4 Conclusions
The results of this study clearly demonstrate seasonal re-assembly of
floodplain aquatic communities across seasons. Although the aquatic
communities were largely shaped by the landscape hydrological
connectivity gradient after the transition, the effect weakened over
time, and the communities were reassembled based more on the local
environments. Animals observed in the floodplain responded differently
but well to the seasonally varying environment based on their mode of
dispersal and life history, and their responses led to the overall
seasonal re-assembly of the community across seasons.
Taxa level analysis showed large contributions of amphibious animals
such as amphibians and aquatic insects to the seasonal re-assembly of
the floodplain aquatic communities. In spring, Rana pirica (frog)
and Hynobius retardatus (salamander), and Limnephilidae
(caddisfly) and diverse aquatic insect taxa contributed to the β
diversity along the hydrological connectivity gradient. We have
previously shown that their habitat selection during the flood recession
period aligns their spatial distribution along a hydrological
connectivity gradient after the flood (Uno et al., 2022). In summer, as
oxygen at many lentic habitats drops to the threatening level for many
benthic aquatic animals (Gaufin et al., 1974), many amphibians and
aquatic insect taxa disappeared from the ponds. For many amphibian and
aquatic insect taxa, their disappearance from the water indicates their
emergence to land as adults (Kishida et al., 2013; Kawai & Tanida
2005). Yet, some taxa such as Apataniidae, Nemouridae, Leptophlebidae
and Ameletiidae disappeared only in ponds but not in the flowing sites,
implying their selective mortality in ponds due to low oxygen level (if
not other factors such as predation). Some Hynobius retardatus were left in the waterbody in summer and autumn after most individuals
left the water, which previous study indicates are overwinter
populations exhibiting phenotypic plasticity response to species
interactions (Kishida et al., 2013). In autumn, other aquatic insects
such as Ephemeridae, Ephemerellidae, Leptophlebiidae and Nemouridae
appeared in the waterbodies. Their habitat selection, based on the local
environment in autumn and environmental filtering for the remaining
assemblages, has likely re-structured the aquatic assemblages in
floodplain ponds based on a stronger association with local
environmental conditions than with the connectivity gradient.
Seasonal re-assembly of planktons and fishes were not detected in the
taxa-level analysis. Planktons were instead quantitatively responding to
the local environmental change and the effects of hydrological
connectivity were not detected in this study. On the other hand, many
fishes were ontogenetically shifting their habitat based on the body
size of individuals occupying different habitats as described well
elsewhere (Bellmore et al., 2013; Jeffres et al., 2008). Among 11
species observed in our study, Rhynchocypris percnurus andMisgurnus anguilicaudata seemed to complete their life history in
floodplain ponds as individuals with all life stages, including juvenile
to mature adults, found in floodplain ponds. In contrast, Cottus
nozawae was only in sites with constant flow. Eight other taxa,
including Rhinogobius sp., Oncorhynchus masou ,Gymnogobius sp., Pseudaspius spp., Lethenteron
reissneri , Salvelinus leucomaenis , Parahucho perryi , andBarbatula oreas temporarily occupied the floodplain ponds as
juveniles. In floodplain ponds, we observed their juveniles including
young of the year individuals and older cohorts, but not mature adults.
Many of their mature adults were observed in the mainstem river and/or
downstream lake. Our observations indicate these fish species are using
the floodplain waterbodies as nurseries for the juvenile fishes (Endo et
al., 2023). Because juveniles of these fishes occupy the floodplain
waterbodies for more than one year, and some cohorts of respective
species stayed in the waterbodies all year, their movements did not
contribute to the taxa level re-assembly across season. Seasonal
movements of fishes between floodplain waterbodies and rivers need to be
determined with more careful observation of their cohorts or direct
observations of their movements (Kanno et al., 2022).
Natural floodplains are characterized by their seasonal and/or temporal
transitions between aquatic-terrestrial and lotic-lentic environments
(Junk et al., 1989; Tockner et al., 2000). Temporal occupation of such
dynamic and ephemeral floodplain waterbodies by amphibious or mobile
animals observed in this study has been commonly and widely observed
(Whiles & Goldowitz 2001; Jeffres et al., 2008; Holgerson et al.,
2019). Therefore, the patterns described in this study of seasonal
re-assembly of aquatic communities in floodplain waterbodies from
spatial structures aligned with landscape level hydrological
connectivity gradients during environmental transition periods to
distributions corresponding to the local environment in stable periods,
maybe widespread. Taking into account such community structures,
maintenance of the diverse aquatic organisms in floodplain waterbodies
requires co-existence of waterbodies of various hydrological
connectivity to the river, as well as waterbodies of various local
environments as characterized by size of waterbodies, temperature, and
dissolved oxygen level.
Our study highlighted seasonal dynamics of community spatial structures.
Although community structures of a given ecosystem are commonly measured
and diagnosed by one-time spatial surveys, this study shows that the
spatial structure of communities may change temporally, particularly in
such dynamic systems as ecotones. Spatial heterogeneities of the
environment at different times of a year can interact, and the spatial
connectivity can also restrict the transition of the seasonal
re-assembly of animals across seasons. The temporal and spatial dynamics
of the natural environment structure the communities in combination, and
their holistic understanding would be required for the understanding of
such dynamic ecosystems. Furthermore, while this study only examined
change in the spatial patterns, the communities at one season would
interact with those in another season as ecological legacies, through
various factors such as population dynamics of each member of the
community, species interactions, nutrient dynamics, and so on
(Richardson 1991; Power et al., 2008; Yang 2008). Further studies would
show spatially and temporally interacting natural community structures
in dynamic environments.
Author contribution
Hiromi Uno: lead all the research activity including Conceptualization,
Data Curation, Formal Analysis, Funding Acquision, Investigation,
Methodology, Project Administration, Resources, Supervision, Validation,
Visualization, Writing-original draft, and review and editing. Shunsuke
Utsumi: equally Conceptualization, Formal analysis, Investigation and
review and editing. Kentaro Morita equally contributed to the
conceptualization, investigation, and review and editing. Osamu Kishida
equally contributed to the conceptualization, investigation, and review
and editing. Md. Khorshed Alam equally contributed to investigation, and
review and editing. Junjiro Negishi equally contributed to the
conceptualization, investigation, supervision, and review and editing
Data availability statement
Authors confirm that we will deposit our data in a public repository and
indicate the repository of our choice upon acceptance.
Funding statement
KAKENHI Grant Number 22K14915 and 22KJ0004 to H.U. from the Japan
Society for the Promotion of Science.
Conflict of interest disclosure
Authors claim no conflict of interest for this manuscript
Ethics approval statement
Our work conforms to the guidelines for the proper conduct of animal
experiments in Japan. Field survey of aquatic animals was approved by
the fishery and forestry department of Hokkaido (ID 610).
Patient consent statement: nothing to declare
Permission to reproduce material from other sources: nothing to
declare
Clinical trial registration: nothing to declare