Life in the slow drain: Landscape structure affects farm ditch water quality

Agrichemical contamination is a major threat to aquatic ecosystems in farmland. There is a need to better understand the influence of the surrounding landscape on farm wetlands to recommend land management options that minimize water quality impacts from agricultural practices. We tested hypothesized relationships between landscape structure and multiple water quality measures in farm drainage ditches in a multi-landscape study in Eastern Ontario, Canada. We measured physicochemical water quality (levels of atrazine, glyphosate, neonicotinoid insecticides, inorganic nitrogen, and dissolved oxygen), and biological water quality indicators (aquatic macroinvertebrate richness, leaf litter decomposition, and Ceriodaphnia dubia population responses) in 27 farm ditches, and measured the amounts of forest cover and high-intensity crop cover (landscape composition), and field edge cover (landscape configuration) in 1-km radius landscapes surrounding each ditch sampling site. We used confirmatory path analysis to simultaneously model the direct and indirect relationships between the landscape predictors and water quality variables. Landscape composition measures were the strongest predictors of water quality: pesticides decreased as surrounding forest cover increased, and nitrogen increased with increasing amounts of high-intensity crop cover. Crop cover was also indirectly negatively related to macroinvertebrate richness via its effects on nitrogen and dissolved oxygen. We found no effects of landscape configuration on agrichemical levels, but there was some support for a positive relationship between macroinvertebrate richness and field edge cover. Our results indicate that aquatic macroinvertebrate richness is strongly impacted by fertilizer use in our region, and that macroinvertebrate richness is a more sensitive biotic indicator of farmland water quality than leaf litter decomposition or C. dubia responses. We conclude that, in our region, landscape management to improve farmland water quality should focus primarily on landscape composition. Such management should aim to increase amounts of non-crop cover such as forest, and reduce amounts of crop cover with high agrichemical inputs.