The seasonality of macroinvertebrate β diversity along the gradient of hydrological connectivity in a dynamic river-floodplain system

Anthropogenic disturbance to natural hydrological connectivity, both longitudinal, lateral, and vertical, is threatening the ecological integrity of the freshwater realm. River-floodplain system is particularly adversely affected by the reduction in lateral hydrological connectivity (LHC), representing one of major biodiversity hotspots under increasingly pressure. Many studies have demonstrated that LHC has great influence on the spatial variations of flora and fauna communities (i.e. spatial beta diversity) through facilitating dispersal. However, to fully understand the impact of anthropogenic disturbances on biodiversity, we must also understand how ecological communities change over time (i.e. temporal beta diversity, TBI) and the underlying processes. To evaluate the processes structuring ecological communities, we examined the macroinvertebrate TBI in habitats along the gradient of LHC for an entire hydrological cycle in West Dongting Lake, a Ramsar-listed floodplain wetland at the middle reach of Yangtze River. Our results showed that the total spatial beta diversity fluctuated with water level and peaked at high-water phase, and LHC was the driving forcing affecting both species richness and abundance in all hydrological periods. In particular, species richness and abundance were highest in habitats with medium LHC levels for water-rising and high-water periods reflecting the intermediate disturbance hypothesis except for water-recessing, during which there was no clear pattern. While replacement determined beta diversity in most sites at water-rising and high-water phases, the contribution of nestedness were high during water-withdrawing phase. From water-rising to high-water, macroinvertebrates from other habitats spread to the modified mudflats, which had the lowest LHC, along with the floods. During water-withdrawing period, beta diversity and its turnover component of all habitats were low compared with other hydrological phases. Temporal beta diversity analysis illustrated that the species dispersal was the main mechanism underlying the temporal and spatial variations in the observed community patterns. These findings demonstrated that hydrological connectivity was critical to maintaining the ecological integrity of river-floodplain ecosystems.