Sub-Antarctic fjord circulation and associated icefish larval retention in a changing climate

• Strong seasonality found in the drivers of fjord oceanographic variability. • Winds drive short-term and spatial variability in fjord circulation. • Oceanographic variability drives interannual variability in particle transport. • Meltwater runoff timing in relation to icefish larvae hatching is key for retention. • Icefish larvae retention increased by regular occurrence of föhn wind events. Climate change is impacting high-latitude fjord circulation with consequences for the transport of marine biota 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, and is a key spawning site for the ecologically and commercially important mackerel icefish. Through the use of a high-resolution three-dimensional hydrodynamic model, the seasonal cycle in Cumberland Bay is found to be driven by a combination of boundary forcing influencing shelf exchange and deep inflow, atmospheric forcing influencing near surface temperatures and flows and freshwater forcing via subglacial discharge driving upwelling and strong outflow. There is a complex three-dimensional flow structure with a high degree of variability on short timescales due to wind forcing. 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. Successful retention of mackerel icefish larvae is found to be sensitive to complex circulation patterns driven by winds, freshwater and fjord-shelf exchanges and to changes in physical processes linked to climate change such as meltwater runoff and föhn wind events. This study highlights the importance of oceanographic variability in influencing ecological processes in fjords in our changing climate.