Abstract
The Arctic spring phytoplankton bloom has been reported to commence under a melting sea ice cover as transmission of photosynthetically active radiation (PAR; 400–700 nm) suddenly increases with the formation of surface melt ponds. Spatial variability in ice surface characteristics, i.e., snow thickness or melt pond distributions, and subsequent impact on transmitted PAR makes estimating light-limited primary production difficult during this time of year. Added to this difficulty is the interpretation of data from various sensor types, including hyperspectral, multispectral, and PAR-band irradiance sensors, with either cosine-corrected (planar) or spherical (scalar) sensor heads. To quantify the impact of the heterogeneous radiation field under sea ice, spectral irradiance profiles were collected beneath landfast sea ice during the Green Edge ice-camp campaigns in May–June 2015 and June–July 2016. Differences between PAR measurements are described using the downwelling average cosine, μd, a measure of the degree of anisotropy of the downwelling underwater radiation field which, in practice, can be used to convert between downwelling scalar, E0d, and planar, Ed, irradiance. A significantly smallerμd(PAR) was measured prior to snow melt compared to after (0.6 vs. 0.7) when melt ponds covered the ice surface. The impact of the average cosine on primary production estimates, shown in the calculation of depth-integrated daily production, was 16% larger under light-limiting conditions when E0d was used instead of Ed. Under light-saturating conditions, daily production was only 3% larger. Conversion of underwater irradiance data also plays a role in the ratio of total quanta to total energy (EQ/EW, found to be 4.25), which reflects the spectral shape of the under-ice light field. We use these observations to provide factors for converting irradiance measurements between irradiance detector types and units as a function of surface type and depth under sea ice, towards improving primary production estimates.
Highlights
In the Arctic Ocean, under-ice phytoplankton blooms can contribute significantly to spring primary production and have been documented more frequently in the last decades (e.g. Fortier et al, 2002; Mundy et al, 2009; Arrigo et al, 2014; Assmy et al, 2017; Oziel et al, 2019)
Experiments to determine underwater irradiance distributions have been undertaken by Voss (1989), Berwald et al (1995) and Leppäranta et al (2003); they have not been examined in detail for ice-covered conditions, where the ice cover plays an important role in the measurements of light availability for primary production estimates, as algal cells indiscriminately absorb light from any direction
In mid-June 2016, air temperatures were above the freezing point, and the ice surface was covered with a relatively wet snow cover displaying a mean albedo of 0.73 in the photosynthetically active radiation (PAR) spectral range, which corresponds to reported albedo values of melting snow (Figure 4.1B; Perovich, 1996)
Summary
In the Arctic Ocean, under-ice phytoplankton blooms can contribute significantly to spring primary production and have been documented more frequently in the last decades (e.g. Fortier et al, 2002; Mundy et al, 2009; Arrigo et al, 2014; Assmy et al, 2017; Oziel et al, 2019). Lower surface albedos of melting snow, developing melt ponds and white ice areas increase total under-ice PAR levels and drive primary production beneath the ice cover (Arrigo et al, 2014; Mundy et al, 2014). The Arctic Ocean has undergone a significant decline in the previously dominant thick multi-year ice cover leading to predictions of an ice-free (sea‐ice area
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