The response of phytoplankton biomass to transient mixing events in the Southern Ocean

We examine the sensitivity of surface chlorophyll-a concentrations (as a proxy for phytoplankton biomass) to short-term, transient Mixed-Layer variability in the High Nutrient-Low Chlorophyll Southern Ocean during the austral summer. To investigate this sensitivity, we use satellite remote sensing data, Ocean reanalyses, and the biogeochemical model NEMO-PISCES. We find an unexpectedly large spatial variability in the nature of the chlorophyll-a response to mixing events in both the observations and the model outputs and our results suggest that the degree of iron limitation (relative to light) seems to be instrumental in dictating these patterns. We propose a conceptual model that provides a first order explanation of the regional and seasonal differences in the response of Chlorophyll-a anomalies to transient Mixed-Layer-Depth (MLD) variability. Overall, we suggest that it is the relationship between the seasonal MLD and the corresponding light and iron vertical distributions that dictates the degree of light and iron limitation. This balance then governs the sign of the response to phytoplankton biomass to transient mixing events.