Abstract Due to the increasing constraints on using the radioactive isotope 14C to measure primary productivity (14C-PP), we determined the surface carbon fixation rates in the Mediterranean Sea, using the alternative stable isotope 13C method (13C-PP). Rates obtained (13C-POCp) were compared with simultaneous 14C-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 (14C method). 13C-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 14C-PP in 1.2-L samples (two sample t-test, t = 1.035, df = 22, P = 0.31). However, we found that 14C-POCp rates derived from 72-mL incubations were 46% lower than those measured with 13C-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 13C-PP provide comparable results to 14C-PP even in waters of low productivity in the Mediterranean Sea.

中文翻译：

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13C 方法作为基于 14C 的地中海初级生产力测量的可靠替代方法

摘要 由于使用放射性同位素 14C 测量初级生产力 (14C-PP) 的限制越来越多，我们使用替代稳定同位素 13C 方法 (13C-PP) 确定了地中海的表面碳固定率。将获得的速率 (13C-POCp) 与不同体积（72 mL 和 1.2 L）的样品中的同步 14C-POCp 测量值进行比较。我们还使用有机和无机过滤器（14C 方法）测试了溶解初级生产 (PER) 与总生产力的百分比变化。13C-POCp 速率范围从爱奥尼亚盆地的 0.4 到利古里亚地区的 1.5 mgC m-3 h-1。这些结果与 14C-PP 在 1.2-L 样品中发现的结果一致（两个样品 t 检验，t = 1.035，df = 22，P = 0.31）。然而，我们发现 72-mL 孵育产生的 14C-POCp 率比使用 13C-PP 测得的低 46%。大体积和小体积孵化之间的差异可能是由于群落内大型浮游植物细胞数量的差异。使用银膜过滤器测量的 PER 值与使用聚碳酸酯过滤器获得的结果相似。我们的研究结果表明，当仔细选择样本量时，即使在地中海生产力较低的水域中，13C-PP 也能提供与 14C-PP 相当的结果。