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)).