Oceanographic variability in Cumberland Bay, South Georgia: Implications for glacier retreat and fisheries management

Abstract

Climate change is impacting high-latitude fjord circulation with consequences for the melting of marine-terminating glaciers, which contributes to sea-level rise, and for the transport of biological material essential for supporting local ecosystems. Currently, little is understood about oceanographic variability in sub-Antarctic island fjords such as Cumberland Bay, the largest fjord on the island of South Georgia in the Southern Ocean. Cumberland Bay is split into two arms, West Bay and East Bay, each with a large marine-terminating glacier at the head. These glaciers have shown asymmetrical retreat rates over the past century, and it is not yet understood why. With a combination of oceanographic data and the development of a new high-resolution hydrodynamic model, including a novel parameterisation of subglacial plumes, we gain the first understanding of the drivers of temporal and spatial oceanographic variability and elucidate the role of such variability in the rate of glacier retreat. We find a strong seasonal cycle in Cumberland Bay driven by a combination of boundary, atmospheric and freshwater forcing. A complex 3-dimensional flow structure is described including cross-fjord circulation features such as eddies, with a high degree of variability on short timescales due to wind forcing, including f\"{o}hn winds. The buoyancy-driven outflow resulting from subglacial plumes is a dominant driver of seasonal and spatial variability in the circulation regime. Bathymetric differences between the fjord arms are also key to the spatial variability, with a postulated shallow inner sill in West Bay potentially contributing to rapid glacier retreat.

The model provides a tool for exploring oceanographic influences on larval retention for the ecologically and commercially important mackerel icefish. Using model flow fields to drive an individual-based model parameterised for mackerel icefish larvae spawned in Cumberland Bay, we identify West Bay as a key retention zone and highlight how larval retention is sensitive to the complex circulation patterns driven by winds, freshwater and fjord-shelf exchanges. Notably, the model predicts higher retention in 2005 and 2006, years with observed high larval abundances in Cumberland Bay, which suggests that the oceanographic variability captured by the model is a key contributor to mackerel icefish recruitment variability. Thus, as understanding of mackerel icefish ecology advances and with further model development, there is potential for the model to provide a predictive tool for fisheries management.

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ORCID for Ivan Haigh: orcid.org/0000-0002-9722-3061

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