Functional traits drive the inter-annual differences in the diversity of benthic macroinvertebrate communities associated with agricultural ditches

Ditches are a key feature of the agricultural landscape, especially in relation to rice cultivation. Despite being affected by anthropogenic pressures in terms of flow regulation and channelisation, they can play an important role as habitats for aquatic species, thus maintaining biodiversity and ecosystem services at both the local and regional scale. Although their well-known role as consumers and bioindicators in freshwater ecosystems, benthic macroinvertebrates of agricultural ditches have received far less attention compared to other aquatic ecosystems. Previous studies have focussed almost exclusively on changes in taxonomic diversity across spatial gradients or under different management practices, while temporal variations in the functional diversity and composition of macroinvertebrate communities in agricultural ditches remain largely unexplored. In this study, benthic macroinvertebrates were sampled from eight sampling sites across five agricultural ditches and one river in October (autumn) of two consecutive years: 2022 and 2023. The two years were characterised by different climatic and hydrological conditions, with significantly lower precipitation and discharge in 2022 than in 2023. On average, the taxon richness, total abundance of benthic macroinvertebrates and functional richness were significantly higher in 2023 than in 2022; while the opposite trend was observed for the heterogeneity in the taxonomic composition of macroinvertebrate communities, as well as for functional evenness. Differences in the number and identity of significant associations between trait modalities of benthic macroinvertebrates and local environmental parameters were found between the two years, with seven and three significant correlations observed in 2022 and 2023, respectively. Overall, these results suggest that the taxonomic and functional diversity of ditch macroinvertebrate communities changed from 2022 to 2023 in this study. They also highlight the importance of considering the temporal dynamics of macroinvertebrate communities alongside anthropogenic practices and flow-related variations when evaluating the biodiversity of agricultural ditches in future research.