Current Research - Claudia Benitez-Nelson
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 (sources, composition, and sinks) 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."
"The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny ...'"
Sediment composition and accumulation in and groundwater flow from the eastern coast of South Africa (current)
My laboratory has become involved in an international collaboration with Dr. Marc Humphries to examine sediment accumulation and geochemistry along the eastern coast of South Africa. One of the benefits of wetlands is their ability to act as sinks for sediment and various chemicals. At the same time, these regions can also be a source of material to the coastal ocean via submarine groundwater discharge (SGD). While wetland floodplains wetlands are common features of rivers in southern Africa, they have not been well studied from a geomorphological perspective. Indeed, the geology of the ecosystem is quite complex. At the same time, while there have been a number of studies that focus on the role of SGD in estuarine and coastal biogeochemistry, only a few of these studies have occurred in the southern hemisphere, and none in southern Africa. Stay tuned as we continue to seek funding and collect initial samples ....
In the meantime, check out our latest collaborations:
Humphries, M., A. and C. R Benitez-Nelson (2013). Recent trends in sediment and nutrient accumulation rates in coastal, freshwater Lake Sibaya, South Africa, Marine and Freshwater Research, http://dx.doi.org/10.1071/MF12313
Humphries, M., A. Kindness, W. N Ellery, J. C. Hughes, and C. R Benitez-Nelson (2010). Accumulation rate of clastic and chemical sedimentation and their role in the long-term development of the Mkuze River floodplain, South Africa, Geomorphology, 119, 88–96.
A photo of our study area makes the cover! Photo by Marc Humphries
Marine Phosphorus Cycle
Phosphorus (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 funded by several National Science Foundation projects to study 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, distribution, and composition of dissolved and particulate inorganic and organic P within oxic, suboxic, and anoxic anoxic waters over seasonal, annual, and interannual timescales using ED/RO, culture experiments, 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 work in Science Magazine, May 2, 2008: Marine Polyphosphate: A Key Player in Geologic Phosphorus Sequestration and in Nature Geoscience, September 20, 2009, A microbial source of phosphonates in oligotrophic marine systems
Argonne National Laboratory
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.
Individual colonies of single-celled phytoplankton Trichodesmium
shown here, are visible to the naked eye; where currents and winds gather many colonies together, the aggregation can be seen from orbiting satellites.
(Photo courtesy of MIT/WHOI Joint Program student Abby Heithoff)
Particle Export and Remineralization
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. More recently, my laboratory was 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 aboutSharmila'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.
Read about some of our preliminary results published in Science Magazine: Arrigo, KR, and 31 authors (including C.R.Benitez-Nelson and Sharmila Pal) (2012) Massive phytoplankton blooms under Arctic sea ice. Science, DOI: 10.1126/science.1215065
Persistence and Fate of the Harmful Algal Toxin, Domoic Acid, in Marine Systems.
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. This project is now complete. To see published papers resulting from this work, click HERE.
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!
Learn more about Eddies and E-Flux at our Science Corner.
Mesoscale Eddies Drive Increased Silica Export in the Subtropical Pacific Ocean Published in Science Magazine May 18, 2007