Seasonal and spatial dynamics of in-channel water bodies on a river sandbar and associated macroinvertebrate communities

In-channel water bodies (ICWBs) on river sandbars, including backwaters and riverine ponds shaped by hydrological disturbance, remain poorly understood with respect to their seasonal and fine-scale spatial dynamics. We investigated the environmental conditions and benthic macroinvertebrate assemblages in ICWBs formed around traditional wood-based river training structures (“Seigyu”) within a single sandbar in a regulated river in central Japan. Environmental conditions showed marked seasonal variation and fine-scale spatial gradients across habitat types over distances of tens of meters. Water temperature emerged as one of the strongest environmental gradients, exhibiting seasonal variation and longitudinal spatial structure within the sandbar. Macroinvertebrate communities in ICWBs were distinct from those in the adjacent main channel, and many taxa were restricted to ICWBs. Community composition varied among habitat types and across seasons, indicating that temporal processes interact with fine-scale spatial heterogeneity to structure assemblages. These results suggest that ICWBs on sandbars function as dynamic habitat patches whose ecological roles are shaped by the interaction between seasonal hydrological dynamics and fine-scale spatial heterogeneity.