Pond mitigation in dissolved and particulate pesticide transfers: Influence of storm events and seasonality (Auradé agricultural catchment, SW-France).

In agricultural headwater catchments, wetlands such as ponds are numerous and well known to partly dissipate contamination. Most of the pesticides are transferred from soils to the aquatic environment during flood events. This study reports the annual/seasonal behaviour of 6 pesticides (metolachlor, boscalid, epoxiconazole, tebuconazole, aclonifen and pendimethalin) in such an environment. Because it is rarely considered, the study focussed on the high frequency of the distribution of pesticides between dissolved and particulate phases, as well as the main controlling factors of their upstream-downstream transfer. The pond removal rate was calculated to evaluate the wetland efficiency in pesticide mitigation. We conducted a one-year high frequency hydrochemical survey, with particular emphasis on flood events, in the upper Auradé catchment (SW-France), an area of long-term conventional agriculture on highly erosive carbonated soils. The inlet and outlet of the pond were instrumented for water level measurements and water sampling. The highest concentrations were observed for tebuconazole and, in general, the presence of the molecules during the year depended on the season. The pond showed satisfactory efficiency in pesticide attenuation for the six molecules considered, although the removal rate depended on the molecule and the bearing phase (from 28.4% for boscalid to 89.4% for aclonifen in the dissolved phase and from 22.1% for pendimethalin to 96.8% for metolachlor in the particulate fraction). Interestingly, the more hydrophilic the molecule (low LogKOW), the more efficient the pesticide removal rate was for its particulate fraction, and the opposite for hydrophobic molecules (high LogKOW). Flood events carried a large amount of Total Suspended Solid (TSS) bearing hydrophobic molecules from a major legacy of upper catchment soils, although 52% of the pesticides were transported by the dissolved fraction. Significant resuspension of TSS from the pond was evidenced by the annual mass balance with four tons of TSS released, while the positive rate of pesticide removal involved other effective mechanisms such as exchange and complexation. Although these constructed wetlands may be beneficial for pesticide mitigation, the results highlighted the need for improved land management in the upstream catchment during the different seasons to avoid bare soils that pose a risk of high surface water contamination, especially due to the presence of hydrophobic molecules in combination with a high erosive context.

Role of Biofilms in Contaminant Bioaccumulation and Trophic Transfer in Aquatic Ecosystems: Current State of Knowledge and Future Challenges.

In freshwater environments, microbial assemblages attached to submerged substrates play an essential role in ecosystem processes such as primary production, supported by periphyton, or organic matter decomposition, supported by microbial communities attached to leaf litter or sediments. These microbial assemblages, also called biofilms, are not only involved in nutrients fluxes but also in contaminants dynamics. Biofilms can accumulate metals and organic contaminants transported by the water flow and/or adsorbed onto substrates. Furthermore, due to their high metabolic activity and their role in aquatic food webs, microbial biofilms are also likely to influence contaminant fate in aquatic ecosystems. In this review, we provide (1) a critical overview of the analytical methods currently in use for detecting and quantifying metals and organic micropollutants in microbial biofilms attached to benthic substrata (rocks, sediments, leaf litter); (2) a review of the distribution of those contaminants within aquatic biofilms and the role of these benthic microbial communities in contaminant fate; (3) a set of future challenges concerning the role of biofilms in contaminant accumulation and trophic transfers in the aquatic food web. This literature review highlighted that most knowledge on the interaction between biofilm and contaminants is focused on contaminants dynamics in periphyton while technical limitations are still preventing a thorough estimation of contaminants accumulation in biofilms attached to leaf litter or sediments. In addition, microbial biofilms represent an important food resource in freshwater ecosystems, yet their role in dietary contaminant exposure has been neglected for a long time, and the importance of biofilms in trophic transfer of contaminants is still understudied.

Light and temperature influence on diuron bioaccumulation and toxicity in biofilms.

Variations of temperature and photoperiod throughout different seasons can affect aquatic communities such as biofilms. Biofilms, generally present at the base of trophic chains in freshwaters, are also subject to organic contamination, and are especially affected by herbicides. Many studies have investigated the effect and interactions of herbicides and environmental factors on biofilms, but never with a toxicokinetic point of view. The objective of this study was to assess structural and functional changes in biofilms exposed to diuron, and to link them with contaminant accumulation, under the influence of temperature and light variations. To this aim, biofilms were exposed to all possible combinations of three concentrations (0, 5 and 50 µg L-1) of diuron, two temperatures (10 and 26 °C), and two light/dark photoperiods (16/8, 10/14), for durations of 0, 1 and 3 days. Diuron accumulation in biofilms was quantified and structural descriptors (protein and polysaccharide contents, dry weight) and functional endpoints (photosynthetic and enzymatic activities) were analyzed. The results obtained mainly highlighted the influence of temperature on diuron bioaccumulation and the associated toxic impact on biofilms. Bioaccumulation in biofilms exposed during three days at 10 °C, at the highest diuron concentration, was in average 1.4 times higher than bioaccumulation on biofilms exposed to 26 °C. Accordingly, the photosynthetic yield was more inhibited at lower than at higher temperatures. Temperature was also the highest impacting factor for metabolism regulation; for example, at 26 °C after three days of exposure, polysaccharide production was boosted under both photoperiods tested.

Metabolomics insight into the influence of environmental factors in responses of freshwater biofilms to the model herbicide diuron.

Freshwater biofilms have been increasingly used during the last decade in ecotoxicology due to their ecological relevance to assess the effect(s) of environmental stress at the community level. Despite growing knowledge about the effect of various stressors on the structure and the function of these microbial communities, a strong research effort is still required to better understand their response to chemical stress and the influence of environmental stressors in this response. To tackle this challenge, untargeted metabolomics is an approach of choice because of its capacity to give an integrative picture of the exposure to multiple stress and associated effect as well as identifying the molecular pathways involved in these responses. In this context, the present study aimed to explore the use of an untargeted metabolomics approach to unravel at the molecular/biochemical level the response of the whole biofilm to chemical stress and the influence of various environmental factors in this response. To this end, archived high-resolution mass spectrometry data from previous experiments at our laboratory on the effect of the model photosynthesis inhibitor diuron on freshwater biofilm were investigated by using innovative solutions for OMICs data (e.g., DRomics) and more usual chemometric approaches (multivariate and univariate statistical analyses). The results showed a faster (1 min) and more sensitive response of the metabolome to diuron than usual functional descriptors, including photosynthesis. Also, the metabolomics response to diuron resulted from metabolites following various trends (increasing, decreasing, U/bell shape) along increasing concentration and time. This metabolomics response was influenced by the temperature, photoperiod, and flow. A focus on a plant-specific omega-3 (eicosapentaenoic acid) playing a key role in the trophic chain highlighted the potential relevance of metabolomics approach to establish the link between molecular alteration and ecosystem structure/functioning impairment but also how complex is the response and the influence of all the tested factors on this response at the metabolomics level. Altogether, our results underline that more fundamental researches are needed to decipher the metabolomics response of freshwater biofilm to chemical stress and its link with physiological, structural, and functional responses toward the unraveling of adverse outcome pathways (AOP) for key ecosystem functions (e.g., primary production).

Do pesticides degrade in surface water receiving runoff from agricultural catchments? Combining passive samplers (POCIS) and compound-specific isotope analysis.

Pesticides lead to surface water pollution and ecotoxicological effects on aquatic biota. Novel strategies are required to evaluate the contribution of degradation to the overall pesticide dissipation in surface waters. Here, we combined polar organic chemical integrative samplers (POCIS) with compound-specific isotope analysis (CSIA) to trace in situ pesticide degradation in artificial ponds and agricultural streams. The application of pesticide CSIA to surface waters is currently restricted due to environmental concentrations in the low µg.L-1 range, requiring processing of large water volumes. A series of laboratory experiments showed that POCIS enables preconcentration and accurate recording of the carbon isotope signatures (δ13C) of common pesticides under simulated surface water conditions and for various scenarios. Commercial and in-house POCIS did not significantly (Δδ13C < 1 %) change the δ13C of pesticides during uptake, extraction, and δ13C measurements of pesticides, independently of the pesticide concentrations (1-10 µg.L-1) or the flow speeds (6 or 14 cm.s-1). However, simulated rainfall events of pesticide runoff affected the δ13C of pesticides in POCIS. In-house POCIS coupled with CSIA of pesticides were also tested under different field conditions, including three flow-through and off-stream ponds and one stream receiving pesticides from agricultural catchments. The POCIS-CSIA method enabled to determine whether degradation of S-metolachlor and dimethomorph mainly occurred in agricultural soil or surface waters. Comparison of δ13C of S-metolachlor in POCIS deployed in a stream with δ13C of S-metolachlor in commercial formulations suggested runoff of fresh S-metolachlor in the midstream sampling site, which was not recorded in grab samples. Altogether, our study highlights that the POCIS-CSIA approach represents a unique opportunity to evaluate the contribution of degradation to the overall dissipation of pesticides in surface waters.

Diuron sorption isotherms in freshwater biofilms.

Biofilms are excellent bioindicators for water quality assessment because of their ability to integrate contamination, and their position at the base of the trophic chain in aquatic environments. Pesticides are ubiquitous in aquatic environments and can constantly interact with aquatic organisms, including those that make up biofilms, at fluctuating concentrations. The aim of this study was to describe pesticide behaviour in biofilms. Previous research highlighted that contaminant sorption was not always linear, but no study considered organic bioaccumulation isotherms and toxic impacts to biofilms concurrently. In order to characterize pesticide sorption isotherms in biofilms and the mechanisms involved in the uptake process, we simultaneously assessed bioaccumulation and toxic impact of diuron (a photosynthesis inhibiting herbicide) at the water-biofilm concentrations equilibrium. Mature biofilms grown on glass slides during one month were subsequently exposed in channels to 7 increasing concentrations of diuron from 1 to 500 µg·L-1, plus a control condition, for 2 h with a flow velocity of 2 cm·s-1. Then, a Langmuir isotherm equation was fitted to the bioaccumulation data, and an Emax model to toxic impact results. This study established that diuron bioaccumulation in biofilm is nonlinear, and allowed to calculate the Langmuir constant and maximal concentration of diuron potentially accumulated in biofilm (up to 17,771 µg·g-1). In turn, we found that photosynthetic inhibition followed classical dose-response patterns with diuron concentrations in the water, and that EC50 could be established at 75 µg·L-1. A continuous diffusion phenomenon was thus demonstrated but it was not linearly correlated to bioaccumulation, highlighting complex uptake mechanisms operating within the matrix. The coupling of toxicokinetic and toxicodynamic approaches provided original information about pesticide behaviour and impact in periphytic microorganisms.

Flow conditions influence diuron toxicokinetics and toxicodynamics in freshwater biofilms.

Biofilms are considered as good bioindicators of contamination by means of their capacity to react quickly to xenobiotics exposure, and their pivotal role in sustaining the aquatic trophic web. The exchanges of dissolved substances between water column and biofilm can be modulated by flow velocity. This study deals with toxicokinetic (transfer mechanisms) and toxicodynamic (effects) modelling of pesticides under two contrasted flow conditions. Diuron was used to run a 2-h kinetic study on mature biofilms in river channels. Two flow conditions were considered (⋘1 cm·s-1: lentic environments such as ponds, 2 cm·s-1: lotic environments such as watercourses). Three concentrations were tested in order to estimate contamination levels in biofilms: 0, 5 (environmentally relevant concentration) and 50 (to determine the concentration effect) µg·L-1. The effect of the above-mentioned factors was also assessed on biofilms photosynthesis inhibition. For successive sampling times between 0 and 2 h, the raw biofilms and EPS tightly bound to cells plus microorganisms (T-EPS-M), were physically separated and analysed for diuron accumulation and structural and functional microbial descriptors. Diuron amounts accumulated in biofilm increased with increasing diuron exposure. Biofilms accumulated higher amounts of diuron at the lower flow velocity compared to high flow for raw biofilms, while accumulation in the T-EPS-M fraction was similar between flow conditions. Consequently, both flow velocity and diuron exposure had an influence on diuron bioaccumulation and distribution. Photosynthesis inhibition over time was directly linked to the exposure concentration of diuron recorded in the T-EPS-M fraction. These results suggest that flow causes a loss of organic matter in biofilms, decreasing the total accumulation of diuron, especially within diffusible EPS. As pesticide distribution in biofilm is a major factor in the onset of toxicity, the novel fractioning method presented here will improve further toxicokinetic and toxicodynamic studies dealing with biofilms exposed to organic toxicants.