Land use and soil drainage interactions drive macroinvertebrates and diatoms composition but not their diversity.

Attaining and keeping good water quality is key to allow agriculture producing while lessening environmental impacts. Biological indicators (macroinvertebrates and diatoms) respond to diffuse pollution associated with agriculture by changes in their diversity and composition. With 14 years monitoring of six intensive agricultural catchments in Ireland, we observed that macroinvertebrates diversity decreased through time and was higher in the spring than in autumn, and higher in catchments dominated by well-drained soils compared to those with poorly drained soils. Both macroinvertebrates and diatoms composition varied in function of an interaction between the main land use and soil drainage. While streams in grasslands with poorly drained soils tended to present lower abundances of macroinvertebrates species tolerant to organic pollution, they also presented higher abundances of diatoms species favoured in high to very high nutrients concentrations. Streams in well-drained catchments presented a variable composition with high abundances of both species tolerant and sensitive to organic pollution. Our findings indicate that improving biological indicators of water quality in intensive agricultural catchments require that mitigation measures consider land use and soil drainage capacity.