When floods meet dispersal: Unravelling macroinvertebrate community dynamics in a large subtropical monsoonal river basin.

Disentangling the mechanisms underlying community assembly is a central topic in community ecology and an important prerequisite for bioassessment. The relative importance of deterministic and stochastic processes is expected to change among organisms relying on different dispersal modes and may vary considerably through time. However, how seasonal change and dispersal modes will interplay to influence community assembly remains to be demonstrated. Here, we examined seasonal dynamics in the relative role of environmental filtering and potential dispersal pathways (overland vs. watercourse) on community assembly of aquatic macroinvertebrates in a Chinese river experiencing monsoon oscillations. We also investigated if and how community assembly processes are dependent on dispersal modes. To do this, we divided the entire metacommunity into three groups: strictly aquatic dispersers (G1), aquatic/aerial dispersers with weak dispersal ability (G2), aquatic/aerial dispersers with strong dispersal ability (G3). Our results showed strong changes in environmental conditions and community structure before, during and after the monsoon. Watercourse distances were more relevant for strictly aquatic dispersers (i.e., G1), whereas overland distances were more strongly related with taxa able to fly during their adult phase (i.e., G2 and G3). Environmental filtering was consistently predominant across seasons. Moreover, mechanisms underlying community assembly of strictly aquatic dispersers remained relatively stable among seasons. On the other hand, ecological correlates of aquatic/aerial dispersers exhibited pronounced seasonal dynamics, probably as a result of their changing dispersal abilities in the transition from larvae to adulthood. Overall, our findings suggest that the relative importance of environmental filtering and spatial processes are mediated by dispersal modes and the hydrological dynamics imposed by seasonal monsoons. Hence, we argue that incorporating the temporal aspect of intra-annual hydrological variation and considering multiple dispersal modes are crucial for unravelling the dynamic nature of community assembly and for improving river bioassessment and management.