Exploring the relationship between sea-ice and primary production in the Weddell Gyre using satellite and Argo-float data

Abstract. Some of the highest rates of primary production across the Southern Ocean occur in the seasonal ice zone (SIZ), making this area of prominent importance for both local ecosystems and the global carbon cycle. There, the annual advance and retreat of ice impact light and nutrient availability, as well as the circulation and stratification, imposing a dominant control on phytoplankton growth. In this study, the drivers and variability of net primary production (NPP) in the Weddell Gyre SIZ were assessed using satellite and autonomous biogeochemical float data. Although the highest daily rates of NPP are consistently observed in the continental shelf region (water depths shallower than 2000 m), the open ocean region's larger size and longer ice-free season mean that it dominates biological carbon uptake within the Gyre, accounting for 95 % of the Gyre's total annual NPP. Variability in the summer maximum ice-free area is the strongest predictor of inter-annual variability in total NPP across the Weddell Gyre (R² = 62 %), with greater ice-free area resulting in greater annual NPP. In the shelf region, the return of sea-ice cover controls the end of the productive season. In the open ocean, however, a decline in NPP occurs before the end of the ice-free season (∼80 to 120 days after sea-ice retreat), suggesting that, later in the summer season, additional factors such as nutrient availability limit NPP. These results indicate that in a warmer and more ice-free Weddell Gyre, notwithstanding compensating changes in nutrient supply, NPP is likely to be enhanced only up to a certain limit of ice-free days.