Publications

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2018
Stephens, BM, Porrachia M, Dovel S, Roadman M, Goericke R, Aluwihare LI.  2018.  Nonsinking Organic Matter Production in the California Current. Global Biogeochemical Cycles. 32:1386-1405.   10.1029/2018gb005930   AbstractWebsite

Productive eastern boundary upwelling systems such as the California Current Ecosystem (CCE) are important regions for supporting both local and remote food webs. Several studies have reported on the temporal and spatial variability of primary production and gravitational export in the CCE. However, few studies have quantified the partitioning of net primary and new production into other reservoirs of detrital organic matter. This study tested the hypothesis that nonsinking detrital reservoirs are an exportable reservoir of new production in the CCE with samples collected by the California Cooperative Oceanic Fisheries Investigation survey between 2008 and 2010. Water column gradients in nitrate (NO3-) and total organic carbon (TOC; which excludes sinking particulate organic carbon) were used to estimate potential rates of new production (P-New) and TOC production (P-TOC), respectively. The P-TOC:P-New varied between 0.16 and 0.56 and often increased with indicators of enhanced autotrophic production. At times, surface stratification was also correlated with elevated P-TOC:P-New. In the most productive, inshore region, P-TOC exceeded previously reported sinking export rates, which identified TOC as a quantitatively significant repository of exportable carbon in the CCE. However the sum of P-TOC and sinking export for these productive regions was less than both P-New and oxygen-based estimates of net community production. These results imply that nonsinking reservoirs alone are not sufficient to explain observed imbalances between production and export for the most productive CCE regions. Plain Language Summary The ocean's biological pump is typically quantified as the organic carbon that quickly sinks, that is, is exported, out of the surface lighted zone to be subsequently sequestered in the deep ocean. Recent studies have shown that other forms of organic matter produced by phytoplankton can also contribute to carbon export. In this study, we quantified how much new production and net primary production was channeled into nonsinking reservoirs such as dissolved organic carbon and suspended particulate organic carbon in the productive eastern boundary California Current Ecosystem. To match the data coverage provided by our organic carbon measurements we used satellite data to calculate net primary production and used measured depth profiles of nitrate together with model-derived upwelling velocities, to determine new production. We quantified the amount of nonsinking organic matter that accumulated in surface waters following production and found that the timescale of accumulation enabled this reservoir to participate in export. In some regions, as much carbon was present in the accumulated nonsinking reservoir as was quantified as sinking particulate carbon. We also found that lateral export from the productive coastal region was a potentially important pathway that could carry nutrients and carbon in organic matter to less productive waters.

2017
Stukel, MR, Aluwihare LI, Barbeau KA, Chekalyuk AM, Goericke R, Miller AJ, Ohman MD, Ruacho A, Song H, Stephens BM, Landry MR.  2017.  Mesoscale ocean fronts enhance carbon export due to gravitational sinking and subduction. Proceedings of the National Academy of Sciences of the United States of America. 114:1252-1257.   10.1073/pnas.1609435114   AbstractWebsite

Enhanced vertical carbon transport (gravitational sinking and subduction) at mesoscale ocean fronts may explain the demonstrated imbalance of new production and sinking particle export in coastal upwelling ecosystems. Based on flux assessments from U-238:Th-234 disequilibrium and sediment traps, we found 2 to 3 times higher rates of gravitational particle export near a deep-water front (305 mg C.m(-2).d(-1)) compared with adjacent water or to mean (nonfrontal) regional conditions. Elevated particle flux at the front wasmechanistically linked to Fe-stressed diatoms and high-mesozooplankton fecal pellet production. Using a data assimilative regional ocean model fit to measured conditions, we estimate that an additional similar to 225 mg C.m(-2).d(-1) was exported as subduction of particle-rich water at the front, highlighting a transport mechanism that is not captured by sediment traps and is poorly quantified by most models and in situ measurements. Mesoscale fronts may be responsible for over a quarter of total organic carbon sequestration in the California Current and other coastal upwelling ecosystems.