We here present conductivity‐temperature‐depth and chemical (CO2, PO4, O2, alkalinity, andpH) observations across an anticyclonic (fresh‐core) ring in the Oyashio Current near Bussol Strait, Kuril Islands. These measurements suggest that the core of the ring is a region of enhanced vertical mixing, with attendant effects on both spatial and temporal patterns of CO2exchange. In particular, the difference between atmospheric and oceanicpCO2at the air‐sea boundary shows a pronounced seasonal reversal in sign. In winter,pCO2is everywhere higher in the ocean than in the atmosphere; however, because mixing within the core of the ring brings additional amounts of deep, CO2‐rich water upward the highest values ofpCO2occur above the ring. In summer,pCO2outside the ring boundary is also higher in the ocean than in the atmosphere; however, because enhanced mixing within the ring now promotes high values of nutrient flux, primary productivity, and CO2uptake,pCO2values there are lower than in the atmosphere. These data suggest the importance of including mesoscale physical processes in the interpretation of oceanic CO2exchange and demonstrate the usefulness of biogeochemical properties in studies of ring dynamics and