The 13C method as a robust alternative to 14C-based measurements of primary productivity in the Mediterranean Sea

Due to the increasing constraints on using the radioactive isotope 14 C to measure primary productivity (14 C-PP), we determined the surface carbon fixation rates in the Mediterranean Sea, using the alternative stable isotope 13 C method (13 C-PP). Rates obtained (13 C-POCp) were compared with simultaneous 14 C-POCp measurements in samples of different volumes (72 mL and 1.2 L). We also tested the variation of the percentage of dissolved primary production (PER), to the total productivity using organic and inorganic filters (14 C method). 13 C-POCp rates ranged from 0.4, in the Ionian basin, to 1.5 mgC m -3 h -1 in the Ligurian region. These results agreed with those found with the 14 C-PP in 1.2-L samples (two sample t-test, t = 1.035, df = 22, P = 0.31). However, we found that 14 C-POCp rates derived from 72-mL incubations were 46% lower than those measured with 13 C-PP. The discrepancy between large and small volume incubations was likely due to differences in the number of large phytoplankton cells within the community. PER values measured with silver membrane filters yielded similar results than those obtained using polycarbonate filters. Our findings showed that when the sample size is carefully chosen, the 13 C-PP provide comparable results to 14 C-PP even in waters of low productivity in the Mediterranean Sea.