Detecting phenology change in the mayfly E phemera danica : responses to spatial and temporal water temperature variations

Rising water temperatures under climate change are expected to affect the phenology of aquatic insects, including the mayfly E phemera danica M üller which is widespread throughout E urope. 2. To assess temporal and spatial variability in mayfly emergence, E . danica were monitored at two thermally contrasting reaches in the R iver D ove, E nglish P eak D istrict over the period 2007–2013. Inter‐annual variations in growing degree days ( GDDs ) were modelled for an upstream site with intermittent spring flow supplementing main channel flow ( B eresford D ale) and downstream site dominated by near constant discharges of cool groundwater ( D ovedale). 3. A strong association exists between the emergence cycle of E . danica and GDDs at each site. Beresford D ale accumulated on average 374 more GDDs than D ovedale. After warm summers E . danica emerged after only 1 year in B eresford D ale but began to revert to a bi‐annual cycle after the particularly wet/cool year of 2012. In D ovedale, E . danica maintained a 2‐year cycle throughout the monitoring period in spite of the phenology changes observed 8 km upstream. 4. Data from the present study suggest that habitats near cool groundwater may provide important refugia for populations of insects, potentially delaying permanent shifts in phenology under climate change. However, an ability to detect changes in the thermal triggers and phenological response may be hindered by conventional spot sampling protocols.