Light transmittance and potential solar heating of the ocean water column following record low sea ice extents across the Distributed Biological Observatory in the Pacific Arctic Region

Record lows in seasonal sea ice cover in the northern Bering Sea have been observed over the past several years, including in 2018 when ice south of St. Lawrence Island was present for only 25 days from January through June (118 fewer days than the 1981–2010 mean of 143 days). Sea ice cover in 2019 over the same months recovered somewhat, but was still well below the mean at 96 days. These recent low ice extents have been due to both later winter freeze-up and earlier spring breakup, which in turn have important cascading impacts on the physical, biological, and biogeochemical state of the overall marine environment throughout the region. Measurements of the transmittance of solar radiation through the ocean water column is one of the critical elements for understanding the potential implications of these recent shifts in sea ice, including impacts on primary production, damaging effects of UV radiation on phytoplankton, photodegradation of dissolved organic matter, and upper ocean heating. We present observations of downwelling irradiance and upwelling radiance profiles in the top ~30–50 meters of ocean waters, collected at 36 discrete stations across the northern Bering and Chukchi Seas. Profiles were collected during July 2018 and 2019 as part of the Distributed Biological Observatory (DBO) program onboard the CCGS Sir Wilfrid Laurier, and represent first observations of the optical consequences of these extremely low ice years. The profiling radiometers collected measurements with 19 channels (320, 340, 380, 395, 412, 443, 465, 490, 510, 532, 555, 560, 625, 665, 670, 683, 710, and 780 nm, as well as photosynthetically active radiation/natural fluorescence), with concurrent measurements made by a surface reference radiometer with identical channels. The potential heating owing to absorption by light-absorbing materials were then compared to our optical observations through radiative transfer modeling, allowing for observation vs. model comparisons. Continued monitoring of the transmittance of solar radiation through the water column at these DBO sites will be crucial for understanding changes in the optical and underwater light field as the duration of the open water season continues to lengthen with declining seasonal sea ice cover.