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Mcdonnell and Buesseler a New Method for Estimation of Sinking

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Mcdonnell and Buesseler a New Method for Estimation of Sinking LIMNOLOGY and Limnol. Oceanogr.: Methods 10, 2012, 329–346 OCEANOGRAPHY: METHODS © 2012, by the American Society of Limnology and Oceanography, Inc. A new method for the estimation of sinking particle fluxes from measurements of the particle size distribution, average sinking velocity, and carbon content Andrew M. P. McDonnell 1,2* and Ken O. Buesseler 1 1Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA, USA 02543 2Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Universitätstrasse 16, 8092 Zürich, Switzerland Abstract We describe a new method for estimating sinking particulate carbon fluxes at high spatial and temporal res - olutions from measurements of the particle concentration size distribution taken with an in situ camera system, in this case an autonomous video plankton recorder (VPR). Paired measurements of polyacrylamide gel traps and the VPR result in depth- and size-resolved parameterizations of the average sinking velocity, which enable the estimation of the flux size distribution from the concentration size distribution. Comparisons between the gel traps and the bulk carbon flux allows for the parameterization of the particle carbon content as a function of size. Together, these parameterizations permit the estimation of carbon fluxes from high-resolution VPR sur - veys. This method enables greater spatial, vertical, and temporal resolution of flux measurements beyond what is possible with conventional sediment traps. We tested this method in the Sargasso Sea and found that it was capable of accurately reproducing the fluxes measured in sediment traps while offering substantial improve - ment in the accuracy of the estimated fluxes compared to previous global and regional parameterizations. Our results point to the importance of local calibrations of the average sinking velocity and particle carbon content when estimating carbon fluxes from measurement of the concentration size distribution. This method holds important oceanographic potential for elucidating regional or basin scale carbon flows and providing new mechanistic insights into the function of the biological pump. The persistent rain of marine particles sinking from the impact the distributions of carbon and other biogeochemi - euphotic zone to greater depths is a dominant component of cally important elements throughout the oceans (Yamanaka the ocean’s biological pump. The magnitude of particle fluxes and Tajika 1996; Howard et al. 2006; Matsumoto 2007; Mari - at the base of the euphotic zone as well as the scaling of these nov et al. 2008). This process also plays a significant role in fluxes with respect to depth are important parameters that controlling the concentration of CO 2 in the atmosphere (Sarmiento and Toggweiler 1984; Kwon et al. 2009). Despite its importance, our present understanding of the *Corresponding author: E-mail: [email protected] biological pump is limited. This is largely due to the experi - Acknowledgments mental and logistical challenges of quantifying particle fluxes We are grateful to the captains, crews, and scientific personnel of the in such a dynamic and inaccessible ecosystem. For the past R/V Atlantic Explorer for their support during our cruises. Scott Gallager, several decades, various forms of conventional sediment traps Mick Follows, Phoebe Lam, Carl Lamborg, and Phil Boyd provided com - have been deployed in subsurface waters, enabling the direct ments and suggestions on early versions of the manuscript while James collection of sinking particulate material (Knauer and Asper Valdes made possible much of the underwater instrumentation associat - ed with the drifting sediment trap array. Thanks to Stephanie Owens, 1989). Studies employing these techniques have revealed a Steven Pike, and Kelsey Collasius for their sampling efforts during the wide dynamic range of flux magnitudes and attenuation cruises. We thank Associate Editor Benitez-Nelson and three anonymous length scales that exists throughout the oceans (Berelson reviewers for their constructive reviews that greatly improved the quality 2001; Lutz et al. 2002; Buesseler et al. 2007b). Given that only of this manuscript. This project was made possible through funding modest changes to the global average flux attenuation length from the National Science Foundation Carbon and Water Program scale can have significant effects on atmospheric CO concen - (06028416), the Woods Hole Oceanographic Institution Academic 2 Programs Office, ETH Zürich, and the Scurlock Bermuda Biological trations (Kwon et al. 2009), it is imperative that we aggres - Station for Research Fund. sively quantify sinking particle fluxes in time and space to bet - DOI 10.4319/lom.2012.10.329 ter understand their role in earth’s biogeochemical dynamics 329 McDonnell and Buesseler Estimation of sinking particle fluxes and climate. However, in addition to being difficult and time- Although few measurements of sinking velocity exist for consuming to deploy, sediment traps also tend to yield only a the open ocean, it is clear that sinking velocities are highly few data points, making them poorly suited for widespread variable and can range over several orders of magnitude, oceanographic application and incapable of capturing the full depending on location, depth, time, or particle type (Turner range of spatial and temporal variations in particle fluxes that 2002; Stemmann et al. 2004b; Armstrong et al. 2009; McDon - exists throughout the oceans. Therefore new strategies, tech - nell and Buesseler 2010). This variability is significant even nologies, sampling methods, and data analysis techniques are within regional and seasonal scales, suggesting that a single required to gain further insight into these dynamic processes. global relationship is likely not appropriate for the accurate Fortunately, the oceanographic toolbox for studying parti - estimation of flux from cn at any particular site or time cle fluxes is expanding. Measurements of the particle reactive (McDonnell and Buesseler 2010; Jouandet et al. 2011). In fact, tracer 234 Th and recent advancements in its sampling methods several studies that employed the Guidi et al. (2008) global and data interpretation are enabling higher resolution esti - parameterization of m·wavg found that estimated carbon fluxes mates of export fluxes and even provide some insight into par - were up to a factor of ten different from those measured by ticle remineralization immediately below the euphotic zone sediment traps at their sites (Iversen et al. 2010; Jouandet et al. (Benitez-Nelson et al. 2001; Buesseler et al. 2009; Maiti et al. 2011). Additionally, unless all particle imaging devices used in 2010). Autonomous floats have also become a capable plat - such studies are meticulously intercalibrated to give equiva - form for remote observations of carbon biomass and export lent results for cn (Jackson et al. 1997; Picheral et al. 2010), the (Bishop et al. 2004; Bishop 2009; Bishop and Wood 2009). use of a single global parameterization for the calculation of With the development of improved digital photography, particle flux will lead to biases that arise from inter-instrument image processing capabilities, and oceanographic instrument variation in particle detection and differences in the image design, in situ imaging of particles is emerging as a new and analysis algorithms used to calculate cn. important tool in the study of particle concentrations and size To improve the local accuracy of the approach developed distributions in the water column (Stemmann et al. 2004b; by Guidi et al. (2008), Iversen et al. (2010) applied the same McDonnell and Buesseler 2010; Picheral et al. 2010). These minimization procedure using a small set of carbon flux and instruments are capable of quickly and easily making observa - cn data pairs from Cape Blanc. Their study resulted in a differ - tions of marine particles at high spatial and temporal resolu - ent set of best-fit parameters, A and b, from those determined tions, providing new opportunities for methodological devel - from the global dataset of Guidi et al. (2008). This regional opments and observation-based insights into marine particle approach greatly improved the accuracy of flux estimates at dynamics and the ocean’s biological pump. this site. However, even in this regional context, the method In an important and novel study, Guidi et al. (2008) devel - still requires a single parameterization of m·wavg to be applied oped a global relationship between the particle concentration over the entire range of locations, depths, or times used in the size distribution ( cn) and measurements of bulk fluxes obtained optimization procedure. In this manner, these methods have with sediment traps. They described the total carbon flux ( FC, no mechanism to deal with variability of sinking velocity mmol C m –2 d–1 ) as an integral over all particle diameters, d: within the study area and also impose a single power law model for the product between m and wavg . Fcdmdwdd() () ()d (1) Many types of direct measurements of particle sinking C = ∫ navg velocities are not appropriate for converting particle concen - –3 –1 where cn (No. m µm ) is the number concentration of parti - trations into fluxes because they often consider only the sink - cles in a given small size range d d, m (mmol C) is the particle ing fraction of particulate matter in the water column, and –1 carbon content, and wavg (m d ) is the average sinking veloc - have no way to account
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