Ocean models, atmospheric data, and ocean observations indicate that the Southern Ocean sink for atmospheric CO2 has substantially weakened in the last few decades. The goal of the project is to quantify and understand the spatio-temporal variability in Southern Ocean biogeochemistry using data collected in the Drake Passage measurement program, data collected in other parts of the Southern Ocean, and output from an ocean biogeochemical and ecological model. This study will help to confirm these trends and understand the cause of decreasing CO2 uptake in the Southern Ocean in recent decades. In the proposed study, variability and trends in nutrients and carbon isotopes will be examined at multiple scales and locations in the Drake Passage to understand the processes driving both short-term variability and long-term trends. These trends will be quantitatively compared to those observed in other Southern Ocean locations. The study will employ the Massachusetts Institute of Technology ocean general circulation model with embedded biogeochemistry and ecology, and will reconfigure this model to include the explicit representation of carbon isotopes. Output from hindcast simulations and sensitivity studies will help place the biogeochemical observations into a larger spatiotemporal context, and will help frame our mechanistic understanding of variability and trends.