Pergamon Deep-Sea Research II, Vol. 44, No. l-2, pp. 129-142, 1997 8 1997 Elsevier Science Ltd PIk SO967-0645(96)00073-2 Printed in Great Britain. All rights reserved 0967-0645/97 $17.oO+O.Oil ti3C of Southern Ocean suspended organic matter during spring and early summer: regional and temporal variability F. DEHAIRS,* E. KOPCZYNSKA,t P. NIELSENJ C. LANCELOT,5 D. C. E. BAKKER,T W. KOEVEI( and L. GOEYENS* (Received 12 January 1995; in revisedform 17 October 1995; accepted 15 June 1996) Abstract-Observations are presented for stable carbon isotope abundance (g13C) 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 gt3C values of suspended organic matter in the Polar Front were high, exceeding values predicted from the relationship with CO+s, concentration observed in other areas of the Southern Ocean. Later in the season a13Cof 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 g13C of phytoplankton. The possible additional impact of shifts in nitrogen uptake regime is discussed. 0 1997 Elsevier Science Ltd. All rights reserved INTRODUCTION Like all autotrophic organisms, marine phytoplankton discriminate to a varying extent against the heavy 13C isotope during photosynthetic activity. The carbon isotopic composition of photosynthesising organisms generally is the result of kinetic isotope effects during (1) carbon mass transport through the cell membrane; and (2) carbon fixation during the enzymatic carboxylation step (O’Leary, 1981; Fry and Sherr, 1984; Boutton, 1991 and references therein). For marine phytoplankton the variations in the degree of fractionation are currently attributed to two main processes. One is related to the physico-chemical conditions (temperature and salinity) that control the CO,,,,, concentrations in seawater. Several authors have discussed the inverse proportional relationship between concentration of CO,,,,, and ai3C of suspended organic matter (SPOM), suggesting that the larger the ambient CO,,,,, p 001 becomes, the greater the effect of discrimination against 13C by the phytoplankton (Rau et al., 1989,199 1, 1992; Francois et al., 1993). This observation has led * Analytische Chemie, Vrije Universiteit Brussel, Pleinlaan 2, Brussels B-1050, Belgium. t Department of Antarctic Biology, Polish Academy of Sciences, Ustrzycka 10, Warszawa 02141, Poland. $ Geochronologie, Vrije Universiteit Brussel, Pleinlaan 2, Brussels B-1050, Belgium. §Groupe de Modelisation des Milieux Aquatiques, Universite Libre de Bruxelles, Boulevard du Triomphe, Brussels B-1050, Belgium. 7 Netherlands Institute for Sea Research, N.I.O.Z., 1790 AB Den Burg, Texel, The Netherlands. IlInstitut fiir Meereskunde, Abt. Marine Planktologie, Universitlt Kiel, Diirstenbrooker Weg 20, Kiel D-2300, Germany. 129