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Samo, TJ, Pedler BE, Ball GI, Pasulka AL, Taylor AG, Aluwihare LI, Azam F, Goericke R, Landry MR.  2012.  Microbial distribution and activity across a water mass frontal zone in the California Current Ecosystem. Journal of Plankton Research. 34:802-814.   10.1093/plankt/fbs048   AbstractWebsite

Ocean fronts with accumulated biomass and organic matter may be significant sites of enhanced microbial activity. We sampled a frontal region (the A-Front) separating oligotrophic and mesotrophic water masses within the California Current Ecosystem (CCE) to assess the influence of frontal hydrography on several microbial parameters. Samples for heterotrophic bacterial, viral and flagellate abundance, dissolved and particulate carbon and nitrogen, transparent particles and bacterial carbon production were collected at 6 depths from the surface to 100 m with 59 conductivity/temperature/depth casts along a 26-km northerly transect across the front. Relative to adjacent oligotrophic and mesotrophic waters, the frontal transition displayed peaks in the mean estimates of cell-specific bacterial carbon and bulk bacterial production, particulate organic carbon and particulate organic nitrogen concentrations, and the abundance and size of transparent particles. Bacterial carbon production increased approximate to 5-fold northward from oligotrophic waters to the frontal zone, in agreement with an increase in the frequency of dividing cells, but bacterial abundance was lower than at adjacent stations. This may be partially explained by high chlorophyll, elevated virus:bacteria ratios and low nanoflagellate grazer abundance at the front. Our data suggest that CCE fronts can facilitate intense biological transformation and physical transport of organic matter, in sharp contrast to adjacent low productivity waters, and harbor dynamic microbial populations that influence nutrient cycling.

Sherwood, BP, Shaffer EA, Reyes K, Longnecker K, Aluwihare LI, Azam F.  2015.  Metabolic characterization of a model heterotrophic bacterium capable of significant chemical alteration of marine dissolved organic matter. Marine Chemistry. 177:357-365.   10.1016/j.marchem.2015.06.027   AbstractWebsite

The marine bacterium Alteromonas sp. AltSIO was previously found to consume an equivalent magnitude of surface coastal marine dissolved organic carbon (DOC) as diverse bacterial assemblages (Pedler et al., 2014). In this study, we sought to investigate the potential of AltSIO to alter the chemical composition of marine DOC by characterizing its capacity to metabolize a broad suite of environmentally relevant model substrates. Results showed that AltSIO had a particularly broad capacity to degrade carbohydrates relative to other marine bacteria characterized as generalist heterotrophs. Growth in seawater incubations amended with model neutral sugars and radiolabeled substrates showed that AltSIO preferentially utilized la-galactose and disaccharides, but showed little to no biomass incorporation or respiration of D-glucose. Lastly, analysis of ambient dissolved organic matter (DOM) from time-course mesocosms by ultrahigh resolution mass spectrometry showed that both AltSIO grown in pure culture and a mixed bacterial community significantly altered ambient DOM, yet the alteration appeared uniform across chemical classes. for both treatments. This study provides insight into the physiological mechanisms of a globally distributed generalist bacterial taxon that has the capacity to significantly alter the geochemistry of marine DOM. (C) 2015 Elsevier B.V. All rights reserved.

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.

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.

Stukel, MR, Kelly TB, Aluwihare LI, Barbeau KA, Goericke R, Krause JW, Landry MR, Ohman MD.  2019.  The Carbon:(234)Thorium ratios of sinking particles in the California current ecosystem 1: relationships with plankton ecosystem dynamics. Marine Chemistry. 212:1-15.   10.1016/j.marchem.2019.01.003   AbstractWebsite

We investigated variability in the C:Th-234 ratio of sinking particles and its relationship to changing water column characteristics and plankton ecological dynamics during 29 Lagrangian experiments conducted on six cruises of the California Current Ecosystem Long-Term Ecological Research (CCE-LTER) Program. C:Th-234 ratios of sinking particles collected by a surface-tethered sediment trap ((CThST)-Th-:234) varied from 2.3 to 20.5 mu mol C dpm(-1) over a depth range of 47-150 m. C:Th-234(ST) was significantly greater (by a factor of 1.8) than C:Th-234 ratios of suspended > 51-mu m particles collected in the same water parcels with in situ pumps. C:Th-234 ratios of large (> 200-mu m) sinking particles also exceeded those of smaller sinking particles. C:Th-234(ST) decreased with depth from the base of the euphotic zone through the upper twilight zone. C:Th-234(ST) was positively correlated with several indices of ecosystem productivity including particulate organic carbon (POC) and chlorophyll (Chl) concentrations, mesozooplankton biomass, and the fraction of Chl > 20-mu m. Principal component analysis and multiple linear regression suggested that decaying phytoplankton blooms exhibited higher C:Th-234(ST) than actively growing blooms at similar biomass levels. C:Th-234(ST) was positively correlated with indices of the fractional contribution of fecal pellets in sediment traps when the proportion of fecal pellets was low in the traps, likely because of a correlation between mesozooplankton biomass and other indices of ecosystem productivity. However, when fecal pellets were a more important component of sinking material, C:Th-234(ST) decreased with increasing fecal pellet content. C:Th-234(ST) was also positively correlated with the Si:C ratio of sinking particles. Across the dataset (and across depths) a strong correlation was found between C:Th-234(ST) and the ratio of vertically-integrated POC to vertically-integrated total water column Th-234 (C-v:Th-234(tot)). A mechanistic one-layer, two-box model of thorium sorption and desorption was invoked to explain this correlation. Two empirical models (one using C-v:Th-234(tot); one using depth and vertically-integrated Chl) were developed to predict C:Th-234 ratios in this coastal upwelling biome. The former regression (log(10)(C:Th-234(ST)) = 0.43 x log(10)(C-v:Th-234(tot)) + 0.53) was found to also be a reasonable predictor for C:Th-234(ST) from diverse regions including the Southern Ocean, Sargasso Sea, Subarctic North Pacific, and Eastern Tropical North Pacific.