Drivers of plankton phenology under the current climate
Under the current climate, simulated OAB and TDM show similar geographic
variation across Europe. Both events occur later at higher latitudes and
altitudes but are only weakly affected by longitude (Fig. 1a-h; Fig.
2a-b, d-e, g-h; Fig. S1-2). Still, with increasing continentality
(eastern longitude), OAB gets slightly delayed whereas TDM shifts
marginally forward in time (Fig. 2 d-e). OAB occurs earlier than TDM and
varies considerably more among lake types and geographic locations (Fig.
1a-h; Fig. 2m-n; Fig. S1-2). The Europe-wide overall median values are
Julian day-of-year 87 vs. 157 for OAB and TDM, respectively, and the
corresponding 20th-80th percentiles
are day-of-year 53-130 (OAB) vs. 140-183 (TDM). At a given geographic
location, OAB can vary up to 46 days with lake type (Fig. 2m). A major
driver of this variation is a lake type’s optical depth (Fig. 2j). In
contrast, TDM at a given geographic location varies much less with lake
type (≤ 26 days, Fig. 2n) and is independent of optical depth (Fig. 2k).
Consequently, variance in TDM is almost exclusively explained by
latitude, longitude, and altitude, whereas optical depth contributes
almost 20% to the Europe-wide variance in OAB (Fig. 2p-q).
The similarities and differences between the phenologies of OAB and TDM
can be explained by the proximate factors controlling them, i.e.
underwater light availability and surface water temperature,
respectively. Both increase seasonally, which explains the common
latitudinal and altitudinal patterns in OAB and TDM (Fig. 2a-b, g-h, see
also Fig. S5 in (Gronchi et al. 2021) and Supplement S3 Model
validation). While the seasonal increase in surface water temperature is
well described by the seasonal increase in air temperature and largely
independent of lake type (Toffolon et al. 2014), underwater light
availability also depends on ice cover, water transparency, and mixed
layer depth, all of which vary with lake type. Depending on geographic
location and lake type, the dominant process controlling OAB can
therefore be the seasonal increase in incident radiation, the timing of
ice-off, or the onset of thermal stratification (Fig. 1m-p; Fig. S3;
(Gronchi et al. 2021)).