Elemental and isotopic characterization of dissolved and particulate organic matter in a unique California upwelling system: Importance of size and composition in the export of labile material

Abstract

We report the interseasonal variation in bulk elemental and stable isotopic (δ13C, δ15N) composition of dissolved and particulate organic matter (DOM, POM) during a high‐resolution time series (2007–2009) on the Big Sur coast. In addition to interseasonal variations, we explore the relationships between physical size and reactivity (i.e., composition) of exportable organic matter (OM) pools, and characterize the elemental and isotopic composition of size‐fractionated POM and DOM pools within this potentially high‐nutrient, low‐chlorophyll (HNLC) California upwelling region. Average POM concentrations were low (< 5.2 µmol C L−1 and < 0.7 µmol N L−1), and all OM pools had low C : N ratios (DOM average = 15.0 and POM < 7.8)—indicating that this upwelling center may represent an important source of N‐rich material to offshore environments. Seasonal “excess” dissolved organic carbon (DOC) production was significant (~ 30–80 µmol L−1 DOC); however, dissolved organic nitrogen (DON) production and cycling was largely decoupled from both DOC and physical processes. Overall, our results are consistent with this region representing an “HNLC upwelling system.” The distinct bulk elemental and isotopic OM composition and seasonality vs. that of high‐productivity upwelling regions highlights the need to better understand the biogeochemical diversity of upwelling systems. Finally, we observed a quantifiable size–composition relationship across both POM and DOM size classes, perhaps representing a powerful new tool for modeling N vs. C fluxes in ocean biogeochemical cycles.