Hydrological connectivity and local environment alternately drive spatial structure of floodplain aquatic community across season

Seasonal changes in the environment often strongly influence biological
communities. In environmental transition zones, or ecotone, the
environment fluctuates over time between two different types of
environments, and the seasonal change is more pronounced. Although
emphasis has been placed on the spatial variation of biota along
environmental gradients, seasonal change has not been well studied despite
the seasonal nature of many ecotones. In this study, we investigated
seasonal shift in aquatic community structures of floodplain waterbodies
characterized as transitions between lotic and lentic environments, and
further investigated the biological processes behind the shift. We
observed a clear seasonal shift of community structure in floodplain
waterbodies. From the spring snowmelt season to the summer low flow
season, the aquatic community structure was largely driven by the
hydrological connectivity to the river, represented as the timing of the
lotic-lentic transition during the seasonal flood recession. In contrast,
after a few months of summer low flow period, the effect weakened over
time, and the communities were structured based more on the basis of the
local environment. The seasonal shift was largely explained by the change
in amphibian and aquatic insect larvae, the main members of the floodplain
aquatic assemblage, which metamorphose and emerge from the water during
the summer period and then re-distribute in different ways more strongly
influenced by local environmental factors such as water body size,
temperature, and dissolved oxygen levels. Given that biota in ecotones
occupy the habitat for a limited time due to the severe environmental
fluctuations, such seasonal changes as we observed in this study may be
widespread in ecotones. Landscape and local environmental conditions could
alternately shape community structures in different seasons. Further
attention to the temporal aspects of community structure is needed for
community studies as well as for conservation.