2.2 Survey
To examine how the post-flood aquatic communities shaped by the snowmelt
flood shift seasonally, we conducted a broad field survey of floodplain
waterbodies for three times; spring (14-18 June 2021), summer (25-29
July 2021), and autumn (27 September - 1 October 2021). A total of 25
waterbodies (including both paleo- and extant side channels) with
different degrees of connectivity to the mainstem in five spatial blocks
along a 10-km segment of the Butokamabetsu River were selected (Fig. 2).
These are the same set of sites described in Uno et al (2022).
To evaluate the hydrological connectivity of each study site to the
river mainstem, we set a time-lapse camera at the upstream end of each
studied paleo- or extant side channel prior to the snowmelt flood season
(early May) and monitored whether each waterbody was flooded by the
river water, and if so for how many days. We have previously confirmed
that once mainstem river water flowed in from the upstream end, the
water flowed through the waterbodies and out from the downstream end.
Based on these observations, we categorized the study sites into four
connectivity categories (Fig. 2). ”No flow” sites were never flushed by
floodwaters because the upstream end of the paleo-side channel was
always closed. At the ”Early”, and ”Late” sites, each waterbody was
flushed by flow at peak discharge and isolated from the mainstem at low
flow but differed by the timing of flow cessation. Flow cessation
occurred before May 30th, 2021 at “Early” sites, and
after June 1st, 2021 at “Late” sites. At “Flowing” sites, flow was
continuous throughout the study period. Among the 25 sites, six were
categorized as “No flow”, six as “Early”, nine as “Late”, and four
as “Flowing” sites.
The survey targeted four faunal groups: plankton, benthos, nekton
(fishes), and amphibians. Benthic invertebrates were sampled with Surber
net samplers (25 cm × 25 cm). Three samples were collected at each site
on each sampling date and combined. Benthic samples were immediately
sieved through a 0.5 mm mesh and preserved in 99% ethanol for later
sorting. Fish were captured with a backpack electrofishing unit (Model
12B, Smith-Root Inc., Vancouver, Washington, USA) using a pulsed direct
current setting (300–400V, DC). A crew of two or three study
participants sampled in an upstream direction. The entire area of small
waterbodies or the first 20–120 m of paleo-side channels at the site
longer than 120 m were sampled by the two-pass method. All fish
collected in the surveys were identified to species and released back to
the same site alive. The fish catch per unit effort was calculated by
dividing the fish count by the total habitat area sampled. The density
of amphibian larvae was binomial (very high or zero), because their
presence depend on whether their adults lay egg-mass, which contains
many eggs. Therefore, for amphibians, we recorded presence/absence of
their larvae rather than their density by visual investigation during
the electrofishing survey. For plankton, 10 L of water was filtered
through a 70-μm-mesh plankton net and preserved in 2% Glutaraldehyde
for zooplankton analysis.
As environmental variables, we measured pH and conductivity with a
portable pH/COND meter (D-74, HORIBA, Kyoto, Japan) and dissolved oxygen
and water temperature with a portable multimeter (HQ-30d, HACH,
Loveland, Colorado, USA) during each sampling event. Furthermore, to
characterize the physical environment, we estimated the pond area,
average depth and flow in ponds each time. Additionally, during the
autumn survey, we recorded presence/absence of external surface water
input from the mountain side, mean grain size of the sediment, and depth
of organic layer at each site.