Reply on RC2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> In the Arctic Ocean region, methane concentrations are higher than the global average; high concentrations of dissolved CH₄ are detectable especially across many subarctic and Arctic continental shelf margins. Yet the Arctic Ocean appears to emit only minimal methane fluxes to the atmosphere across the air-sea interface, suggesting water column oxidation of methane may be an important process. Here we paired thermohaline, chemical, and biological data collected during the Northwest Passage Project transit through the Canadian Arctic Archipelago (CAA) waters in the Summer of 2019, with in-situ and in-vitro methane data. Our results showed high meltwater (meteoric water + sea ice melt) throughout the Western CAA and Croker Bay in the East, and these surface meltwaters showed methane excess. The meteoric waters showed a strong correlation with chlorophyll-&alpha; fluorescence (r=0.63), as well as a correlation between dissolved [CH₄] and chlorophyll-&alpha; fluorescence (r=0.74). Methane oxidation rate constants were highest in Wellington Channel and Croker Bay surface waters (av. 0.01&plusmn;0 d^-1), characterized by meltwaters and Pacific-origin waters. The average oxidation rates in meteoric and Pacific waters were respectively 24.4 % and 12.6 % higher than the entire survey average. Moreover, Pacific and meteoric waters hosted microbial taxa of Pacific-origin that are associated with methane oxidation, <em>Oleispira</em> (&gamma;-proteobacteria), and <em>Aurantivirga</em> (Flavobacteria). The deeper layers were characterized by low methane concentrations and low methane oxidation rate constants (av. 0.004&plusmn;0.002 d^-1). Sea ice covered much of the Western CAA, in the same region with high sea ice meltwater concentrations. These waters also hosted higher average methane oxidation rates (av. 0.007&plusmn;0.002 d^-1). To the east, open coastal water coincided with methane enrichment, but low chlorophyll fluorescence and weak methane oxidation. These results suggest that methane production in ice-associated Arctic blooms may be quickly oxidized by microbes that are also found in these waters, associated with seasonal biology.