δ13C of Southern Ocean suspended organic matter during spring and early summer: regional and temporal variability

Observations are presented for stable carbon isotope abundance (δ13C) and organic carbon and nitrogen content of suspended organic matter from the Southern Ocean (Circumpolar Current and Polar Front) during spring and early summer. The Polar Front Zone was characterized by elevated plankton biomasses and phytoplankton activity, which also increased significantly over the one-month investigation period. From the beginning of the phytoplankton bloom δ13C values of suspended organic matter in the Polar Front were high, exceeding values predicted from the relationship with CO2(aq) concentration observed in other areas of the Southern Ocean. Later in the season δ13C of suspended organic matter in the Polar Front became more negative despite continued high biomass and productivity. Ambient CO2 concentration and cell growth rate, therefore, are not the only factors controlling the δ13C of phytoplankton. The possible additional impact of shifts in nitrogen uptake regime is discussed.