4.1 Fish ponds contamination in relation to crop area and pesticides uses
We analysed contamination by pesticides in fish ponds from areas cropped mainly with winter cereals and maize along a gradient of crop area in catchments. Contamination was predominantly related to six active ingredients which were low soil adsorption herbicides, except AMPA which is the first glyphosate metabolite and Imidacloprid which is an insecticide used for seeds protection. Only 38% of water samples taken were free of pesticide residues. Our hypothesis of a correlation between cropland area and global frequency of contamination in fish ponds was confirmed (Fig. 6). The most frequent and highly concentrated active ingredients found were related to current pesticide uses in maize and winter cereals (Table 2). In previous studies surface water contaminations were also associated with current used pesticides (Du Preez et al. 2005; Ulrich et al. 2018; Cui et al. 2020). Some pesticides residues such as S-metolachlor and Dimethenamid were also found in previous studies in runoff water that supplies fish ponds with water in the same area (Sarrazin et al. 2011).
AMPA residues was found in 8 out 10 ponds and Glyphosate on 3 out 10 ponds, both sometime with high concentrations (Table 2), and its frequency of contamination was strongly related to crop area in pond catchments (Fig. 6). Glyphosate is sprayed by farmers before crops planting in order to clean the fields from weeds that developed during the period between two crops or to destroy winter cover crops. Its use has increased as a result of the development of reduced or no tillage farming. Glyphosate and AMPA are now very frequent in topsoils of cropped fields in Europe (Silva et al. 2018) or other farming nations like Argentina (Aparicio et al. 2013). Silva et al. (2018) indicated that cereal crops show the highest exports for Glyphosate through water erosion, but also for AMPA being more persistent than glyphosate, its mother compound.
Spraying periods for winter cereals are usually October or more frequently late February/early March with pre-emergent herbicides. Fungicides are sprayed in spring, and very few insecticides are used in the study region. We found principally Isoproturon and Chlorotoluron that are both pre-emergent herbicides. The temporal pattern of the high contamination levels is consistent with cumulative rainfall after spraying at the out of winter after (Fig. 5). Ponds are exposed to these residues especially in early spring.
The spraying periods for maize were late April/early May with pre-emergent herbicides just after sowing. The monitoring we conducted demonstrated that pesticide applications in maize is responsible for the highest quantified contamination in fish ponds from S-metolachlor, Acetochlor and Dimethenamid that are pre-emergent herbicides (Table 2). S-metolachlor was also the most frequent maize herbicide found in the ponds. This substance was regularly detected in surface waters in various countries. Following a recent review on occurrences of pesticides in surface waters for the period 2012–2019 it is the second most frequent herbicide found (De Souza et al. 2020). The dissipation rates of Metolachlor from maize fields were measured as only a few percent (Ng et al. 1995; Rose et al. 2018), but the dosage of the herbicide product is relatively high. Maize was the dominating crop in the crop rotations of farmers in our case study (Fig. 7). The temporal pattern of maize pesticides contaminations was particularly well consistent with the maize chemical protection schedule which coincides with the middle of the fish production season in May/June (Fig. 5). Peak concentrations occurred in late spring, and decreased over the remaining part of the production season. Soil cover in maize fields was very low at this time as compared to winter cereals. It may have facilitated surface runoff with pesticide transfers to the ponds. Finally, strong correlations between maize crop area and related pesticides contamination frequency confirmed that frequent pesticide transfer can occurs from this crop toward fish ponds. It was less in 2015, probably because the rainfall was very low in spring (Table 4).
Few studies have been dedicated on non-point source pollution of fish ponds by pesticides. In a previous study in barrage ponds of northeastern France, five ponds were monitored in spring and fall (Lazartigues et al. 2012). Few active ingredients exceeded the threshold of 1µg/l for a single substance but the total pesticide level in October reached 0.17 to 8.81 µg/l depending on the pond. Herbicides as Isoproturon and Chlorotoluron were frequently quantified, which were also among the most important ones in our study. The total contaminations of water observed in March followed the gradient of cultivation area percentages in the catchment (Lazartigues et al. 2012).
4.2. Rain fall and connectivity as most important factors for pesticide transfer
We found that higher rainfall events in the preceding days correlated well with higher frequency of contamination as well as higher concentrations of pesticides found in the fish ponds. Hydrological parameters are more relevant than pesticides characteristic to explain pesticide transport toward water bodies (Ulrich et al. 2018). Our fish pond landscape is dominated by loamy-clayey loess soils that are very prone to structural degradation by compaction or rainfall impact. Consequently, they are vulnerable to Hortonian runoff (Reichenberger et al. 2007). Saturation excess runoff can also occur in fields around the ponds, because of soils with impermeable horizons. And preferential flow can contribute to the transfer of pesticides when drainage systems were installed (Reichenberger et al. 2007). As pointed out by a previous study focused on nutrients status of Dombes fish ponds in relation to cropland area in pond catchments (Wezel et al. 2013), our results also illustrated the impacts of the strong hydrological connectivity between crop fields and ponds. In such a relatively flat area, the drainage network is particularly well developed because artificial drains or ditches are essential part of the channel network. In such a high drainage density context, our results confirmed the ones of Collin et al (2000) showing that the total surface area of the catchment must be taken into account as contributive areas for herbicides contamination in surface water. The artificial drainage network managed by farmers and fish producers (Fig. 1) is consequently a very important way of pesticides transfers.
Several parameters can influence surface water contamination as rainfall, spraying dates, type of pesticides (Boithias et al. 2014). As found in a recent monitoring of ten small water bodies in Germany (Ulrich et al. 2018), our results on ten fish ponds were also strongly variable (Fig. 4) because of combination of rainfall temporal pattern, spraying time and type of pesticides used. Sites characteristics like slope or soil types could also locally influence the results but they are relatively homogeneous in the Dombes area which is a plateau of glacial origin. However, the dense artificial drainage network can be spatially variable in its ability to transfer pesticide residues. According to their characteristics and management practices, ditches could also be an asset in controlling the flow of pesticide residues (Lagacherie et al. 2006).
Numerous studies concluded to the importance of precipitations playing a major part in the herbicide transfer after their application (Garmouma et al. 1997) and indicate application time as more important than application rate (Holvoet et al. 2005). The time between the rain and the treatment is essential in the contamination process of surface waters (Reichenberger et al. 2007), and this is also the case for the contamination of water bodies (Ulrich et al. 2018). Our results were consistent with these findings because we found significant positive correlation between cumulated antecedent rainfall for 2 days and the contamination frequency (p<0,05) but not for previous cumulated rainfall over longer periods. In contrast, the correlations were higher for the high contamination frequency with longer cumulated antecedent rainfall periods. This can be explained by the temporal pattern of the saturation of the soil with water during the spraying period in spring. Due to the high frequency of heavy rainfall in 2013 (Fig. 3), rainfall-runoff events occurred close to maize sowing and pre-emergent herbicides spraying. Conditions were drier and clearly different in 2014 and 2015. Our results also showed that contaminations can persist for several weeks after the application periods as it can be observed in streams (Garmouna et al. 1997). Overall, we must remain cautious about the interpretation of our data regarding detecting peak exposure and maximum contamination in fish ponds caused by rainfall as this would need a higher frequency sampling (Lefrancq et al. 2017).
4.3. Impacts on fish pond aquatic ecosystem and human health
Different studies show impacts of pesticides on different components of aquatic ecosystem. Smedbol et al. (2018) showed effects of Glyphosate in freshwater phytoplankton community, but the concentrations tested were much higher than those observed in the Dombes fish ponds. Current herbicides such as Metolachlor can affect phytoplankton cultures and communities at environmentally relevant concentrations (Beaulieu et al. 2020). The authors pointed out the risk to the health of phytoplankton in lakes at concentrations below current national guidelines. The increasing sensitivity of some Chlorophyceae algae after being previously exposed to several pulse of Metolachlor was also demonstrated (Copin et al. 2016). Thunissen et al. (2020) assessed ecological risks of imidacloprid to aquatic species. Aquatic insects were found to be particularly sensitive. A mesocosm study revealed effect of Imidacloprid on aquatic invertebrates (Rico et al. 2018). Calculated no observed effect concentration were 0.2 µg/l for some taxa which is less than the maximal concentrations we observed in one of our fish ponds. We should add here that the mixture of pesticides can be more toxic than the individual active ingredient although this type of impact is currently poorly understood (De Souza et al., 2020).
Fish ponds themselves can be used as pesticides mitigation in surface waters. They can contribute to limit downstream pollution by pesticides in case of barrage fish ponds (Gaillard et al. 2016). However, fish ponds in Dombes are in most cases enclosed basins of water with a very low water turn over during the fish production season. Pond fish such as carp or pike may be intended for human consumption. Nevertheless, the risk of contaminating the fish seemed unlikely according to previous studies. Sarrazin et al. (2011) found no contamination in the flesh of the fish, but only with low concentration in the liver which is normally not consumed. Current uses pesticides were also monitored in sediments and fish of some northeastern French fishponds (Lazartigues et al. 2013a). Among other substances, Isoproturon was present in all sediments samples and in some fishes at very low levels. In the same area, another study led to the conclusion that none of the active ingredients found would probably be bioaccumulated within aquatic food webs (Lazartigues et al. 2013b).
4.4. Operational perspectives and mitigation measures
Using contamination frequency as outcome variable, we demonstrated the consequence of increased crop area in pond catchments on decreasing pond water quality. As the work was carried out with local stakeholder, results should support operational decision making and giving recommendations for improved management. In the hydrological context of the Dombes area, the frequency of contamination could be controlled by taking better account of the rainfall history before pesticide treatments in particular to control high contamination of pond water. Deeper changes in cropping systems such as longer and more diversified crop rotations which support having less pest or disease incidence (Wezel et al. 2014) could also help to reduce the contamination frequency of fish ponds as treatment frequency, type and amount of pesticides used could diminish. The list of the active ingredients involved in treatments could also help farmers to select herbicides with less mobility.
Numerous mitigation measures can be considered to prevent pesticide transfer from runoff such as buffer strips, hedge rows and forest buffer, constructed wetlands, tillage practice, cover crops, and intercropping. Beyond local implementation, it is necessary to look for additive effect in the larger catchment with a combination of mitigation measures on total pesticide losses (Reichenberger et al. 2007; Carluer et al. 2017). It is essential to design vegetated buffer zones adapted to the specific context of the area following water pathways and local practices (Syversen and Bechmann 2004; Carluer et al. 2017). Many ponds benefit from vegetation belts, however riparian buffer strips are probably much less effective than edge-of-field buffer strips because riparian buffer strips are known as better for drift control than for runoff control (Lacas et al. 2005). Grass strips are known to be effective but their efficiency depends on the context (Reichenberger et al. 2007). Attention must be paid on the risk of channeling of surface flow within the strip which could considerably reduce efficiency (Lacas et al. 2005). Constructed wetlands are considered as good tools to clean surface waters from pesticides residues in farmland landscapes (Tournebize et al. 2013). It is well suited to catch water from artificial drainage network in agricultural catchments (Passeport et al. 2013) and the presence of plants enhances pesticide retention (Vymazal et al. 2015). It could also be small detention ponds that are cheap and can easily be created with on-farm machinery (Fiener et al. 2005). But all these measure are just mitigation measure and are not tackling the origin of the problem which is too high or too frequent pesticide application. Some policies are in place for this (e.g. Ecophyto programme and policy in France), but new ones might have a strong impact on farmers’ practices such as the EU Farm to Fork strategy which sets a target to reduce pesticide use by 50% by 2030 (European Commission 2020).