Sea Surface Kinetic Energy as a Proxy for Phytoplankton Light Limitation in the Summer Pelagic Southern Ocean

The pelagic Southern Ocean is a high-nutrient, low-chlorophyll ecosystem. Here, phytoplankton growth is colimited by iron supply and light availability. This creates a general expectation that when light is available in the austral summer (shallow mixing depths), phytoplankton concentrations may be high or low depending on the delivery of iron to the surface layer. When light is not adequate (deep mixing depths), phytoplankton concentrations will likely be low, even if iron is available. Here we show that low surface kinetic energy behaves like a necessary but not sufficient condition for high chlorophyll concentrations. In high kinetic energy conditions, high chlorophyll concentrations are rare. Conversely, under low kinetic energy conditions, both high and low chlorophyll concentrations were observed. We show that higher kinetic energy conditions are related to deeper mixed layers, which is likely a proxy for local light conditions. Probabilistic models of chlorophyll based on surface kinetic energy were able to describe 30% of the spatial variability in monthly chlorophyll climatologies. This means that local light availability, proxied by mixing through kinetic energy, significantly shapes the spatial distribution of chlorophyll in the Southern Ocean. We suggest that regions with consistently higher kinetic energy may not be as sensitive to iron inputs compared to historic iron addition experiments, which were conducted in low surface kinetic energy conditions. Plain Language Summary Phytoplankton are at the base of the food web in most ocean ecosystems. In the Southern Ocean, they are critical for converting carbon dioxide into organic matter. Some of that organic matter is exported to the deep ocean and sequestered from the atmosphere. Therefore, factors that affect phytoplankton concentrations are important for understanding their impact on both marine ecology and global carbon budgets. Here we show that surface kinetic energy may be an important indicator of phytoplankton concentrations in the Southern Ocean. We propose that increased surface kinetic energy results in deeper vertical mixing, limiting phytoplankton access to light. We show that phytoplankton blooms are not common in high kinetic energy environments in the Southern Ocean in the summertime. Key Points Kinetic energy is a proxy for mixed layer depth, operating as a necessary but not sufficient condition for phytoplankton blooms High kinetic energy appears to suppress phytoplankton blooms in the Southern Ocean The spatial pattern of kinetic energy in the Southern Ocean explains up to 30% of the variability in the spatial distribution of chlorophyll