Abstract
The Peruvian upwelling system (PUS) is among the most productive regions in the ocean, with high rates of primary production and an intense oxygen minimum zone (OMZ). The main perturbation of this system is associated to “El Nino” (EN), which affects water mass distribution and reduces primary production. Previous studies in the PUS provided first insights into the dynamics of dissolved organic matter (DOM), but high-resolution studies involving the molecular characterization of the DOM pool to reveal the processes that affect the carbon cycle in this highly productive system are lacking. We characterized the molecular composition of solid-phase extractable DOM (SPE-DOM) in the coastal upwelling system off Central Peru and linked it to specific processes that affect DOM cycling. Seasonal sampling (April, August, and December) was carried out off Central Peru (12°S) during 2015, a low productivity year marked by EN conditions. The DOM molecular composition was obtained via Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Solid-phase extractable dissolved organic carbon (SPE-DOC) concentrations showed significant differences (p < 0.05) between the water masses present off central Peru. In order to explore if changes in SPE-DOC concentrations were the result of water mass mixing, we applied a conservative mixing (CM) model. The model revealed a non-conservative behavior of SPE-DOC and allowed us to identify two distinct groups of samples with increased and decreased SPE-DOC concentrations, respectively, and one group of samples inside the CM range. Differences in environmental parameters characterizing these groups were in accordance with respective processes associated to production and degradation of SPE-DOC. The trends observed for molecular parameters revealed the imprint of processes related to DOM production and DOM degradation, both biotic (microbial degradation) and abiotic (photodegradation). Our study suggests that even under low productivity conditions like EN, there is an active cycling of the DOM pool off central Peru.
Highlights
Dissolved organic matter (DOM) represents one of the largest reactive pools of organic carbon in the ocean (Carlson and Hansell, 2014)
In addition to the molecular formulae (MF) that significantly decreased in intensity (n = 315), we identified a total of 840 MF that were absent in the samples that lost Solid-phase extractable dissolved organic carbon (SPE-dissolved organic carbon (DOC)) but present in the conservative mixing (CM) group (Table 3)
We identified that 14% of the MF absent in the group that gained SPE-DOC but present in the CM group corresponded to aromatics
Summary
Dissolved organic matter (DOM) represents one of the largest reactive pools of organic carbon in the ocean (Carlson and Hansell, 2014) This pool of dissolved molecules plays a crucial role in biogeochemical cycles, since it constitutes the link between production and decay of organic matter in the oceanic water column. There are four major Eastern Boundary Upwelling Ecosystems characterized by high primary production (Fréon et al, 2009). These areas show active upwelling and are associated with the subtropical gyres of the Atlantic and the Pacific (Chavez and Messié, 2009). Large amounts of DOC are produced and processed in the coastal upwelling area off the north western part of the Iberian Peninsula (Álvarez-Salgado et al, 2001); coastal upwelling areas have been suggested to play an important role in the export of labile DOC to the adjacent ocean (Álvarez-Salgado et al, 2001; Lønborg et al, 2010)
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