment. SEM suggests that, by reducing the shading effects of macrophytes (bottom-up effect) and decreasing herbivores (top-down effect) early in the experiment, chlorothalonil indirectly increased phytoplankton and periphyton late in the experiment. The SEM also suggests that these algal blooms drove the change in ecosystem functions observed late in the experiment. Alternative hypotheses for modifications in ecosystem functions were not supported. These results suggest that ecologically relevant concentrations of the most commonly used synthetic fungicide can decimate freshwater biodiversity, which consequently can cause major shifts in freshwater ecosystem functions.