| Summary: | Observations are presented for stable carbon isotope abundance (δ 13 C) 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 δ 13 C values of suspended organic matter in the Polar Front were high, exceeding values predicted from the relationship with CO 2(aq) concentration observed in other areas of the Southern Ocean. Later in the season δ 13 C of suspended organic matter in the Polar Front became more negative despite continued high biomass and productivity. Ambient CO 2 concentration and cell growth rate, therefore, are not the only factors controlling the δ 13 C of phytoplankton. The possible additional impact of shifts in nitrogen uptake regime is discussed.
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