RESEARCH ARTICLE Stream macrophytes increase invertebrate production and fish habitat utilization in a California stream Robert A. Lusardi 1 | Carson A. Jeffres 2 | Peter B. Moyle 2 1 Center for Watershed Sciences/California Trout, University of California, Davis, California, USA 2 Center for Watershed Sciences, University of California, Davis, California, USA Correspondence R. A. Lusardi, Center for Watershed Sciences/ California Trout, University of California, One Shields Avenue, Davis, California 95616. Email: ralusardi@ucdavis.edu Funding information California Trout Abstract Stable flow and thermal regimes, coupled with geologically derived nutrients, are drivers of enhanced productivity in volcanic spring‐fed rivers. However, little informa- tion exists on biotic mechanisms or species interactions contributing to elevated pro- ductivity at higher trophic levels. In a California stream, juvenile steelhead trout were observed to preferentially select macrophyte habitat at a rate three times greater, on average, than five other habitat types. To understand the potential rearing benefits associated with macrophytes, we conducted a manipulative experiment to determine how macrophytes affect invertebrate prey availability and stream water velocity. Macrophytes supported up to nine times greater abundance of invertebrates than adjacent open gravel habitats. They also doubled invertebrate drift rates and reduced water velocity by up to 42‐fold. The results show that aquatic macrophytes are an important stream habitat feature that may be bioenergetically more favourable for rearing salmonids than more traditional lotic habitats. We suggest that macrophytes have the potential to enhance growth rates of juvenile salmonids when compared with other habitat types. Habitats that confer growth and size advantages may ulti- mately improve fitness and contribute to conservation of imperilled salmonids. KEYWORDS invertebrate production, macrophytes, positive species interactions, salmonid habitat, spring‐fed rivers 1 | INTRODUCTION Macrophytes are a dominant habitat feature in many lowland, nutrient rich, lotic ecosystems (Kaenel & Uehlinger, 1999). Their physical and biological effects on the stream micro‐environment are extensive. Macrophytes enhance stream bed stability, reduce water velocity, and increase water depth (Cotton, Wharton, Bass, Heppell, & Wotton, 2006; Fritz, Gangloff, & Feminella, 2004; Gregg & Rose, 1982; Kaenel & Uehlinger, 1999). They provide habitat complexity by directly increasing available surface area (via plant morphology) for inverte- brates and epiphytic algae colonization and by providing refuge from predators (Rozas & Odum, 1988; Taniguchi, Nakano, & Tokeshi, 2003). They are known to retain fine particulate organic matter and detritus, important energy sources in food web dynamics (Elger, Barrat‐Segretain, & Willby, 2006; Gregg & Rose, 1982; Harrod, 1964; Sand‐Jensen, 1998). The importance of macrophytes in lotic ecosystems is clearly substantial; however, few studies have examined their effect on secondary production and drift of invertebrates and their potential importance for rearing of Pacific salmonids (Oncorhynchus spp.). Macrophytes are particularly extensive in spring‐fed rivers in volcanic arc terrains; such rivers can be important for rearing of juvenile salmonid fishes (Lusardi, Bogan, Moyle, & Dahlgren, 2016). Macrophyte abundance in these systems is largely a product of water chemistry, stable flow, constant thermal regimes, open canopy, and low gradient. Although macrophytes are known to be important habitat modifiers and provide spatial heterogeneity in streams for invertebrate and epiphytic algae colonization, their role in food webs and other ecological processes is not well defined (Taniguchi et al., 2003). Abiotic variables such as water temperature and flow Received: 5 October 2017 Revised: 28 June 2018 Accepted: 5 July 2018 DOI: 10.1002/rra.3331 River Res Applic. 2018;1–10. © 2018 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/rra 1