Consistent effects of pesticides on community structure and ecosystem function in freshwater systems

Freshwater ecosystems, despite their high biodiversity and vital ecosystem services, are threatened by pesticide contamination. Predicting ecosystem responses to pesticides is hindered by two main challenges: the lack of knowledge on the consistency of pesticide effects across species and ecosystem functions, and the unclear role of pesticides in biodiversity-ecosystem function relationships. Tens of thousands of synthetic chemicals are registered, making comprehensive assessments extremely complex. If pesticide effects show consistency within classes (similar chemical structures) or types (targeting similar pests), risk assessment could be significantly simplified, focusing on exceptions to general patterns. Prior studies often manipulated single trophic levels, overlooking the non-random, multi-trophic level effects of anthropogenic factors like pesticide contamination. This study aimed to: (1) evaluate the consistency of pesticide effects across types, classes, and individual pesticides; (2) assess whether effects stemmed from sublethal, non-target effects or targeted taxa changes; and (3) determine if changes in functional group composition, abundance, and richness mediate pesticide effects on ecosystem functions. Three hypotheses were proposed: consistent ecosystem process responses within pesticide types due to functional redundancy; consistent community responses within pesticide classes due to taxa-specific sensitivities; and mediation of pesticide-induced ecosystem process disruptions by functional group changes.

The literature review highlights the importance of freshwater biodiversity and the threats posed by pesticide contamination. It emphasizes the challenges in predicting the ecological consequences of pesticide exposure due to the sheer number of chemicals and their complex interactions within ecosystems. Studies on the relationship between biodiversity and ecosystem function are discussed, highlighting the limitations of previous approaches that focused on single trophic levels and the need for a more holistic understanding of the complex interplay between biodiversity, ecosystem functioning and anthropogenic stressors such as pesticides. Previous work also suggested the potential for simplifying risk assessment by focusing on pesticide classes or types and their effects on groups of organisms with similar functions and sensitivities. The existing literature laid the groundwork for this study by highlighting the gaps in our understanding and the need for a more comprehensive approach to assessing pesticide impacts on freshwater ecosystems.

A large-scale mesocosm experiment using 72 outdoor mesocosms was conducted to evaluate the effects of 18 treatments (12 pesticides, 4 simulated pesticide treatments, and 2 controls) on tri-trophic temperate pond communities. The pesticides were nested within four classes (organophosphates, carbamates, chloroacetanilides, and triazines) and two types (insecticides and herbicides). Pesticides were applied singly at environmentally relevant concentrations calculated using U.S. EPA software GENEEC v2, simulating pesticide runoff. Simulated pesticide treatments involved top-down or bottom-up manipulations of food webs to mimic the effects of herbicides and insecticides. The experiment ran for four weeks. Measurements included ecosystem processes (pH, respiration, decomposition, turbidity, phytoplankton, and periphyton biomass), zooplankton community composition and abundance, and tri-trophic community responses (survival, mass, reproductive rates of various organisms). Statistical analyses included permutational analyses of variance (PERMANOVA) to assess consistency of effects across pesticide types, classes, and individual pesticides, distance-based redundancy analysis (dbRDA) to visualize multivariate responses, and structural equation modeling to link changes in functional groups to ecosystem functions.

Pesticide type explained a significant portion of the variation in ecosystem function (46%). Herbicides decreased phytoplankton primary production and respiration (bottom-up effect), while insecticides increased phytoplankton production and respiration (top-down effect on zooplankton). Pesticide type also explained a substantial portion of the variation in zooplankton community structure (44.2%). Insecticides led to the elimination of cladocerans and increased copepod abundance, while herbicides decreased overall zooplankton abundance. In the tri-trophic community, variation was more evenly distributed among pesticide type, class, and individual pesticide. Insecticides generally reduced predator survival, leading to increased prey survival and growth. Simulated pesticide treatments rarely produced results similar to the actual pesticide treatments, highlighting the difficulty of mimicking complex pesticide effects through simple manipulations. Structural equation models showed that herbicide effects on respiration and primary productivity were mediated by bottom-up effects on phytoplankton abundance, while insecticide effects were driven by top-down effects on zooplankton composition and abundance.

The consistent responses of ecosystem functions to pesticides within types, despite less consistent community-level responses, suggest functional redundancy in species roles. The greater consistency in ecosystem-level responses compared to community-level responses points to the importance of considering functional groups in ecological risk assessment. The results support simplifying risk assessment by focusing on pesticide classes or types and their effects on functional groups, rather than evaluating individual pesticides and species. This simplification would allow for more efficient use of resources and improved focus on exceptions to these general patterns. The study emphasizes the broader detrimental effects pesticides can have on complex ecological systems, even if the effects show some consistency across pesticide types and classes.

This study demonstrates consistent effects of herbicides and insecticides on freshwater ecosystem functions, mediated by changes in functional groups. These findings suggest a simplified approach to ecological risk assessment, focusing on pesticide classes/types and functional groups. Further research should explore additional pesticides and ecosystems to assess the generalizability of these findings and identify any exceptions. The study highlights the need for a more holistic, ecosystem-level approach to understanding and managing the impacts of pesticides on freshwater biodiversity and ecosystem services.

The study focused on single pesticides, not mixtures, limiting the understanding of real-world scenarios. Simulated pesticide treatments did not perfectly mimic the effects of actual pesticides, potentially underestimating the complexity of pesticide impacts. The generalizability of findings might be limited to similar temperate pond communities. While the study provides a framework for a simplified risk assessment, the complexities of specific pesticide-species interactions and other environmental factors were not entirely captured. Long-term effects of multi-generational exposures were not considered.