Research Interests

I have participated in a multitude of research avenues ranging from stratospheric/tropospheric exchange to sediment accumulation rates in the Black Sea. One of the leading questions that I am currently seeking to answer is "What are the processes that dominate natural and/or anthropogenically induced climate change." My present research focuses on understanding the biogeochemical cycling of phosphorus (P) in the ocean and what controls particle formation and organic carbon (C) fluxes from the surface ocean to the deep floor. Please feel free to email me with questions and/or comments.

Contents:

The Role of Eddies

My laboratory just completed a large scale study, E-Flux, on the role of eddies in the biogeochemistry of the oceans. Several studies have suggested that mesoscale (100+ km) eddies play a major role in supplying new nutrients to the surface ocean, thereby enhancing biological production and carbon export in otherwise nutrient-deficient systems. Eddies are ubiquitous features throughout the oceans, with observations in the Gulf of Alaska (Crawford and Whitney 1999), the North Pacific Subtropical Gyre (Falkowski et al. 1991, Letelier et al. 2000), the Gulf Stream region (Warm-core Rings Program; e.g., Smith & Baker 1983), the Sargasso Sea (McGillicuddy & Robinson 1997, McGillicuddy et al. 1998, McNeil et al. 1999, Siegel et al. 1999, Conte et al. 2001, Dickey et al. 2001), the North Atlantic (e.g. NABE and PRIME; Robinson et al. 1993, Savidge & Williams 2001), the California (Simpson et al. 1984) and East Australia Currents (Nilsson & Cresswell 1981), and the Arabian Sea (Dickey et al. 1998, Honjo et al. 1999, Fischer et al. 2002). However, most studies of eddies have utilized models (with or without satellite data) with limited in situ data sets or have involved unplanned observations from cruises or moored instrumentation. Few have directly targeted plankton community structure or export. As a result, the biogeochemical significance of eddies has remained enigmatic and controversial. Current estimates suggest that 10 to 50% of global new primary production is due to eddy-induced nutrient fluxes. This wide range reflects the paucity of direct field observations of the biological and biogeochemical impacts of eddies, along with difficulties in putting the scattered existing observations into a broader context.

In our study, we sampled eddies that formed in the lee of the Hawaiian islands focusing predominantly on their biogeochemical aspects. Why Hawaii? Off of Hawaii, cyclonic eddies are formed about once per month during the winter. These eddies were typically visible with satellite imagery and have lifetimes of 3-8 months!

The Phosphorus Cycle

P is an essential nutrient utilized by all living organisms. Yet, we have very little understanding of the sources and sinks of this essential nutrient in the marine realm. We have even less information regarding the actual cycling of P by marine biota. Why is this important? The predominant view of oceanic P is that this nutrient only limits primary production over geologically long timescales in marine systems (>1000 years). On shorter timescales, it is believed that primary production is limited by nitrogen (N). However, recent research suggests that N₂-fixing organisms (organisms that can convert atmospheric N into more bioavailable forms) are becoming more numerous, possibly as a result of a shift in climate. Since these organisms can obtain N from the atmosphere, their growth may be limited by other elements, such as iron (Fe) and P. Regardless, understanding how nutrients, such as N and P, control the primary production of organisms in the upper ocean is essential if we are to elucidate the oceanic role in anthropogenic CO₂ uptake and hence, global climate change.

Particulate P removal via the sinking of biologically produced marine organic matter is a major removal mechanism of P from the upper ocean (e.g. see reviews by Benitez-Nelson, 2000; Delaney, 1998). A number of studies have demonstrated that sinking particles have rapid P turnover rates, indicating that at least a fraction of this material is comprised of compounds that are bioavailable (Benitez-Nelson and Buesseler, 1999; Benitez-Nelson and Karl, 2002; Waser et al., 1994). Thus, remineralization of particulate P occurs rapidly and is an important process for the regeneration of inorganic and organic P compounds to the dissolved phase. Furthermore, the particulate P component that remains and reaches the seafloor will play a subsequent role in benthic community production and in global productivity over geologic time. Several studies have suggested that alternating periods of oxic and anoxic (no O₂) conditions have greatly impacted the efficiency of P recycling during the geologic past, and hence the availability of nutrients for plankton growth (Ingall, 1996; Van Capellen and Ingall, 1994; Van Capellen and Ingall, 1996). Despite the significance of particulate P remineralization, there have been few studies that have examined sinking particles for P composition or concentration (Loh and Bauer, 2000; Payton et al., 2003), and virtually nothing is known about water column organic P cycling in anoxic environments.

My laboratory is currently studying the composition of P within dissolved and particulate phases in a range of environments, such as the Cariaco Basin, San Pedro Basin, Santa Barbara Basin, Guaymas Basin, Effingham Inlet, the Sargasso Sea and Lake Superior. Our current goals are to:

1) Characterize the magnitude and distribution of particulate inorganic and organic P within oxic, suboxic, and anoxic anoxic waters over seasonal, annual, and interannual timescales using continuously moored sediment traps and in situ large volume filtration pumps.

2) Elucidate the chemical composition of these particles and the dissolved phase of P using sequential chemical extraction techniques and solid state and liquid state ³¹P NMR.

3) Link variations in particulate P speciation to changes in oxygen content, overlying production, and other biological parameters such as C, N, opal, and silica export, as well as to the chemical composition of P in underlying sediments.

Polyphosphate in natural diatoms. Cells collected from the coastal waters of Effingham Inlet, British Columbia, were fixed and stained with DAPI. In these samples, DAPI not only revealed cell nuclei (blue), but many large intracellular polyphosphate inclusions (yellow to green), as seen in (A) Skeletonema spp. and (B) a solitary centric diatom. This image shows that natural, non-cultured plankton synthesize polyphosphate at nonenriched, sub-micromolar dissolved phosphate concentrations that are typical of many regions in the global ocean. Scale bars are 10 um. Pictures courtesy of Julia Diaz from GaTech.

In general, organic carbon export (and hence CO₂ sequestration) is dominated by spatially and temporally discrete events. Thus, short-term sampling may often “miss” an important export event. Analysis of the seasonal pattern in particulate ²³⁴Th activity and its ratio to particulate carbon (PC), particulate phosphorus (PP), and particulate nitrogen (PN) pools allows for the in situ determination of the export fluxes of these nutrients (Buesseler et al., 1992a; Buesseler et al., 1995, 1998; Buesseler, 1998). ²³⁴Th is a naturally occurring particle-reactive radionuclide which has been commonly used to study particle scavenging in the upper ocean (e.g. Buesseler, 1998 and references therein). Since the half-life of ²³⁴Th is 24.1 days, the disequilibrium between its soluble parent ²³⁸U and the measured ²³⁴Th activity reflects the net rate of particle export from the upper ocean on time scales of days to weeks. In the upper ocean, both the formation of fresh particle surfaces (proportional to primary production) and the packaging of particles into sinking aggregates (export or new production) are reflected in the observed ²³⁴Th distribution.

I am currently using ²³⁴Th as part of several National Science Foundation funded programs to understand the temporal variability of C export in the ocean. In 2006, I organized an international conference to study ²³⁴Th and it's role application in aquatic systems. Results were published in the August 2006 volume of Marine Chemistry. To find out more information about the conference and the papers produced, please visit the Fate Conference Home Page. My laboratory is currently involved in a large scale project to study export and remineralization processes in the Gulf of California using both ²³⁴Th:²³⁸U disequilibria in combination with ²¹⁰Po:²¹⁰Pb, another radioactive tracer pair with similar scavenging properties, but has direct uptake by biota as well. We are also involved in the NASA funded ICESCAPE Program to examine linkages between changes in sea ice cover with biological production, community structure and export. Read an article about Sharmila's 37-day cruise in the Arctic onboard the coast guard vessel: the Healy, where she collected samples for ²³⁴Th:²³⁸U disequilibria and Phosphorus dynamics.

Toxic blooms of a variety of algal species (harmful algal blooms (HABs)) have been documented throughout the world’s coastal oceans, ultimately impacting shellfish, finfish, marine mammals and birds over large areas. Several species within the genus of Pseudo-nitzschia, a group of marine diatoms that produce the neurotoxin domoic acid (DA), have been identified as common members of algal assemblages along the coast of California. Key questions in HAB research include not only what causes toxic Pseudo-nitzschia spp. to bloom, but what happens to that bloom after its demise. Causative factors ranging from coastal eutrophication to increased upwelling to resuspension of seed populations from sediments have all been hypothesized, but remain enigmatic, mainly due to a paucity of integrative data. The fate of DA producing Pseudo-nitzschia blooms remains even more elusive, and yet there is increasing evidence of substantial DA concentrations in benthopelagic feeders and benthic organisms both along the coast and offshore. Our current research seeks to build upon intriguing and exciting preliminary data that suggests that sinking particles are a major vertical transport mechanism of DA from surface waters to sediments, with DA fluxes exceeding 50,000 ng DA m^-2 d^-1 at depths in excess of 500 m. We therefore hypothesize that DA is rapidly transported to sediments and likely persists on timescales of a few weeks to several months, well after the demise of a Pseudo-nitzschia bloom. The goal of our funded National Science Foundation program is to create a regional observation and modeling program focused on the Santa Barbara Basin (SBB) that specifically: 1) Examines the temporal relationship between Pseudo-nitzschia blooms, DA toxicity, and the vertical transport efficiency of Pseudo-nitzschia and DA to the benthos, 2) Investigates the incorporation of DA into the sediments, and 3) establishes a historical record of DA toxicity spanning the past decade, and potentially, the last century. To achieve this goal, we are combining monthly survey cruises of water column biogeochemistry (in conjunction with the Plumes and Blooms program), two continuously moored sediment traps (150 and 550 m; USC Marine Sediments Laboratory), regional surface and down core sediments records (in conjunction with the Southern California Coastal Water Research Project Bight 2008 campaign), and satellite imagery. Ultimately, we hope to develop a numerical model that examines and reproduces the surface timing of toxic Pseudo-nitzschia blooms and the vertical export of DA to the seafloor. This transformative research will fundamentally change current views on the persistence of DA toxicity in marine systems and will provide the groundwork for similar measurements of other harmful toxins.

A satellite image of a Pseudo-nitzschia bloom that occurred in October 2005. Areas of high chlorophyll fluorescence (yellow to red) dominate the entire Santa Barbara Basin and correspond to high domoic fluxes observed in the 540 m sediment trap. Image courtesy of Erik Fields with data processing using Feldman, G. C., C. R. McClain, Ocean Color Web, AQUA Reprocessing 1.1, NASA Goddard Space Flight Center. Eds. Kuring, N., Bailey, S. W. 28-Feb-2006. http://oceancolor.gsfc.nasa.gov/.	Eric Tappa, Dr. Bob Thunell, and Emily Sekula-Wood recover the Santa Barbara Basin sediment trap floats onboard the NOAA vessel, R/V Shearwater. Image courtesy of Captain Charlie Lara and Emily Sekula-Wood.	Santa Barbara Basin sediment trap samples immediately after mooring recovery. Image courtesy of Eric Tappa.
A scanning electron micrograph of Pseudo-nitzschia collected within the Santa Barbara Basin sediment traps in April 2004. Image courtesy of Dr. Steve Morton and Emily Sekula-Wood.	A blown up scanning electron micrograph of Pseudo-nitzschia australis isolated from a widespread toxic bloom that resulted in hundreds of bird and marine mammal deaths in 2003. SEM image courtesy of Drs. J. Weaver and C. Anderson.	A sea lion recovering from domoic acid poisoning after being treated by Dr. Lauren Palmer at the Marine Mammal Care Center at Fort McArthur, San Pedro, CA. Image courtesy of Dr. A. Schnetzer

Hear two interviews with Emily Sekula-Wood on Carolina minute. Interview1 and Interview2

CLAUDIA R. BENITEZ-NELSON

Research Interests

Contents:

The Role of Eddies

The Phosphorus Cycle