Received: 4 January 2019 Revised: 13 May 2019 Accepted: 14 May 2019 DOI: 10.1002/rra.3464 RESEARCH ARTICLE Water temperature drives variability in salmonfly abundance, emergence timing, and body size Heidi E. Anderson1 | Lindsey K. Albertson1 | David M. Walters2 1 Department of Ecology, Montana State University, Bozeman, Montana Abstract 2 U.S. Geological Survey, Columbia Freshwater organisms are disproportionately impacted by climate change and human Environmental Research Center, Columbia, disturbance, resulting in shifts in species' distributions and life histories. We coupled Missouri contemporary and historical datasets documenting physical and ecological variables Correspondence over four decades to quantify changes in the abundance, emergence timing, and body H. E. Anderson, Department of Ecology, Montana State University, Bozeman, MT size of salmonflies (Pteronarcys californica) in the Madison River in southwest Mon- 59717. tana. In contemporary surveys, water temperature was the main driver of salmonfly Email: [email protected] abundance, emergence timing, and body size. Salmonfly densities were negatively Funding information correlated with summer water temperature, which explained 60% of variation in lar- Montana State University; U.S. Geological Survey Montana Water Center, Grant/Award val density among sites, whereas substrate type played a negligible role. Emergence Number: G16AP00067 occurred 20 days earlier, and male and female exuvia length were 13.8% and 11.3% shorter, respectively, at the warmest site relative to the coolest site (4°C difference). These patterns were supported by historical data. For example, a 1.2°C increase in mean annual water temperature in the Madison River between 1977 and 2017 coin- cided with evidence for upstream range contraction. Between 1973 and 2017, emer- gence timing varied widely among years, occurring up to 41 days earlier in years when spring water temperatures were relatively warm. As climate change progresses, we predict that salmonflies could be extirpated from an additional 28 km of currently occupied habitat, representing a 22.6% reduction in suitable habitat along the Madi- son River. This study provides evidence of long‐term biological change of an aquatic insect and highlights the importance of combining spatial and historical datasets to better understand species' responses to environmental stressors across both space and time. KEYWORDS aquatic insect, distribution, long‐term, macroinvertebrate, phenology, Pteronarcys 1 | INTRODUCTION insects are expected to develop faster, transitioning into their adult stage earlier in the year and thus maturing at a smaller adult body size Climate change has been linked to observed and predicted changes in (Dallas & Ross‐Gillespie, 2015). Changes to these biological character- species' distribution, phenology, and growth (Parmesan & Yohe, 2003; istics have important implications at both population and ecosystem Sheridan & Bickford, 2011). In aquatic ecosystems, water temperature levels of organization. Reduced aquatic insect body size can affect is a major driver of ecological processes and one of the primary factors multiple aspects of life history, including fecundity and dispersal ability influencing aquatic organisms' distributions and life histories (Vannote (McCauley & Mabry, 2011; Sweeney, Funk, Camp, Buchwalter, & & Sweeney, 1980). In particular, as water temperatures rise, aquatic Jackson, 2018), whereas changes in their phenology can influence River Res Applic. 2019;1–10. wileyonlinelibrary.com/journal/rra © 2019 John Wiley & Sons, Ltd. 1 2 ANDERSON ET AL. the strength and outcome of predator–prey interactions (Miller‐Rush- relatively high proportions of fine bed sediments. We predicted that ing, Høye, Inouye, & Post, 2010). salmonfly emergence would occur earlier and body size would be The distribution of freshwater macroinvertebrates is also highly reduced at sites where water temperature was relatively warm. When temperature dependent as influenced by physiological processes (Dal- possible, we paired contemporary surveys with historical datasets to las & Ross‐Gillespie, 2015), niche partitioning (Ward & Stanford, better understand how these responses to variable environmental 1982), and biotic processes such as food availability (Sweeney & conditions may vary across space and time. We predicted that as Vannote, 1986). As climate change progresses, many cold‐water obli- water temperatures increased over time, larval density would decline gate species of aquatic insects are experiencing upstream and pole- and emergence timing would advance. Finally, we provided further ward range shifts or contractions (Giersch et al., 2014; Hassall & context for how changes in salmonfly populations in the Madison Thompson, 2010; Sheldon, 2012). These changes often exceed River may continue throughout the 21st century by using predictive responses of their terrestrial counterparts (Ricciardi & Rasmussen, models of water temperature change. 1999; Sala et al., 2000) as most are constrained within river networks, which limits their ability to track suitable habitat. | Negative responses to warming water temperatures can be further 2 METHODS amplified when other anthropogenic stressors, such as excessive fine sediment deposition, are present (Piggott, Lange, Townsend, & 2.1 | Study site Matthaei, 2012). Human activities such as agriculture, grazing, for- estry, and urbanization can artificially increase fine sediment deposi- This study was conducted on the mainstem of the Madison River in ‐ tion in rivers and streams (Waters, 1995). Many benthic southwest Montana. The Madison River originates in north western macroinvertebrates are impaired by increased levels of sedimentation, Yellowstone National Park and then flows approximately 210 km with negative responses that include habitat loss due to filled intersti- northward through broad valleys to join the Jefferson and Gallatin Riv- tial spaces, feeding inhibition, and increased drift among others (Jones ers and form the Missouri River near Three Forks, Montana. The Mad- et al., 2012). ison River passes through Hebgen Reservoir and Ennis Reservoir at We examined how changing temperature and substrate conditions 177 and 59 km above its mouth. The dams forming Hebgen and Ennis influenced the abundance, emergence timing, and body size of a large‐ Reservoirs were constructed in 1914 and 1906, respectively. Ennis bodied aquatic insect, the river stonefly (Pteronarcys californica, also Reservoir is formed by a small hydroelectric dam that operates under ‐ ‐ ‐ known as the salmonfly), in the Madison River in southwest Montana. a run of the river configuration with surface water release. Ennis Res- Salmonflies are iconic insects in the rivers of western North America ervoir is relatively shallow and warms during the summer, which leads due to their large size (often exceeding 6 cm in length as adults) and to elevated summer water temperatures in the Madison River down- popularity among anglers. Salmonflies spend between 2 and 5 years stream of the dam (Fraley, 1978). Above Hebgen Reservoir, water in their aquatic larval stage depending on the thermal regime of the temperatures are influenced by geothermal activity in Yellowstone river they inhabit (DeWalt & Stewart, 1995; Freilich, 1991; Townsend National Park and are therefore much warmer than expected for the & Pritchard, 1998). In early summer, they emerge in massive, synchro- location along a typical Rocky Mountain stream continuum. We char- nized hatches that draw anglers from around the world and provide a acterized physical and biological stream parameters at 10 representa- ‐ significant nutrient pulse to both aquatic and terrestrial consumers tive 100 m reaches distributed along the length of the river, with two including birds, fish, spiders, and ants (Rockwell & Newell, 2009). sites upstream of Hebgen Reservoir, six between the two reservoirs, Mounting anecdotal evidence suggests that salmonfly populations and two downstream of Ennis Reservoir (Figure 1). Historical data could be declining throughout the American West (Nehring, Heinhold, were collected at a subset of these sites (Table S1). & Pomeranz, 2011), including in the Madison River (Stagliano, 2010). Increasing water temperatures and fine sediment additions are a major 2.2 | Environmental conditions threat to this charismatic species, as salmonflies have a narrow ther- mal tolerance and are sensitive to fine sediment deposition that Water temperature was recorded at 30‐min intervals from April 1 to smothers high‐quality riffle habitat by filling the interstices between September 30, 2017, at all 10 sites using a HOBO pendant tempera- cobbles and gravels (Bryce, Lomnicky, & Kaufmann, 2010; Relyea, ture logger (Onset Computer, Bourne, Massachusetts) secured to a Minshall, & Danehy, 2012). boulder midchannel and shaded with a UV‐safe PVC tube. Particle size We documented physical and ecological parameters to determine and level of embeddedness were determined at 10 sites with a how variable water temperature and substrate conditions influence Wolman pebble count (Wolman, 1954) in August 2017 during salmonfly abundance, emergence phenology, and body size. We baseflow when the river was accessible via wading. Percent fine sed- hypothesized that higher water temperatures and proportions of fine iment was calculated as the proportion of 200 observations that were bed sediments would lead to lower larval density and that warmer recorded as fine sediment (<2 mm). At each