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Summary 1. Understanding mechanisms behind the distribution of organisms along a gradient of hydrological connectivity is crucial for sustainable management of river–floodplain systems. We tested the hypothesis that frequency of flood pulses exerts a direct influence on the distribution of freshwater mussels (Unionoida) by creating a local environment that limits their fitness. 2. Multiscale habitat analyses combined with transplant‐rearing experiments were carried out with a focus on abundance, presence/absence, survival rates and growth rates of mussels. Sixty‐nine floodplain waterbodies (FWBs) were surveyed within a 15‐km lowland segment of the Kiso River in Japan. 3. The abundance of mussels significantly increased with increased frequency of inundation associated with flood pulses at the among‐FWB scale, while the probability of occurrence of mussels was negatively predicted by the amount of benthic organic matter at the within‐FWB scale. 4. Field‐rearing experiments showed that survival rates were low and growth rates nearly zero in infrequently inundated FWBs (these FWBs had no naturally occurring resident mussels). In such FWBs, hypoxia (DO < 2 mg L−1) was frequently observed near the bottom when temperature was optimal for mussel growth (>15 °C). 5. These findings demonstrated that flood pulse frequency was the most important factor in determining mussel distribution in FWBs because it directly limits mussels’ fitness by mediating local environmental factors, possibly dissolved oxygen (DO) levels. Successful restoration efforts for mussel habitat conservation should focus on processes that lead to improved local conditions.

This study tested the applicability of airborne scanning laser altimetry (LiDAR) for characterizing surface connectivity of floodplain water bodies by comparing it with in situ measurements of water levels, and examined whether LiDAR derived data accurately predicted the occurrence of globally imperilled unionid mussels. We intensively examined 10 isolated and 3 connected floodplain water bodies (IWBs and CWBs, respectively) located within a 15‐km lowland segment of the Kiso River in Japan. Using a digital elevation model (DEM) of floodplain ground surface, which is derived from LiDAR, and water surface DEM, which is obtained from records of water level fluctuations, the frequency of surface connectivity between IWBs and the main channel (inundation frequency) was calculated. Inundation frequency of IWBs was also measured in situ using water level and temperature probes. Also, the occurrence of mussels in CWBs and IWBs were examined. LiDAR derived data well predicted in situ derived one with a high level of accuracy (r2 > 0.77), validating the LiDAR‐based approach. Some errors existed in the predictive model, indicating that the applicability of LiDAR data is limited by its spatial resolution and snapshot nature. The occurrence of mussels was positively explained by an increasing level of inundation frequency, and the high accuracies of empirical models were validated using data for other 67 water bodies within the study segment. This study overall demonstrated a high potential of LiDAR data for efficiently monitoring hydrological and biological conditions of floodplain water bodies in the Kiso River and beyond. Copyright © 2010 John Wiley & Sons, Ltd.

Anthropogenic modifications have disrupted aquatic ecosystems by modification of the flow regime and transport of sediment. The impacts of dams are believed to be very large, but, despite the abundance of dams in monsoonal Asia, great scientific uncertainty still exists about the effect of dams on macroinvertebrates. Therefore, we studied macroinvertebrate assemblages at three reaches (downstream of a dam, downstream of a confluence and on a tributary) of the Yahagi River, central Japan, to confirm the impact of long-term impoundment on the relationships between macroinvertebrate assemblages and biotic and abiotic environmental factors and also the role of a major tributary in community shift.Four Surber samples and associated physical measurements (depth, velocity and substrate composition) were taken from four study sites (riffles) at each study reach during the period from 10 to 21 February 2004. Drifting materials (zooplankton, POM and bedload sediment) and periphyton were also sampled. Significant differences were found in the macroinvertebrate fauna of these different reaches. Faunal distributions downstream of the dam were severely altered, with high macroinvertebrate abundance and low taxa richness in contrast to those in the tributary. The two-way indicator species analysis (TWINSPAN) classification clearly distinguished these samples from the others. Based on nonmetric multidimensional scaling (NMS) ordination and correlation analysis (environmental variables vs. NMS axis score), these differences in community structure reflected changes in substrate composition and quantity and quality of suspended and benthic FPOMs. Our observations suggest that while dam-derived zooplankton and reduced sediment transport had a great impact on the fauna, the tributary inflow, with a catchment size of less than 20% of that above the dam, acted as a major source of sediment input and facilitated macroinvertebrate community recovery.

Using a large experimental channel, five artificial high flows with different flow regimes were launched to reveal the precise temporal sequences of drifting particulate matter and organisms during the high flows. Drifting fine particulate matter and organisms were collected by sampling bottles and a water pump, respectively. The peaks of drift abundance occurred before the peak discharges, and the abundance declined quickly within several minutes during the rising phase of high flows. The major determinant of drift abundance of particulate matter and organisms was periods of stable conditions before each high flow (tested by determination coefficients in a correlation analysis). The drift of macroscopic plant material also accelerated the drifts of epiphytic fauna and infauna, which drifted with plants and bed sediments. The magnitude (discharge and duration of peak flows) of high flows only slightly affected drift abundance. The fauna more resistant to high flows were invertebrates that fasten their cases or retreats by silk threads on the substrata. Invertebrate taxa having sucking apparatus or high swimming activity were also resistant to high flows.