Spatially variable hydrological and biological processes shape diverse post‐flood aquatic communities

Abstract

1. Diverse aquatic environments in floodplains support high biodiversity, including plankton, benthos, nekton (fish), and amphibians. Variation in aquatic communities among waterbodies should be explained not only by the spatial variation in the environment at low flow but also by the hydrological dynamics and biological responses over flood and recession periods. However, very few studies have examined the formation processes of floodplain aquatic communities over flood periods. 2. In this study, we aimed to obtain a mechanistic understanding of the assembly of aquatic communities on a temperate floodplain by conducting intensive fieldwork during and after seasonal flooding. The study was conducted in the Butokamabetsu River catchment in northern Hokkaido, where snowmelt causes annual spring floods. At peak flow, floodwater not only flow through main stream river and the extant side channels, but also flow through some palaeo-side channels, which are usually isolated from the mainstream river and harbour stagnant water. As the floods recede, palaeo-side channels become isolated from the mainstream river and their flow cease, but the timing of the flow cessation varied among palaeo-side channels. We hypothesised that this spatial asynchrony in the timing of flow cessations shapes diverse floodplain aquatic communities. Therefore, we conducted field surveys of four faunal groups (plankton, benthos, nekton [fish], and amphibian) during and after the spring flood in 2019. 3. The post-flood aquatic communities varied spatially, according to the flow at peak flood, the timing of flow cessation, and the flow at low flow. Plankton composition was influenced by the water flow at peak flood, and they were more abundant in waterbodies that were never flushed by floodwaters. Fish composition was also influenced by the water flow at peak flood; in particular, salmonids were more abundant in waterbodies that were hydrologically connected to the main stream at peak flood. The presence of amphibian eggs was influenced by the timing of flow cessation; eggs were laid in waterbodies with stagnant water during breeding season. The benthic composition was influenced by flow at low flow. After the flood had receded, these different distribution patterns of the four biological groups led to gradual variation of aquatic communities among waterbodies on a floodplain. 4. Overall, this study showed that hydrological dynamics during flood recession shapes the post-flood aquatic communities, and the spatial variability in the hydrological dynamics and the different responses of the four faunal groups support diverse aquatic communities on a floodplain. Importance of the natural geomorphological complexities, where paleo-side channels with wide range of hydrological connectivity to the river channel co-occur, as well as the natural hydrological dynamics of snowmelt recession, where the flooding discharge gradually decrease over a certain period in spring for floodplain aquatic communities are highlighted.