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 (d 13 C, d 15 N) composition of dissolved and particulate organic matter (DOM, POM) during a high-resolution time series (2007)(2008)(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 mmol C L 21 and , 0.7 mmol N L 21 ), and all OM pools had low C : N ratios (DOM average 5 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 mmol L 21 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.

Pacific carbon cycling constrained by organic matter size, age and composition relationships

“…Whereas HMW DOC is on average predictably younger than LMW Overall, our core observation of progressively increasing UDOC Δ 14 C values with increasing CF suggests that molecular size and composition are key parameters in determining DOC cycling and persistence in the ocean. These data represent the most detailed evidence for a size-age continuum within the marine DOC pool, significantly expanding upon previous observations of DOC size-bioavailability relationships [Benner and Amon, 2015], and also potential linkages between organic matter size, composition, and Δ 14 C [Walker and McCarthy, 2012;Walker et al, 2014]. Our model results show that molecular size can largely predict DOC cycling, suggesting that the size distribution of DOC molecules likely have a pronounced effect on DOC residence times.…”

The effect of organic carbon on fixed nitrogen loss in the eastern tropical South Pacific and Arabian Sea oxygen deficient zones

“…The C : N ratio of the DOM was high (9.3) compared with both the Redfield ratio (6.6) and the POM collected concurrently in the sediment trap (7.8), suggesting the DOM was more chemically degraded. Labile N-rich proteins and amino acids are preferentially remineralized, leaving behind the more refractory fraction of DOM (Walker and McCarthy 2012). Another possibility is that not enough DOM was added to cause a response.…”

“…Labile N-rich proteins and amino acids are preferentially remineralized, leaving behind the more refractory fraction of DOM (Walker and McCarthy 2012). Another possibility is that not enough DOM was added to cause a response.…”

Midwater zooplankton and suspended particle dynamics in the North Pacific Subtropical Gyre: A stable isotope perspective

“…Suspended particle degradation in midwaters-Suspended particles are a key component of the marine OM sizereactivity continuum (Amon and Benner 1996) and may, through precursor-product relationships (Walker and McCarthy 2012) act as a conduit for degradation pathways between large particles and dissolved organic matter (DOM). Such pathways likely drive the reduction of large particle fluxes in midwaters and the formation of recalcitrant DOM, a major repository of fixed C in the world's oceans.…”