Experimental effects of multiple agricultural stressors on diversity and size structure of subtropical stream macroinvertebrates

Abstract

Agricultural activities cause changes in land use and affect the ecological integrity of streams and rivers, with some of the main impacts being related to increased nutrient and fine sediment inputs. Agricultural water diversion and prolonged droughts due to climate change can also cause alterations in these ecosystems, such as modifications in flow dynamics. However, the interactions between stressors related to agricultural production and climate change are still poorly understood. Therefore, the singular and combined effects of three agricultural and climate change stressors were tested: i. nitrate enrichment (ambient/80 %/140 %/220 %), ii. fine sediment deposition (ambient and 80 % increase), and iii.- flow reduction (ambient and 66 % reduction) on the benthic macroinvertebrate community of a subtropical stream. For this purpose, we used a streamside mesocosm setup consisting of 64 experimental mesocosms. These were continuously supplied with stream water, allowing for comparable physicochemical and climatic characteristics to the adjacent stream. We investigated the individual and combined effects of the stressors on invertebrate community richness, abundance, Shannon diversity, equitability (species dominance), community composition and community size structure (size spectra). Our findings revealed that the macroinvertebrate community was affected by all three stressors, with each one influencing different structural aspects of the community. Nitrate addition had a positive effect on community evenness up to moderate enrichment levels. Sedimentation primarily decreased richness and the abundance of larger individuals, notably impacting chironomid communities that are closely tied to sediment characteristics. Flow reduction altered diversity and equitability, benefiting species such as ostracods that thrive in slow-flow environments. When combined, sedimentation and reduced flow decreased the occurrence of several rarer taxa. These findings suggest that agricultural intensification and climate change may negatively impact macroinvertebrate communities in subtropical streams through single and combined stressor mechanisms.