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

1. Rising water temperatures under climate change are expected to affect the phenology of aquatic insects, including the mayfly Ephemera danica Muller which is widespread throughout Europe. 2. To assess temporal and spatial variability in mayfly emergence, E. danica were monitored at two thermally contrasting reaches in the River Dove, English Peak District 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 (Beresford Dale) and downstream site dominated by near constant discharges of cool groundwater (Dovedale). 3. A strong association exists between the emergence cycle of E. danica and GDDs at each site. Beresford Dale accumulated on average 374 more GDDs than Dovedale. After warm summers E. danica emerged after only 1year in Beresford Dale but began to revert to a bi-annual cycle after the particularly wet/cool year of 2012. In Dovedale, E. danica maintained a 2-year cycle throughout the monitoring period in spite of the phenology changes observed 8km 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.