The importance of spatial and temporal scales in understanding chlorophyll variability in the Southern Ocean

Polar regions are undergoing dramatic, rapid, and possibly irreversible changes. Substantial shifts in patterns of sea ice extent and thickness have cascading effects on polar ecosystems (including phytoplankton), with implications for carbon cycling and global climate. Phytoplankton growth is closely tied to environmental variables such as light and nutrient availability, which are sensitive to climate-induced changes in upper ocean circulation, stratification, and sea ice cover. Recently, Prend et al. (2022, ) investigated temporal and spatial scales of chlorophyll (a proxy for phytoplankton biomass) variability in the Southern Ocean. They demonstrated that the dominant temporal scale of variability is sub-seasonal (similar to 0.5-3 months). The implications of this are two-fold: first, climate oscillations (such as the Southern Annular Mode) are not major drivers of year-to-year variation in chlorophyll; second, intermittent bursts of chlorophyll, generated by small-scale processes such as storms and eddies, dictate the annual mean chlorophyll concentration. Additionally, spatial autocorrelation for chlorophyll concentration varied by time scale: seasonal chlorophyll variability was correlated over much larger areas than were variations in year-to-year chlorophyll concentration. Based on Prend et al. (2022, ), future work should be cognizant of (a) the spatio-temporal scales over which chlorophyll is averaged and (b) the need to focus on small-scale, sub-seasonal events (rather than large-scale climate oscillations) to mechanistically explain chlorophyll variability.