Mercury (Hg) is a contaminant of major concern in Arctic marine ecosystems. Decades of Hg observations in marine biota from across the Canadian Arctic show generally higher concentrations in the west than in the east. Various hypotheses have attributed this longitudinal biotic Hg gradient to regional differences in atmospheric or terrestrial inputs of inorganic Hg, but it is methylmercury (MeHg) that accumulates and biomagnifies in marine biota. Here, we present high-resolution vertical profiles of total Hg and MeHg in seawater along a transect from the Canada Basin, across the Canadian Arctic Archipelago (CAA) and Baffin Bay, and into the Labrador Sea. Total Hg concentrations are lower in the western Arctic, opposing the biotic Hg distributions. In contrast, MeHg exhibits a distinctive subsurface maximum at shallow depths of 100–300 m, with its peak concentration decreasing eastwards. As this subsurface MeHg maximum lies within the habitat of zooplankton and other lower trophic-level biota, biological uptake of subsurface MeHg and subsequent biomagnification readily explains the biotic Hg concentration gradient. Understanding the risk of MeHg to the Arctic marine ecosystem and Indigenous Peoples will thus require an elucidation of the processes that generate and maintain this subsurface MeHg maximum.

Abstract. Ocean acidification driven by the uptake of anthropogenic CO2 by the surface oceans constitutes a potential threat to the health of marine ecosystems around the globe. The Arctic Ocean is particularly vulnerable to acidification and thus is an ideal region to study the progression and effects of acidification before they become globally widespread. The appearance of undersaturated surface waters with respect to the carbonate mineral aragonite (ΩA<1), an important threshold beyond which the calcification and growth of some marine organisms might be hindered, has recently been documented in the Canada Basin and adjacent Canadian Arctic Archipelago (CAA), a dynamic region with an inherently strong variability in biogeochemical processes. Nonetheless, few of these observations were made in the last 5 years and the spatial coverage in the latter region is poor. We use a dataset of carbonate system parameters measured in the CAA and its adjacent basins (Canada Basin and Baffin Bay) from 2003 to 2016 to describe the recent state of these parameters across the Canadian Arctic and investigate the amplitude and sources of the system's variability over more than a decade. Our findings reveal that, in the summers of 2014 to 2016, the ocean surface across our study area served as a net CO2 sink and was partly undersaturated with respect to aragonite in the Canada Basin and the Queen Maud Gulf, the latter region exhibiting undersaturation over its entire water column at some locations. We estimate, using measurements made across several years, that approximately a third of the interannual variability in surface dissolved inorganic carbon (DIC) concentrations in the CAA results from fluctuations in biological activity. In consideration of the system's variability resulting from these fluctuations, we derive times of emergence of the anthropogenic ocean acidification signal for carbonate system parameters in the study area.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; Ocean acidification driven by the uptake of anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; by the surface oceans constitutes a potential threat to the health of marine ecosystems around the globe. The Arctic Ocean is particularly vulnerable to acidification due to its relatively low buffering capacity and, thus, is an ideal region to study the progression and effects of acidification before they become globally widespread. The appearance of undersaturated surface waters with respect to the carbonate mineral aragonite (&amp;#937;&lt;sub&gt;A&lt;/sub&gt;&amp;#8201;&lt;&amp;#8201;1), an important threshold beyond which the calcification and growth of some marine organisms might be hindered, has recently been documented in the Canada Basin and adjacent Canadian Arctic Archipelago. Nonetheless, few of these observations were made in the last five years and the spatial coverage in the latter region is poor. Additionally, the strong variability inherent to this dynamic shelf environment renders the temporal imprint of ocean acidification on carbonate system parameters (pH, pCO&lt;sub&gt;2&lt;/sub&gt;, DIC, &amp;#937;) virtually indistinguishable on decadal timescales. We use a dataset of carbonate system parameters measured in Canadian Arctic Archipelago (CAA) and its adjacent basins to describe the recent state of these parameters across the Canadian Arctic and investigate the amplitude and sources of the system's variability. Our findings reveal that, in addition to the surface of the Canada Basin, the entire water column of the Queen Maud Gulf was undersaturated with respect to aragonite in 2015 and 2016. We also estimate that approximately a third of the interannual variability in surface DIC in the CAA results from fluctuations in biological activity.&lt;/p&gt;

Interactive comment on "The recent state and variability of the carbonate system of the Canadian Arctic in the context of ocean acidification" by Alexis Beaupré-Laperrière et al. Alexis Beaupré-Laperrière et al.

Interactive comment on "The recent state and variability of the carbonate system of the Canadian Arctic in the context of ocean acidification" by Alexis Beaupré-Laperrière et al. Alexis Beaupré-Laperrière et al.