Autoanalyzer 2

DOM_SPE samples were collected between 2016 and 2021 on 10 cruises (see SI Appendix for details), including the BATS and the Hawaii Ocean Time-Series, that covered the following regions: N Pacific Gyre, N Atlantic Gyre, S Pacific Gyre, NE Pacific, NE Pacific Shelf, NE Pacific OMZ, San Pedro Basin (coastal California), and the Caeté Estuary (Amazonian mangroves). Porewater DOM_SPE samples were collected from the intertidal sediments of the mangrove-fringed Caeté Estuary, south of the Amazon Estuary in North Brazil, and a North Sea intertidal flat in Germany. For marine samples, conductivity, temperature, and density casts for physical parameters (i.e., dissolved oxygen, salinity, fluorescence, temperature) were taken at each station and can be found in the SI Appendix (SI Appendix, Figs. S1–S7). Seawater (∼5–20 L samples) was collected from Niskin bottles into acid-washed polyethylene containers and filtered through a 0.80/0.45 µm capsule filter (AcroPak 500) prior to acidification to pH 2 with reagent grade 12 N hydrochloric acid. DOC concentrations were measured prior to SPE via high-temperature combustion on a Shimadzu Total Organic Carbon Analyzer (Shimadzu Corp). Nutrients (nitrate, phosphate, and silicate) were measured via colorimetry on an AutoAnalyzer II (Seal Analytical; see SI Appendix for station-specific methods details and references).