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.