Phenological asynchrony varies greatly across locations and lake
types
The simulated current Europe-wide variation in phenological asynchrony,
expressed as the phenological delay (PLD) between OAB and TDM, is
strikingly large, ranging from 20 to 190 days across geographic
locations and lake types (Fig. 1i-l, Fig. S4, S9). PLD decreases towards
more northern and eastern locations and with increasing elevation (Fig.
1i-l, Fig. 2c, f, i), indicating shorter spring bloom periods in regions
with a higher probability of ice cover in winter. Because OAB and TDM
show strong but similar latitudinal and altitudinal trends and weak but
opposite longitudinal trends, the influence of these three geographical
factors on PLD is of comparable magnitude (Fig. 2a-i). The longitudinal
trends can be explained by the more continental climate at eastern
longitudes, where cold winters delay ice-off and thus OAB while warm
summers promote an earlier TDM (Fig. 1a-l, Fig. 2d-f, Fig. S1, S2, S4).
For both OAB and TDM, optical depth explains much less of the
variability than geographic location (Fig. 2p-q). Yet, because
latitudinal and elevation effects on OAB and TDM are similar, geographic
location explains only a relatively small fraction of the variance in
PLD (Fig. 2r), which instead is primarily driven by the effect of
optical depth on OAB (Fig. 2j, l). With increasing optical depth, OAB
occurs later while TDM remains approximately unchanged (Fig. 2j-k).
Consequently, PLD decreases with increasing optical depth (Fig. 2l). The
differences in the sensitivities of the phenologies to optical depth are
also reflected in location-specific standard deviations, which are small
for TDM and large for OAB and PLD (Fig. 2m-o). Large PLDs are typically
observed in lakes with optical depth ≤ 12 in which OAB occurs early
because it is controlled by the seasonal increase in incident radiation
(Fig. 1; Fig. 2l; Fig S1, S3, S4). In contrast, small PLDs are observed
in lakes with optical depth ≥ 36, where OAB is controlled by the onset
of stratification (Fig. 1l, p; Fig. 2l; Fig S3, S4).