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.

"Every great advance in science has issued from a new audacity of imagination."

-John Dewey

"The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny ...'"

-Isaac Asimov

Contents:

• E:Flux: Eddy Influenced Primary Production and Export
• Upper Ocean Cycling of Phosphorus
• Particle Export and Remineralization

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!

E-Flux Project Website:  For more information on the program, principal investigators, meetings, and data exchange.

Mesoscale Eddies Drive Increased Silica Export in the Subtropical Pacific Ocean  Published in Science Magazine May 18, 2007

Satellite Images:  March 2005 Satellite imagery of (left) sea surface temperature depicting the cold (blue) central waters of Cyclone Opal (to the south of Maui) that are also (right) enriched in chlorophyll a (light blue).  GOES Image is courtesy of Lucas Moxey from NOAA OceanWatch – Central Pacific (EddyWatch Hawaii).  MODIS image is courtesy of Francesco Nencioli at UC Santa Barbara.
A collage of microscopic images depicting the abundant life found within Cyclone Opal.  Images courtesy of Dr. Susan Brown at the University of Hawaii.

          Press Releases:

        USC Press Release: A Forest Blooms in the Marine Desert:  Implications for mitigating climate change and

               on the USC Research page and USC Media relations website.

        This week in Science: Spinning up new production

        New Scientist Environment:  Southern Ocean already losing ability to absorb CO₂ (see the bottom para)

        Science Now:  Don't bet on the Bloomin' Plankton

        Environmental Research Web: Doubt cast on ocean fertilization technique

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.

The Marine P Cycle (From Benitez-Nelson, 2000)

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, 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.

Read about some of our recent work just published in Science Magazine, May 2, 2008: Marine Polyphosphate: A Key Player in Geologic Phosphorus Sequestration

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.

Schematic of how the ²³⁴Th technique works.

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.. 

References (not routinely updated)

Benitez-Nelson, C.R. (2000) The Biogeochemical Cycling of Phosphorus in Marine Systems. Earth Sci. Rev., 51, 109-135.

Benitez-Nelson, C.R., K.O. Buesseler, M. Rutgers van der Loeff, J.E. Andrews, L. Ball, G. Crossin, and M. A. Charette (2000) Testing a new small volume technique for determining ²³⁴Th in seawater. J. Radioanal. and Nuc. Chem., 248, 795-799.

Benitez-Nelson, C.R., K.O. Buesseler, D.M. Karl, J.E. Andrews (2001) A time-series study of particulate matter export in the North Pacific Subtropical Gyre based on ²³⁴Th:²³⁸U disequilibrium.  Deep-Sea Res. I, 48, 2595-2611.

Buesseler, K.O. (1998) The decoupling of production and particle export in the surface ocean. Glob. Biogeochem. Cycles 12, 297-310.

Buesseler, K. O., Bacon, M. P. Cochran, J. K. and Livingston, H. D. (1992)  Carbon and nitrogen export during the JGOFS Bloom Experiment estimated from ²³⁴Th-²³⁸U disequilibria. Deep-Sea Res.I, 39, 1115-1137.

Buesseler, K. O., Andrews, J. A., Hartman, M. C., Belastock, R. and Chai, F. (1995) Regional estimates of the export flux of particulate organic carbon derived from thorium-234 during the JGOFS EQPAC program. Deep-Sea Research II, 42(2-3), 777-804.

Buesseler, K. O., Ball, L., Andrews, J. A., Benitez-Nelson, C. R., Belastock, R., Chai, F. and Chao, Y. (1998) Upper ocean export of particulate organic carbon in the Arabian Sea derived from Thorium-234. Deep-Sea Res. II, Special Arabian Sea Issue, 45, 2461-2488.

Chadwick, O. A., L. A. Derry, P. M. Vitousek, B. J. Huebert, and L. O. Hedin (1999) Changing sources of nutrients during four million years of ecosystem development. Nature, 397, 491-497.

Emerson, S., Quay, ., Karl, D., Winn, C., Tupas, L., ans Landry, M. (1997) Experimental determination of the organic carbon flux from open-ocean surface waters. Nature, 389, 951-954.

Graham, W. F. and R. A. Duce (1982) The atmospheric transport of phosphorus to the Western North Atlantic. Atmospheric Environment, 16, 1089-1097.

Heath, J. A. and B. Huebert (1999) Cloudwater deposition as a source of fixed nitrogen in a Hawaiian montane forest. Biogeochemistry, 44, 119-134.

Huebert, B., P. Vitousek, J. Sutton, T. Elias, J. Heath, S. Coeppicus, S. Howell, and B. B. Blomquist (1999) Volcano fixes nitrogen into plant-available forms. Biogeochemistry, in press.

Karl, D. M., Letelier, R., Hebel, D., Tupas, L., Dore, J., Christian, J., Winn, C. (1995). Ecosystem changes in the North Pacific subtropical gyre attributed to the 1991-1992 El Nino. Nature, 373, 230-234.

Karl, D. M., Lukas, R. (1996) The Hawaii Ocean Time-series (HOT) program:  Background, rationale and field implementation. Deep-sea Res. II, 43, 129-156.

Karl, D. M. and Tien, G. (1997) Temporal variability in dissolved phosphorus concentrations at station ALOHA (22 45’N, 158W). Marine Chemistry, 56, 77-97.

Karl, D. M., Letelier, R., Tupas, L., Christian, J., Hebel, D. (1997)  The role of nitrogen fixation in biogeochemical cycling in the subtropical North Pacific ocean. Nature, 388, 533-538.

Letelier, R. M., and Karl, D. M. (1996). Role of Trichodesmium spp. in the productivity of the subtropical North Pacific Ocean. Mar. Ecol. Prog. Ser., 133, 263-273.

Redfield, G. W. (1999) Quantifying atmospheric deposition of phosphorus: Concepts, constraints, and published rates. Revision of Tech. Pubs. WRE #360. South Florida Water Management District, Wet Palm Beach, FL, 35 p.