Agricultural stressors reduce river structural and functional integrity in a subtropic agricultural watershed

Agriculture exerts a greater impact on water resources than any other economic sector. Among agricultural stressors, nutrients and pesticides represent critical and global threats, however, their integrated consequences on biodiversity loss and ecosystem degradation remain poorly understood, particularly in subtropic river ecosystems. This study evaluated agricultural stressors and macroinvertebrate communities and leaf litter decomposition along agricultural gradient across a subtropic watershed, focusing on how macroinvertebrate community composition (as structural integrity), functional diversity and leaf litter decomposition (both as functional integrity) respond to agricultural stressors. Results revealed that structural integrity responds more strongly to land use change from forest to agriculture, both individual and interactive effect of nutrients and pesticides than functional integrity. Agricultural land use promotes nutrient stressors and pesticide toxicity in adjacent rivers. Though pesticide toxicity reduced the abundance of sensitive taxa, this compositional shift did not translate into a decline in macroinvertebrate-mediated decomposition. In contrast, nutrient stressors indirectly reduced decomposition rates by 30.11%, primarily via a 63.16% decrease in functional diversity. Moreover, pesticides impacted the direction and magnitude of nutrient effects on integrity indicators. These results provide empirical evidence of agricultural stressors' impact on river structural and functional integrity at the watershed scale under field conditions in a highly agricultural area. These findings highlight the need for strategies that balance food production and river conservation in subtropical watershed landscapes.