Definition of TDM and phenological delay (PLD)
It is well established that Daphnia growth in spring is predominantly controlled by water temperature (Gillooly & Dodson 2000; Schalau et al. 2008; Straile et al. 2012). It has been shown that the timing of the annual Daphnia abundance maximum (TDM) can be predicted from a temperature threshold (TT 13,5m), i.e. the first day of the year when the mean temperature in the upper 2 m of a lake exceeds 13°C (Straile et al. 2012):
\(TDM=0.99\cdot\text{TT}_{13,5m}+22.25\ days\) (eq. 2)
This relationship has been successfully applied to predict TDM in 62 northern hemisphere lakes of various depth and trophic status, as well as inter-annual variation in TDM in three lakes ranging from 5 to 100 m mean depth (Straile et al. 2012). We therefore used eq. 2 as our definition of TDM.
Water temperatures required for the calculation of this proxy were obtained from numerical simulations (see below). In years when the simulated mean water temperature in the upper 5 meters did not reach 13°C, the TDM was set to day 366. Lakes where this occurred in ≥16 of the simulated 31 years were excluded from further statistical analyses. Finally, we quantified the degree of phenological synchrony between OAB and TDM as the phenological delay (PLD) between the two phenologies, i.e. the difference in days
\(PLD=TDM-OAB\) (eq. 3)