Microbubbles are very small bubbles with a diameter of 50 m or less. The important characteristics of microbubbles are their large specific area and small buoyancy, and therefore effective dissolution of gas phase and high adsorption rate are expected. The utilization of microbubbles is useful in improving water environments. Research efforts at a number of institutions have been devoted to the development of microbubble generators. However, in previous microbubble generation devices, complex mechanisms and a high power supply have been required. In this study, a compact and low-power microbubble generator has been developed. Microbubbles were generated by inducing local shear stress in the flow of water through a pipe with slits. Three types of model (slit angle = 30 • , 60 • , and 90 • ) were used in order to investigate the effect of slit angle on the purification technique. The proposed microbubble generator with the slit angle = 60 • showed superior performance for the water purification technique in comparison with the other two models. It was confirmed that microbubble generation was affected by slit angle. Also, in this study, we investigated the electrical property of the gas-water interface, and the -potential of microbubbles was measured by electrophoresis. The results of the floatation experiments were affected by the -potential of the microbubbles.
Understanding how groundwater contribution changes with increasing drainage area is an important research challenge for catchment hydrology (Condon et al., 2020;Fan, 2019). Although hydrological and biogeochemical studies of headwater catchments have often assumed that the underlying bedrock was impermeable, many studies have revealed that non-negligible amounts of water and solutes infiltrate into bedrock and leave headwater catchments not detected at the weir (e.g., Aishlin & McNamara, 2011;Frisbee et al., 2016;Iwasaki et al., 2016;Schaller & Fan, 2009). On the other hand, hydrogeology studies have revealed the existence of regional groundwater flows with larger spatial scales than local groundwater flows (e.g., Ono et al., 2019;Tóth, 1962). In the current study, local groundwater contributing to headwater streams and regional groundwater not contributing to them were referred to as shallow and deep groundwater, respectively. Small headwater catchments are perched on top of regional groundwater systems, and deep groundwater usually discharges to lower basins (Fan, 2019;Smerdon et al., 2012). Thus, understanding the scaling relationships of groundwater contributions is essential when applying small headwater catchment study findings to larger catchments.The role of groundwater flow in scaling relationships has been examined by observing specific discharge (discharge per unit drainage area) and stream water chemistry during baseflow periods at multiple points with different drainage areas in a watershed. Shaman et al. (2004), investigating low-flow specific discharge in the Neversink River watershed, USA, which is underlain by sedimentary bedrock, suggested that deep groundwater contributions increase with increasing drainage area in subcatchments smaller than a representative elementary area (REA) of 8-21 km 2 but become constant in subcatchments larger than the REA. Asano et al. (2020) found an increase in baseflow-specific discharge with drainage area in a 93.58-km 2 sedimentary watershed of the Arakawa River, Japan. In addition to baseflow-specific discharge, stream and spring baseflow chemistry have been commonly used to understand groundwater dynamics across spatial scales by separating stream water into shallow and deep groundwater. Two-source separation has shown an increasing ratio of deep groundwater with drainage area in a 5-km 2 sedimentary Inokawa catchment,
We examined the relationship between the annual escapement of salmon and the d 15 N of willow (Salix spp.) leaves to evaluate the contribution of marine-derived nutrients (MDN) to riparian vegetation around the Pacific Northwest and Northeast regions. Foliar d 15 N values ranged from −3.42‰ to 4.65‰. The value increased with increasing density of carcasses up to 500 fish/km and 1500 fish/km. d 15 N values were variable at carcass densities below 500 fish/km. Possible factors affecting the fluctuation of d 15 N at reference sites are: (1) denitrification; (2) the presence of N 2 -fixing trees, such as alder; and (3) agricultural runoff. d 15 N values at the sites with carcass densities over 500 fish/km were consistently high, while a value of d 15 N below zero was observed at only one site (Rusha River; d 15 N = −1.87‰). At this site, most adult pink salmon returned to limited locations near the estuary because steeper channel gradients acted as a migration barrier, resulting in the negative d 15 N value. Nevertheless, we concluded that our results showed evidence of the feedback of MDN to terrestrial vegetation, although the use of the d 15 N value as a terrestrial end member at spawning sites is limited. If the relationship between the enrichment index, which is expressed as the values using a mixing model, and salmon abundance was estimated, the availability of MDN in riparian ecosystems could possibly be evaluated and will lead to the establishment of escapement goals.
Species identification is a fundamental process for ecological studies and conservation practices, and simple nonlethal identification criteria are important for endangered species. This study developed species identification criteria for two endangered freshwater pearl mussels (Margaritifera laevis and Margaritifera togakushiensis) based on linear discriminant functions (LDFs) that were established considering intraspecific regional morphological differences from sites at Hokkaido, northern Japan. We collected a total of 1,110 mussels from 52 rivers across 32 watersheds in two geographical regions (east and west). Shell morphologies (length, height, and width) of the collected mussels were measured, followed by species identification with gel electrophoresis banding patterns of 16S rRNA polymerase chain reaction products. LDFs were constructed for two size classes (≥40 mm and < 40 mm) using forward stepwise procedures to determine key morphological differences between the two species and consideration of whether regional morphological differences improved identification accuracies. The LDFs revealed that the morphologies of the two species were clearly distinguished by the relationship between height and length for both size classes. Region‐specific LDFs produced better identification accuracies. Regardless of region, the maximum length of M. laevis exceeded 100 mm, whereas all M. togakushiensis were < 100 mm in length. Identification accuracies of the established LDFs were high for each of the five length classes (0–20 mm, 20–40 mm, 40–60 mm, 60–80 mm, and 80–100 mm) with 85–96% (mean: 92%) accuracy in the east and 67–96% (mean: 80%) in the west. These criteria for species identification will progress future ecological studies and conservation practices for freshwater mussels.
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