Data

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Methodology

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App. of Stable Isotope Techniques

Samples were analyzed for stable isotopic composition (¹³C and ¹⁵N) using a Micromass Optima continuous flow mass spectrometer coupled to a Carlo Erba elemental analyzer. Samples with carbonate (all algae and sediments) were acidified to remove carbonates. Carbonates were removed from the samples using the acid vapor method (Hedges and Stern, 1984; Yamamuro and Kayanne, 1995), except that the samples were acidified for 18 hrs after being moistened with deionized/distilled water. Results are reported in the usual delta format as permil values (Craig, 1957; Mariotti, 1983): (1) for ¹³C relative to V-PDB, normalized to a scale where NBS-19 is +1.95 permil and NBS-21 is -28.10 permil, and (2) for ¹⁵N relative to air, normalized to a scale where IAEA-N1 is +0.43 permil and IAEA-N2 is +20.41 permil. Analytical precision (1 sigma level) was generally in the range of 0.1 to 0.2 permil for both C and N, but for some samples replication was no better than ± 0.5 permil due to sample heterogeneity.