DOM Isolation.

HMW and LMW DOM fractions were collected using an inline combination of ultrafiltration (UF) and solid-phase extraction (SPE). A detailed explanation of the DOM isolation protocol is described in ref. 5. The removal of HMW DOM, which was shown to be isotopically and compositionally distinct from total DOM (5), allows direct observation of LMW DON without any confounding influence from HMW material. The high molecular weight cutoff (2.5 kD) UF membranes and high concentration factors employed in this study ensured the collection of the highest MW, most labile material possible, but also result in lower average recoveries than UDOM isolated at low concentration factors. The class of SPE sorbents used for the recovery of LMW material were selected based on maximum recovery of nitrogenous compounds from total seawater (e.g., ref. 5). Together, the combined HMW UDON and LMW SPE-DOM fractions represent an average of 40 ± 4% of total DOC and 27 ± 6% of DON throughout the water column. A summary of the recovery of DOC and DON within each fraction is presented in SI Appendix, Table S3.