Physical drivers of long-term chlorophyll-<i>a</i> variability in the Southern Ocean

Despite the critical roles of the Southern Ocean in marine biodiversity and the global carbon cycle, there is a significant gap in understanding how physical drivers shape the long-term variability of phytoplankton at the base of this extensive marine ecosystem and the carbon it exports. Here, we used monthly satellite ocean colour observations to assess seasonal and interannual variability and long-term trends of phytoplankton in the Southern Ocean. Over the 26 years we studied (1998–2023), seasonal variations in the concentration of chlorophyll-a (Chl-a, a proxy for phytoplankton biomass) were statistically significant, with the highest values (mean ± standard error of 0.305 ± 0.001 mg m−3, n = 78) in summer (January–March) and lowest (0.169 ± 0.003 mg m−3, n = 54) in winter (July–September). The highest and lowest annual Chl-a values across the entire Southern Ocean were observed in 2010 and 2018, respectively. A 5-to-6-year cycle was also observed for areas south of the Polar Front. Over the 26 years, 60% of the Southern Ocean showed an increase in Chl-a. We then assessed the concurrent changes in Chl-a and four potential environmental drivers of its variability, namely the sea surface temperature, mixed-layer depth, wind speed, and above-surface photosynthetically available radiation. This analysis revealed complex patterns of either parallel or opposite changes in Chl-a and these four physical variables. When anomalies (de-seasonalised data) of Chl-a were regressed against anomalies of the physical variables, sea surface temperature and photosynthetically available radiation showed stronger correlations with Chl-a than did mixed-layer depth and wind speed in about 75% of the Southern Ocean. Further investigation incorporating additional variables, such as large-scale climate drivers and sea ice variability, is needed to achieve a clearer understanding of multi-decadal phytoplankton variability in the Southern Ocean.