Stream groundwater inputs generate fine‐scale variation in brook trout phenology and growth across a warming landscape

Abstract

Climate change is increasing global atmospheric temperatures, which can reduce abundance and cause range shifts in species that are sensitive to warming. However, fine‐scale thermal heterogeneity can drive highly variable local responses to climate change, especially in freshwater environments that differ in groundwater inputs and geomorphology. We used temperature data collected during 2012–2021 from 10 small, pristine streams in eastern Canada to characterise thermal variation at a small spatial scale (~25 km2). We then used relationships between daily air and stream temperatures to reconstruct stream temperature since 1980, and assessed how thermal variation influenced the phenology and growth of brook trout (Salvelinus fontinalis). Air–stream temperature relationships varied considerably among streams despite their close proximity, with predicted summer temperatures differing up to 9.5°C between warmer rainfall‐dominated streams and cooler groundwater‐dominated streams. Rainfall‐dominated streams warmed more than twice as fast as groundwater‐dominated streams across all seasons since 1980, with nearly four‐fold differences in rates of warming evident during summer months. Fine‐scale thermal heterogeneity also shaped brook trout phenology, as juveniles in rainfall‐dominated streams were estimated to hatch and emerge much later (~70 and 40 days, respectively) and experience faster phenological shifts than in groundwater‐dominated streams. Relationships between juvenile brook trout size and accumulated degree‐days were positive, but slopes differed over two‐fold and did not vary systematically based on stream hydrology, suggesting more idiosyncratic impacts of warming on early growth. Collectively, our study illustrates how species responses to climate change in freshwater environments can be consistent in direction but vary substantially in magnitude owing to the influence of groundwater. Future climate change is likely to increase the thermal stress experienced by brook trout populations in warmer rainfall‐dominated streams, while potentially benefiting those in groundwater‐dominated streams where current temperatures are often suboptimal. Observed differences in the rates and ecological impacts of warming among streams suggest that fine‐scale thermal variation must be considered when forecasting effects of future climate change on stream fish population dynamics.

Microgeographic variation in demography and thermal regimes stabilize regional abundance of a widespread freshwater fish

“…Cape Race brook trout are genetically differentiated despite their close proximity and exposure to similar climate conditions (Fraser et al, 2014). In addition, populations vary in abundance and life history (Fraser et al, 2019;Hutchings, 1993), while streams differ in size, habitat variability, and thermal regimes driven by the prevalence of groundwater seeps (Gallagher & Fraser, 2023;Wood et al, 2014; 1). Specifically, LO and UO are separated by a short (~50 m) boulder field that occasionally floods, while MC receives individuals from both UC and LC (Bernos & Fraser, 2016).…”

“…More broadly, groundwater can generate substantial differences in thermal regimes among neighboring streams (Snyder et al, 2015), and this was evident in Cape Race (Appendix S2: Table S1, Gallagher & Fraser, 2023). For example, the disparate incubation temperatures experienced by Cape Race populations (Figure 6a) likely influenced hatch and emergence timing, with warmer winter temperatures accelerating development in groundwater-dominated streams (Appendix S2: Figure S3).…”