Stable Isotope Analysis of Sargassum and POM

For pieces of Sargassum, approximately 2 g of sample dried powder was weighed in tin capsules for isotopic analysis. For POM, samples were duplicated: Only the subsamples that were not treated were analysed for δ¹⁵N. Stable isotope analyses were performed by continuous flow on a Flash EA 2000 elemental analyser (Thermo Scientific, Milan, Italy) coupled to a Delta V Plus, isotope ratio mass spectrometer (Thermo Scientific, Bremen, Germany) at the Pôle Spectrométrie Océan (IUEM-UBO, Plouzané, France). The reference materials (USGS-61, USGS-62, and USGS-63) and an in house standard Thermo-Acetanilide were used for isotopic corrections and to assign the data to the appropriate isotopic scale: USGS-61 (certified values: δ¹⁵N = −2.87‰ ± 0.04‰ and measured values: δ¹⁵N = −2.89‰ ± 0.06‰), USGS-62 (certified values: δ¹⁵N = +20.17‰ ± 0.06‰ and measured values: δ¹⁵N = +20.17‰ ± 0.08‰), and USGS-63 (certified values: δ¹⁵N = +37.83‰ ± 0.06‰ and measured values: δ¹⁵N = +37.84‰ ± 0.10‰). Results were reported in the δ unit notation and expressed as parts per thousand relative to the international standards atmospheric N₂ for nitrogen. Analytical precision based on replicate measurements (after every five samples) of Thermo-Acetanilide was <0.1 for δ¹⁵N values.