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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.

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Landry, MR, Ohman MD, Goericke R, Stukel MR, Barbeau KA, Bundy R, Kahru M.  2012.  Pelagic community responses to a deep-water front in the California Current Ecosystem: overview of the A-Front Study. Journal of Plankton Research. 34:739-748.   10.1093/plankt/fbs025   AbstractWebsite

In October 2008, we investigated pelagic community composition and biomass, from bacteria to fish, across a sharp frontal gradient overlying deep waters south of Point Conception, California. This northsouth gradient, which we called A-Front, was formed by the eastward flow of the California Current and separated cooler mesotrophic waters of coastal upwelling origin to the north, from warm oligotrophic waters of likely mixed subarcticsubtropical origin to the south. Plankton biomass and phytoplankton growth rates were two to three times greater on the northern side, and primary production rates were elevated 5-fold to the north. Compared with either of the adjacent waters, the frontal interface was strongly enriched and uniquely defined by a subsurface bloom of large diatoms, elevated concentrations of suspension-feeding zooplankton, high bioacoustical estimates of pelagic fish and enhanced bacterial production and phytoplankton biomass and photosynthetic potential. Such habitats, though small in areal extent, may contribute disproportionately and importantly to regional productivity, nutrient cycling, carbon fluxes and trophic ecology. As a general introduction to the A-Front study, we provide an overview of its design and implementation, a brief summary of major findings and a discussion of potential mechanisms of plankton enrichment at the front.

Hopkinson, BM, Seegers B, Hatta M, Measures CI, Mitchell BG, Barbeau KA.  2013.  Planktonic C:Fe ratios and carrying capacity in the southern Drake Passage. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 90:102-111.   10.1016/j.dsr2.2012.09.001   AbstractWebsite

The carbon to iron (C:Fe) ratio of planktonic biomass constrains net production in iron-limited regions of the ocean and is an important parameter for predicting biomass production from iron inputs. On a cruise to the southern Drake Passage in July-August 2006, we used two approaches to determine the C:Fe ratio of planktonic material: dual-radiotracer labeling and net biomass production in iron-limited grow-out experiments. There was variability in C:Fe ratios among experiments, but values from the two methods overlapped with average values of 1.4 x 10(5) (mol:mol) for the radiotracer method and 1.7 x 10(5) for the net biomass production method. This is notable since the net biomass production method is a new approach to determine C:Fe ratios. Although it has potential issues related to bottle effects and sensitivity to trace contamination, the method avoids some of the questions associated with iron speciation and bioavailability since ambient iron supports production. Because light intensity is known to affect C:Fe ratios in phytoplankton through photosynthetic iron demands, we tested the effect of light level on C:Fe in Antarctic assemblages. In contrast to what is seen in many phytoplankton cultures, C:Fe ratios increased at low-light, but we suspect that this is due to initial photoinhibition of the low-light adapted winter assemblages at higher light levels. (c) 2012 Elsevier Ltd. All rights reserved.