Spatiotemporal patterns of emergence phenology reveal complex species-specific responses to temperature in aquatic insects

Aim Climate change is broadly affecting phenology, but species-specific phenological response to temperature is not well understood. In streams, insect emergence has important ecosystem-level consequences because emergent adults link aquatic and terrestrial food webs. We quantified emergence timing and duration (within-population synchronicity) of insects among streams along a spatiotemporal gradient of mean water temperature in a montane basin to assess the sensitivity of these phenological traits to heat accumulation from mid-winter through spring emergence periods. Location Six headwater streams in the Lookout Creek basin, H.J. Andrews Experimental Forest, Oregon, USA. Methods We collected emerging adults of four abundant insect species twice weekly throughout spring for 6 consecutive years. We fit Gaussian models to the empirical temporal distributions to characterize peak emergence timing (mean) and duration (days between 5th and 95th percentiles) for each species/stream/year combination. We then quantified relationships between degree-day accumulation and phenological response. Results Only one of the four species (a caddisfly) showed a simple response of earlier emergence timing in both warmer streams and years. One stonefly had lengthy emergence periods resulting in substantial phenological overlap between warmer and cooler streams/years. Interestingly, two species (a mayfly and a stonefly) responded strongly to temporal (interannual) temperature differences but minimally to spatial differences, indicating that emergence was nearly synchronous among streams, within years. These two species had among-stream differences approaching 500 degree-days from mid-winter to peak emergence. Conversely, duration of emergence was more strongly associated with spatial than temporal differences, with longer duration in lower-elevation (warmer) streams. Main conclusions Emergence phenology has species-specific responses to temperature likely driven by complex cues for diapause or quiescence periods during preceding life cycle stages. We hypothesize a trade-off between complex phenological response that synchronizes emergence among heterogeneous sites and other traits such as adult longevity and dispersal capacity.