At approximate every degree, water samples to measure C assimilation rates were taken from the clean underway flow through system (intake at 6m). Triplicate incubation bottles were inoculated with 20 µmol/L of NaH13CO3. All polycarbonate incubation bottles were acid rinsed three times, rinsed two times with deionized water, and rinsed three times with seawater directly from the sample point prior to incubation. Samples were incubated for 24 hours. C assimilation rates experiments were terminated by filtering each bottle (pressure drop <10 kPa) through a 25 mm precombusted GF/F filter. Natural abundance samples for particulate organic carbon, used as t-zero values, were obtained by filtering 4 L water samples onto pre-combusted GF/F filters. All filters were snap frozen in liquid N and stored at -80 °C. Back on land, the filters were acidified and dried overnight at 60 °C. Samples were analysed at the Isotopic Laboratory at UC Davis, California, with an Elementar Vario EL Cube or Micro Cube elemental analyser (Elementar Analysensysteme GmbH, Hanau, Germany), interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). The external error of analyses was 0.2 per mil for d13C. C fixat assimilation ion rates (rho in nmol/L/h) were calculated following Dugdale and Goering (1967) and Knap et al. (1996).