bioRxiv preprint doi: https://doi.org/10.1101/2020.04.26.062968; this version posted April 28, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. 1 Title 2 Spring-fed streams as flow refugia: Changes in fish assemblages after a rainfall event 3 4 Authors 5 Masaru Sakai1 | Ryoshiro Wakiya2 | Gosuke Hoshi3 6 7 Affiliations 8 1 National Institute for Environmental Studies, Japan (Fukushima Branch), Fukushima, 9 Japan 10 2 Atmosphere and Ocean Research Institute, the University of Tokyo, Chiba, Japan 11 3 Graduate School of Science and Engineering, Chuo University, Tokyo, Japan 12 13 Correspondence 14 Masaru Sakai, National Institute for Environmental Studies, Japan (Fukushima Branch), 15 10-2 Fukasaku, Miharu, Tamura District, Fukushima 963-7700, Japan 16 Email: [email protected] 17 ORCID iD: 0000-0001-5361-0978 18 1 bioRxiv preprint doi: https://doi.org/10.1101/2020.04.26.062968; this version posted April 28, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. 19 Abstract 20 Understanding the structure and function of flow refugia in river networks is critical to 21 freshwater conservation in the context of anthropogenic changes in flow regimes. Safety 22 concerns associated with field data collection during floods have largely hindered 23 advances in assessing flow refugia for fishes; however, spring-fed streams can be safely 24 surveyed during rainfall events owing to their stable flow regimes. In this study, we 25 assessed temporal changes in fish assemblages in a lowland, spring-fed tributary in 26 northern Japan after a 48.5 mm rainfall event. A total of seven fish species were 27 collected, and responses to the flood varied among species. Abundance of salmonids 28 (Oncorhynchus keta and O. masou masou) increased immediately after rainfall but 29 subsequently decreased. Collections of O. keta consisted entirely of mature individuals, 30 while O. masou masou collections included only juveniles, suggesting that the former 31 uses the tributary for spawning whereas O. masou masou may evacuate into the 32 tributary during rainfall events. Benthic species (Cottus hangiongensis and 33 Gymnogobius opperiens) were only observed in the main stream under ordinary flow 34 conditions, and did not appear in the tributary during the flood, suggesting that other 35 flow refugia are available to these species. Our results suggest that spring-fed tributaries 36 may provide flow refugia for nekton species, whereas substrate preferences may be the 37 major determinant of whether spring-fed tributaries function as flow refugia for benthic 38 species. 39 40 KEYWORDS 41 flow refugia, flow regime, flood disturbance, groundwater, lowland, river ecosystem 2 bioRxiv preprint doi: https://doi.org/10.1101/2020.04.26.062968; this version posted April 28, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. 42 1. INTRODUCTION 43 Flood disturbance is a key factor in the assembly of stream communities (Junk, 44 Bayley, & Spark, 1989; Poff et al., 1997; Walton et al., 2017). Flooding is one of the 45 most ubiquitous and frequent disturbances in stream ecosystems, and many stream 46 organisms have developed strategies for coping with this disturbance (Lytle & Poff, 47 2004). For example, stream organisms may avoid the lethal impacts of flooding through 48 strategies such as remaining in their original habitats and enduring the flood, or 49 alternatively, evacuating to locations with relatively mild currents during flood events, 50 and subsequently returning to their usual habitat (Miyake, 2013). These areas of 51 relatively mild currents are called “flow refugia” (Lancaster & Hildrew, 1993a; 1993b). 52 Functionally available flow refugia can be found in shallow bankside areas (Negishi, 53 Inoue, & Nunokawa, 2002; Sueyoshi, Nakano, & Nakamura, 2013), deep pools 54 (Matthews, 1986), complex habitats with woody debris and plant roots (Borchardt, 55 1993; Palmer, Arensburger, Martin, & Denman, 1996), hyporheic zones (Williams & 56 Hynes, 1974), and tributaries (Koizumi, Kanazawa, & Tanaka, 2013), including 57 spring-fed streams. This indicates that maintaining habitat heterogeneity, including a 58 variety of flow refugia, is important for conserving stream biodiversity (Sedell, Reeves, 59 Hauer, Stanford, & Hawkins, 1990). 60 Despite the importance of flow refugia, anthropogenic impacts have steadily 61 degraded stream environments and have introduced both temporal and spatial 62 disruptions to flow regimes. Examples include unpredictable and/or massive flood 63 disturbances that result from dam operations (Choi, Yoon, & Woo, 2005; Magilligan & 64 Nislow, 2005) and climate change (Milner, Robertson, McDermott, Klaar, & Brown, 65 2012; Yin et al., 2018); these may interrupt radical strategies of stream organisms. In 3 bioRxiv preprint doi: https://doi.org/10.1101/2020.04.26.062968; this version posted April 28, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. 66 addition, flood control measures can lead to habitat homogenization and the 67 disappearance of flow refugia such as shallow aquatic-terrestrial transition zones (Junk 68 et al., 1989). In this context, understanding the responses and strategies of stream 69 organisms to flood disturbance, and restoring appropriate flow refugia, are important for 70 the sustainability of stream ecosystems. 71 While it is important to understand evacuation behavior and flow refugia of 72 aquatic organisms, conducting field investigations during floods is generally unsafe and 73 difficult, particularly in the case of fishes. As a result, information regarding the 74 responses of stream fishes to floods has been primarily derived from artificial flood 75 experiments (e.g., Koizumi et al., 2013) and simulation models (e.g., Booker, 2002), 76 and the lack of field-based data has resulted in continued uncertainty regarding the 77 quality and usage of flow refugia. For example, artificial flooding experiments are 78 typically spatially restricted and cannot imitate actual precipitation events in broader 79 stream networks (Miyake, Hiura, Kuhara, & Nakano, 2003; Koizumi et al., 2013), and 80 as a result they often overestimate the responses of stream fishes to floods. Field 81 investigations after floods may not capture short-term responses in which species 82 migrate quickly between flow refugia and their usual habitats. Therefore, investigating 83 the dynamics of stream fishes in flow refugia that can be safely accessed during floods 84 may contribute valuable information that can be applied to the conservation and 85 restoration of aquatic habitats. 86 Spring-fed streams, which are maintained by permanent groundwater discharge, 87 constitute unique ecosystems with stable flow regimes (Mattson, Epler, & Hein, 1995; 88 Lusardi, Bogan, Moyle, & Dahlgren, 2016). Owing to their high stability, spring-fed 89 streams may function as high-quality flow refugia and are suitable for investigating 4 bioRxiv preprint doi: https://doi.org/10.1101/2020.04.26.062968; this version posted April 28, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. 90 community dynamics during flood events. However, substantial information gaps 91 remain regarding the functional role of spring-fed streams in the dynamics of stream 92 fishes during floods. Because springs generally provide purified water, spring-fed 93 streams are frequently conserved for human water supply (LaMoreaux & Tanner, 2001). 94 Elucidating the ecological functions of spring-fed streams in providing flow refugia 95 would further highlight their value. 96 We investigated temporal changes in stream fish assemblages in a spring-fed 97 tributary after a rainfall event. While access to the surrounding non-spring-fed streams 98 was prohibitive at the time, the spring-fed tributary was relatively calm and could be 99 safely accessed for faunal surveys. Here, we report the responses of fishes to the flood 100 event and discuss differences among species. We also assessed the function of 101 spring-fed streams as flow refugia, and propose future perspectives for river network 102 management that will be informative to the conservation of stream biodiversity. 103 104 2. METHODS 105 2.1 Study site 106 The study was conducted in the Shubuto River basin in Kuromatsunai, 107 Hokkaido, Japan (42.64° N, 140.34° E), which encompasses 367 km2 and includes both 108 montane and lowland regions. The region is underlain by sandstone and mudstone, 109 including Cenozoic fossil shells and tuff. Mean annual precipitation between 2009 and 110 2018 was 1615.8 mm, and mean air temperature was 7.5°C, measured at the 111 Kuromatsunai AmeDAS automated weather station, located 4 km northwest of the site. 112 The dominant tree species in riparian zones in the study area are Salix species and 5 bioRxiv preprint doi: https://doi.org/10.1101/2020.04.26.062968; this version posted April 28, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. 113 Quercus crispula, and dominant understory plants are Sasa kurilensis and Reynoutria 114 sachalinensis. 115 Localized areas of groundwater discharge such as flowing wells, seepages, and 116 springs can be found in lowland regions of the Shubuto River system. Among these, 117 significant amounts of groundwater discharge (ca. 0.1 m3/s) have created a spring-fed 118 stream that connects directly to the Shubuto River. This stream has no artificial 119 structures that inhibit longitudinal connections, and stream fishes can thus migrate 120 between the spring-fed tributary and the main stream. We used this spring-fed tributary 121 as a study site and examined its role as a refugium for stream fishes.