Upwelling Irradiance below Sea Ice—PAR Intensities and Spectral Distributions

Lars Chresten Lund-Hansen,Ian Hawes,Michael Bjerg-Nielsen,Tanja Stratmann,Brian K Sorrell

doi:10.3390/jmse9080830

Abstract

Upwelling and downwelling spectral (320–920 nm) distributions and photosynthetic active radiation (PAR) intensities were measured below a first-year land-fast sea ice in a western Greenland fjord with and without a snow cover. Time-series of surface upwelling PAR, downwelling PAR, and under-ice PAR were also obtained. Spectral distributions of upwelling and downwelling irradiances were similar except for reduced intensities in the UV, the red, and NIR parts of the spectrum when the ice was snow-covered. Upwelling PAR amounted to about 10% of downwelling intensities, giving 5.1 µmol photons m−2 s−1 at the bottom of the ice with a snow cover and 8.2 µmol photons m−2 s−1 without. PAR partitioning analyses showed that the upwelling was related to scattering by suspended particles in the water column. A snow melt increased under-ice daily maximum downwelling PAR from 50 to 180 µmol photons m−2 s−1 and overall under-ice PAR of 55 and 198 µmol photons m−2 s−1 with 10% upwelling. It is concluded that upwelling PAR below sea ice might be an important factor regarding sea ice algae photophysiology and performance with a 10% higher PAR; specifically when PAR > Ek the light saturation point of the sea ice algae.

Highlights

10% of the surface of the earth is covered by sea ice, at its maximum extent in both the Arctic and Antarctic [1,2], and sea ice has a significant role in the Earth’s climate due to its high albedo, whereby 50–80% of upcoming radiation is reflected [3]
Active sea ice algae photosynthesis has been observed at just 0.17 μmol photons m−2 s−1 at the bottom of a 1 m thick sea ice cover with 0.9 m snow [10]
In the present work we report we report upwellingbelow and downwelling irradiances the asea ice, both with and upwelling and work downwelling irradiances the sea ice, both with andbelow without snow without a snow cover, measurements ice andincluding snow thickness, sea ice cover, along with measurements of sea along ice andwith snow thickness, sea of icesea properties properties including bulk salinity, brine volumes, and chlorophyll a (Chl a) content bulk salinity, brine and gas volumes, temperature, andand

Summary

Introduction

10% of the surface of the earth is covered by sea ice, at its maximum extent in both the Arctic and Antarctic [1,2], and sea ice has a significant role in the Earth’s climate due to its high albedo, whereby 50–80% of upcoming radiation is reflected [3]. 10% of the total marine-produced carbon [6] They bloom in the early spring when light reaches the bottom of the sea ice, often passing a snow pack of variable thickness. Most of these microalgae are acclimated to low light conditions [7,8,9]. Light is the main limiting factor for sea ice algae in early spring, as nutrient concentrations are still relatively high in the water column [11]. The absorption and scattering in the snow and ice packs are related to snow age, temperature, water content [12], and, especially, brine and gas volumes in the sea ice [13]

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Discussion

Conclusion