Net Community Production and Inorganic Carbon Cycling in the Central Irminger Sea

The subpolar North Atlantic plays an outsized role in the atmosphere-to-ocean carbon sink. The central Irminger Sea is home to well-documented deep winter convection and high phytoplankton production, which drive strong seasonal and interannual variability in regional carbon cycling. We use observational data from moored carbonate chemistry system sensors and annual turn-around cruise samples at the Ocean Observatories Initiative's Irminger Sea Array to construct a near-continuous time series of mixed layer total dissolved inorganic carbon (DIC), pCO2, and total alkalinity from summer 2015 to summer 2022. We use these carbonate chemistry system time series to deconvolve the physical and biological drivers of surface ocean carbon cycling in this region on seasonal, annual, and interannual time scales. We find high annual net community production within the seasonally varying mixed layer, averaging 9.8 +/- 1.6 mol m-2 yr-1 with high interannual variability (range of 6.0-13.9 mol m-2 yr-1). The highest daily net community production rates occur during the late winter and early spring, prior to the observed high chlorophyll concentrations associated with the spring phytoplankton bloom. As a result, the winter and early spring play a much larger role in biological carbon export from the mixed layer than traditionally thought. The subpolar North Atlantic takes in more carbon from the atmosphere than other areas of the ocean relative to its size. This is partially caused by photosynthesis in the surface ocean, which turns inorganic carbon into organic carbon that is transported into the deep ocean in a process known as the biological carbon pump. Using measurements from sensors on moorings in the Irminger Sea, we construct a 7-year time series of the different parts of the inorganic carbon system. Using these, we separate out the forces that impact how much inorganic carbon has the potential to be exchanged with the atmosphere. We find that biological processes remove inorganic carbon from the surface ocean in the spring, summer, and early fall, while in the winter the surface ocean gets deeper and encompasses waters from below that have higher carbon content. The total amount of inorganic carbon removed from the surface ocean each year by biological process is extremely high in the Irminger Sea compared to other global ocean regions. This research highlights the importance of long-term, full year measurements to understand carbon cycle dynamics. We present the first multi-year, near-daily mixed layer total dissolved inorganic carbon time series in the Irminger Sea Annual net community production within the seasonally varying mixed layer is high (9.8 +/- 1.6 mol m-2 yr-1) and has large interannual variability The greatest rates of inorganic carbon removal from the mixed layer via photosynthesis take place prior to mixed layer shoaling