Stable Isotope Methods for d15N-NO3-, d18O-NO3-, d15N-POM, and d15N-NH4+: All filters, glassware, and glass vials used for isotope analysis were pre-combusted for 4 hours at 450C and plastic bottles were acid washed in a 10% HCl bath overnight, to eliminate any crossover N. In preparation for stable isotope analysis, raw seawater was filtered onto a glass fiber filter (25-mm diameter, Whatman GF/D, Cat #: 1823010) to separate particulate organic matter (POM) and dissolved fractions (NO3- and NH4+). Filters were transferred into 20-ml glass vials (Fisher, catalog #05-719-117) and frozen at -20C until isotope analysis of POM (15NPOM). Filtrate, 200 ml, for isotope analysis of dissolved NH4+ (15NNH4+) was processed using a modified version of the NH4+ diffusion method of Holmes (1998), whereby polypropylene membrane filters (25 mm, lot #: 151579, Sterlitech) replaced Teflon filters (Hannon and Bohlke 2008). Isotope analysis of NH4+ and POM was performed on a Finnigan-MAT DeltaPlus Isotope Ratio Monitoring Mass Spectrometer coupled with a Carlo Erba NC 2500 Elemental Analyzer (Model 1108) (Organic Mass Spectrometry Facility, WHOI). Depending on the expected amount of N per sample, the instrument was configured for the typical range of detection, 0.5 to 5 moles N, or modified at the EA combustion furnace to reach a lower detection limit, 0.15 umoles N (Houghton et al. 2000, Holtvoeth et al. 2005, 2006, York et al. 2007). The precision of 15N measurements on this instrument was 0.17‰. Filtrate, 20 ml for N and O stable isotope analysis of dissolved nitrate (15NNO3-) was transferred into duplicate 30-ml HDPE bottles and analyzed by bacteria denitrification assay using ThermoFinnigan GasBench + PreCon trace gas concentration system interfaced to a ThermoScientific Delta V Plus isotope-ratio mass spectrometer (Bremen, Germany) (UC Davis Stable Isotope Facility).

Methods for nutrient quantification: Dissolved nutrient samples were filtered through a glass fiber filter (25-mm diameter, Whatman GF/F, CAT No. 1825-025). The filtrate was collected in acid-washed 20-ml scintillation vials for nutrient analysis and then frozen at -20 deg C until analysis. A SEAL AA3 four-channel segmented flow analyzer (Nutrient Chemical Facility, Woods Hole Oceanographic Institution, WHOI) was used to quantify NH4+, NO3-/NO2-, Silicate, and PO43- in the GF/F-filtered medium using standard methods.