Sea Ice and Upper Ocean Properties around the South Orkney Islands, Antarctica, and Their Influence on Regional and Local Antarctic Krill Distribution

Abstract:

The South Orkney Islands region, situated in the southwest Atlantic sector of the Southern Ocean, is a highly productive environment, a hotspot for Antarctic krill, and a key fishing ground for the commercial krill fishery. Processes influencing the advection and retention of krill around the South Orkney plateau have impacts not only locally, but across a wider region downstream. Here, we present a combination of observations and model experiments to investigate the impact of sea ice on upper ocean hydrography and transport of krill. The circulation around the plateau is dominated by a topographically steered boundary current which transports surface and intermediate water masses along the shelf break. Data from two hydrographic surveys across the plateau in 2016 and 2019 document water mass transformations along the pathway of the boundary current. Upper ocean properties are closely linked to sea ice and atmospheric conditions through freshwater input from ice melt and sea surface temperature anomalies, and the two survey years were characterised by contrasting conditions, likely due to large-scale climate patterns such as El Niño. Transport across the boundary current is vital for advection of krill onto the plateau. Results from an individual based model (IBM), forced by output from the NEMO ocean-sea ice model at 1/12^o and ~2.5 km resolutions, demonstrate the effects of physical drivers and individual behaviour on krill distribution. In particular, association with sea ice had a significant impact on distribution patterns by increasing the probability of transport from the western Antarctic Peninsula to the South Orkney plateau, and decreasing transport times. Strong regional oceanic flows had a weaker influence on transport pathways, and local recruitment onto the plateau and retention tended to decrease. Of particular interest for fisheries is a canyon on the northwest of the plateau where krill aggregations frequently occur. We use in situ data to explore the local hydrography and currents, and investigate the effects of high-frequency variability in the flow field on flux into and retention in the canyon using additional IBM simulations.