Description: The presence of snow on sea-ice can have dramatic effects on the photosynthetically available radiation (PAR) that reaches the ice algae. To better quantify this effect in the high Arctic, an experiment was conducted in Kangerlussuaq, west Greenland, throughout March 2013 where snow was cleared off the sea-ice and measured parameters were compared to a control area. Samples of under-ice algae were then taken to analyse the species composition. The bottom 30 mm of the sampled ice cores were used to determine the stress on the photosystems of the sea ice algae as they experienced the rapid irradiance via the variable fluorescence of photosystem II (PSII). PSII was measured using pulse amplitude modulated (PAM) fluorometry (Walz Imaging-PAM fluorometer). This provides the effective quantum yield of PSII (Φ_PSII), which is a unitless ratio of ambient (F) and maximum (F_m) fluorescence yields. These measurements were taken every 3 hours over the final day of the study. Periodically over the week of the study, similar measurements were taken (in triplicate) with a Phyto-PAM System II Emitter-Detector (Phyto-ED) to determine how F and F_m values changed at a courser temporal resolution. Also measured were rapid light curves (RLCs), which allowed for the derivation of the relative electron transfer rate (rETR and rETR_max; µmol é/m²/s), the slope of the light-limited portion of the RLC (α; mol é /mol photons), and the irradiance at which rETR was light-saturated (E_k; µmol photon/m²/s). 100 ml samples of thawed sea-ice were also collected and stored in the dark to be used for species identification. This was done by first enumerating the ice algae to the Utermöhl method. A Zeiss Axiovert 135M (40×) inverted microscope was used to identify and count species/morphological groups via a total of four diagonals per sample. Relative abundances of algae were estimated as percent of total count, and the biomass was calculated according to the ALGESYS protocol. Throughout the experiment the following variables were collected at 5 minute intervals: Air temperature (°C; sun-screened Campbell 107 temperature probe), upwelling and downwelling PAR (µmol photons/m²s; Li-COR Li-191 PAR sensor) at 0.5 m above the sea-ice surface, and under-ice PAR (µmol photon/m²/s; Li-COR –Li-192 submersible PAR sensor). From these data, transmittance (τ) was calculated as the ratio between under-ice PAR and downwelling PAR at the surface, and albedo (0-1 scale) was calculated as the ratio between upwelling and downwelling PAR at the surface.

Description: The averages over the duration of the experiment for irradiance values (E; mW/m²/nm) per wavelength (λ) for downwelling E above the ice, upwelling E above the ice, and downwelling E below the ice. Also provided are albedo (0-1 scale), as a ratio of downwelling/upwelling E above the ice, and transmittance (τ), the ratio of downwelling E above the ice and below.