Model description, lake types, and climate scenarios
The quantification of OAB and TDM requires knowledge of the timing of
ice off, the seasonal development of the underwater light climate in the
mixed surface layer, and the temperature in the top 5 m of the water
column. We derived this information from numerical simulations of the
seasonal development of ice cover and underwater light and temperature
profiles with the model LAKEoneD. This model combines a one-dimensional
hydrodynamic model with an ice model (Jöhnk & Umlauf 2001; Hutter &
Jöhnk 2004; Yao et al. 2014; Gronchi et al. 2021) (Supplement S2
Hydrodynamic model) and requires meteorological data, lake depth and
water clarity as input variables.
In the reference scenario, LAKEoneD was driven with meteorological data
from the global atmospheric reanalysis dataset ERA-Interim (Dee et al.
2011) produced by the European Centre of Medium-Range Weather Forecasts
(ECMWF). We extracted from this dataset 3-hourly data on wind speed, air
temperature, incident solar radiation, relative cloud cover, and
relative humidity for a total of 1907 terrestrial locations covering
Europe from 35° to 70° North and ‑10° to 20° East at a 0.5° resolution.
All meteorological variables covered the period from 1979-2009 and were
linearly interpolated to hourly values.
In addition to the reference scenario, we explored a warming scenario
that used the same 31 years of meteorological data as the reference
scenario except for assuming an increase in air temperature by +4°C at
all locations and times. Similar simplified warming scenarios have been
employed in earlier lake studies (Peeters et al. 2007; Trolle et al.
2011; Kupisch et al. 2012; Wahl & Peeters 2014; Straile et al. 2015),
and warming by 4°C is within the range of pessimistic projections
(Rajendra K. 2014).
At each location, we considered 16 different lake types defined by the
factorial combination of four maximum lake depths
(zmax = 5, 10, 30, and 100 m) with four light
attenuation coefficients (Kd = 0.3, 0.6, 1.2, and
2.4 m-1). These values of zmaxand Kd cover the ranges encountered in a majority
of lakes >1 ha (Pérez-Fuentetaja et al. 1999; Cael et al.
2017; Seekell et al. 2018). For certain statistical analyses (see
below), we characterized the underwater light climate in each of the 16
lake types by its optical depth (OD) defined as
\(OD=K_d\cdot z_{\max}\) (4)
For each lake type and at each geographic location, we simulated 31
years of vertical temperature profiles for both climate scenarios,
yielding a total of 1,891,744 simulated lake years. All simulations were
performed by first simulating a spin-up period of 5 years using
meteorology from 1979 to 1984. Vertical temperature profiles from the
final day of the pre-run period were used as initial conditions for the
main simulations, which were restarted on the 1st of
January 1979 and run through 2009. In the warming scenario, the +4°C
temperature increase was applied to both the pre-run and the main
simulation periods. Based on these simulations and local incident
radiation from the meteorological data, we generated for each lake type,
geographic location, and climate scenario a 31-year time series of OAB,
TDM, and PLD, respectively. We expressed the impact of climate warming
on the individual phenologies by their respective time differences
(warming minus reference scenario) OABdiff,
TDMdiff, and PLDdiff.