ALBERT

Description: During winter 2017 the semi‐permanent Beaufort High collapsed and the anticyclonic Beaufort Gyre reversed. The reversal drove eastward ice motion through the Western Arctic, causing sea ice to converge against Banks Island, and halted the circulation of multiyear sea ice via the gyre, preventing its replenishment in the Beaufort Sea. Prior to the reversal, an anomalously thin seasonal ice cover had formed in the Beaufort following ice‐free conditions during September 2016. With the onset of the reversal in January 2017, convergence drove uncharacteristic dynamic thickening during winter. By the end of March, despite seasonal ice comprising 97% of the ice cover, the reversal created the thickest, roughest and most voluminous regional ice cover of the CryoSat‐2 record. Within the Beaufort Sea, previous work has shown that winter ice export can precondition the region for increased summer ice melt, but that a short reversal during April 2013 contributed to a reduction in summer ice loss. Hence the deformed ice cover at the end of winter 2017 could be expected to limit summer melt. In spite of this, the Beaufort ice cover fell to its fourth lowest September area as the gyre re‐established during April and divergent ice drift broke up the pack, negating the reversal's earlier preconditioning. Our work highlights that dynamic winter thickening of a regional sea ice cover, for instance during a gyre reversal, offers the potential to limit summer ice loss, but that dynamic forcing during spring dictates whether this conditioning carries through to the melt season.

Description: The environmental factors influencing the microalgal bloom during sea-ice breakup in Hudson Bay were investigated during June 2018, producing the first results ever on the seasonal development of the marine ecosystem in the offshore waters of this vast inland sea. As is typical in the Arctic, primary production was found to commence at the onset of ice melt, with surface nutrient depletion leading to the formation of a subsurface chlorophyll maximum in the open waters of western Hudson Bay. Simultaneously, the melting mobile ice cover in central Hudson Bay created favorable conditions for a diatom-dominated under-ice bloom, with the results of irradiance-photosynthesis curves confirming that phytoplankton cells were acclimated to increasing light levels in the surface water. The high production rates measured in ice-covered and ice-free waters highlight the considerable plasticity of phytoplankton in terms of photosynthetic performance in this highly variable environment. Interestingly, the maximum values of primary production and phytoplankton biomass observed under the sea ice (343 mg C m-2 d-1 and 35.10 mg TChl a m-2) were lower than those observed in open waters during the late-bloom stage in the western region (486 mg C m-2 d-1 and 57.12 mg TChl a m-2), which is attributed to a confined euphotic zone (reduced light availability? Since the euphotic zone in clear waters under the ice can be as thick as elsewhere, but simply receive less irradiance overall) under the ice and low surface concentrations of inorganic nitrogen (〈2 mmol L-1) in central Hudson Bay. However, the highly abundant sub-ice diatom Melosira arctica contributed an estimated additional 287 mg C m-2 d-1 to under-ice production in this region, which implies that this filamentous diatom has a similar role in the seasonally ice-covered sub-Arctic as in the central Arctic Ocean where it significantly contributes to local production. Refining the historical total production estimates of Hudson Bay with our spring observations, we recalculated annual production to be ca. 69 g C m-2, which equates to mean value for interior Arctic shelves.