Supplementary Figure 1. Irradiance level in the primary production measurements in laboratory
and in situ. Incident, reflected, and transmitted spectral plane irradiances were measured
simultaneously with two Ramses-ACC VIS hyperspectral radiometers (TriOS Optical Sensors).
These sensors use cosine collector and have a wavelength range from 320 to 950 nm and sampling
band width of 3.3 nm. Measurements from these two sensors were cross-calibrated in the air to
minimize instrumental errors. Transmitted irradiances under the ice cover were measured using a
floating aluminium arm to position a sensor under the ice 1 m from the incubation hole. Spectral
diffuse attenuation coefficient for downwelling irradiance (Kd) for the whole ice cover was
calculated from the measured downwelling incident irradiance at the surface (Ed(z=0+)) and the
downwelling irradiance (Ed(z)) below the ice cover using an exponential irradiance decay function
(Ed(z) = Ed(z=0) e-Kd z). Kd was measured for snow covered ice (Kd,ice+snow) and snow
removed from the ice (Kd,ice) to calculate Kd for the snow layer on ice (Kd,snow = zice+snow /
zsnow * (Kd,ice+snow – zice / zice+snow * Kd,ice). Kd,in ice for the incubation depth at 10 cm
from the ice surface with 2 cm snow layer on top was calculated as an layer thickness weighted
average from Kd,snow and Kd,ice. Ed in the incubation depth was then calculated using
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exponential irradiance decay with the measured incident irradiance at the surface and the calculated
Kd,inice. This was then averaged over the day light hours (between 0830 and 1730 local time) to
get the average irradiance Ed(z = 12 cm) at the incubation depth. Irradiance in the laboratory was
measured 40 cm from the light source and also averaged over the same day light hours as the
natural light.
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