| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | February 11, 2013 |
| Latest Amendment Date: | February 11, 2013 |
| Award Number: | 1260055 |
| Award Instrument: | Standard Grant |
| Program Manager: |
David Garrison
OCE Division Of Ocean Sciences GEO Directorate for Geosciences |
| Start Date: | April 1, 2013 |
| End Date: | March 31, 2018 (Estimated) |
| Total Intended Award Amount: | $572,998.00 |
| Total Awarded Amount to Date: | $572,998.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
8622 DISCOVERY WAY # 116 LA JOLLA CA US 92093-1500 (858)534-1293 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
CA US 92093-0210 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | BIOLOGICAL OCEANOGRAPHY |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Intellectual merit: Energy dissipation and elemental cycling by protistan consumers in lower trophic levels of ocean food webs are of sufficient magnitude, based on global mean measures of the amount of primary production consumed, to strongly alter the efficiencies of material transfers to higher-level consumers and to export. We presently know very little about these microbial food web steps, how they vary regionally or temporally, or how they might be altered by climate change. Compound Specific Isotope Analysis of Amino Acids (CSIA-AA) offers an approach for advancing our understanding of microbial food web structure and trophic fluxes based on the trophic positions (TP) of mesozooplankton as temporal integrators of the fluxes from direct feeding on phytoplankton and indirect transfers via protistan microzooplankton. Preliminary laboratory experiments to test this idea have demonstrated that the standard application of the method, using labeled phenylalanine as the representative source AA for the primary producer baseline and labeled glutamic acid as the indicator AA for trophic enrichment, does not produce a measureable trophic-step signal for protistan grazers. However, the results have also shown that an alternative high-turnover AA, alanine, strongly enriches in protistan as well as metazoan consumers, and leads to substantially higher TP estimates of mesozooplankton in field-collected specimens than that based on labeled glutamic acid.
This research project will test the hypothesis that labeled alanine provides a quantifiable and consistent index of trophic enrichment for protistan steps in marine food webs. The research will involve three major elements. First, controlled laboratory experiments will be conducted with chemostat systems to compare 15N enrichments of alanine to other AAs for a representative suite of ciliate and flagellate grazers feeding on phytoplankton, and to evaluate the two-step enrichment from phytoplankton via a protistan grazer to a suspension-feeding copepod. Second, field-collected mesozooplankton from four distinct ecological regions of the Pacific Ocean will be analyzed by CSIA-AA to test the transfer of alanine enrichment through a metazoan trophic step (comparing suspension feeding species to primary carnivores) and to assess how the TP index differs with trophic structure over a broad range of ecological conditions. Last, CSIA-AA assessments of TP for size-structured zooplankton will be integrated into inverse models of nitrogen flows in the four regions (equatorial Pacific, subtropical North Pacific, California Current and Costa Rica Dome) as a major constraint for resolving and comparing fluxes through the microbial food web over the range of ecological conditions. A properly calibrated CSIA-AA assessment of mesozooplankton trophic position will provide a new and valuable approach for regional intercomparisons of lower-level food web structure, for assessing temporal and spatial trends in climate change, for ocean ecosystem model validation, and for better understanding of lower food-web energetic constraints on ocean fisheries.
Broader Impacts: This project will involve mentoring relationships at all levels (postdoctoral, graduate, undergraduate and K-12) with an emphasis on advancing the scientific interests and career paths of young underrepresented women in marine ecology and oceanography. Undergraduate researchers will be engaged in hands-on relevant research activities as part of the Scripps Undergraduate Research Fellowship (SURF) and CCE-LTER summer REU programs. At the K-12 level, the investigators will work with the Ocean Discovery Institute, which brings science and education to San Diego urban youth, and Expanding Your Horizons San Diego, a program that provides scientific activities to girls from underrepresented minorities. Results from the study will contribute to understanding the mechanisms structuring the base of the food web, its variability and resilience. The results will have important implications for models of global biogeochemical cycling and ocean fisheries.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
The trophic structure of ocean foods webs is important to understand because its variability is a major determinate of temporal and regional ecosystem differences in nutrient cycling, energy transfer efficiency to higher-level consumers (fisheries), and export of carbon to the deep sea. While trophic (predator-prey) relationships of larger organisms can be reconstructed from dietary analyses of their stomach contents, as is the general approach in fisheries science, this cannot be done at lower levels of the food web, where multiple pathways of energy flows exist within the diverse microscopic community of autotrophic, heterotrophic and mixotrophic bacteria and protists. Compound Specific Isotope Analysis of Amino Acids (CSIA-AA) has opened the possibility of a new and precise technique for determining the mean number of trophic steps in the microbial food web, based on the differences in ∂15N enrichment of “trophic” amino acids, which enrich greatly with each predator-prey transfer, relative to “source” amino acids (AA), which enrich little with each step. The major objectives of the present study were to evaluate whether the amino acid, alanine, can be used as a unique indicator for protistan steps in food webs and to demonstrate how isotope enrichment of field-collected mesozooplankton can be applied to assessing and comparing variability in lower-level trophic structure in marine systems. For the former, results of controlled laboratory experiments in two- and three-stage chemostat systems were consistent with our hypothesis that only alanine shows trophic enrichment for both protistan and metazoan consumers, while the commonly used trophic AA, glutamic acid, enriches significantly only for metazoan consumers. In field applications, we established: 1) that trophic position differences between alanine and glutamic acid similar to those observed in laboratory experiments are also evident in complex natural communities, 2) that there is, on average, about one intermediate trophic step for protistan consumers between phytoplankton and suspension-feeding mesozooplankton in oligotrophic waters of the subtropical Pacific, and 3) that the trophic positions of tropical tunas standard are significantly underestimated by commonly applied gut content and isotope approaches. These results have important implications for interpretations of trophic structure and energy requirements in current models of ocean fisheries, while also validating an analytical approach for acquiring data that will improve their performance in the future. We further used isotope constraints on nitrogen flows in models of the Costa Rica Dome ecosystem to demonstrate the importance of active export by mesozooplankton vertical migration and to elucidate the unique role of the large pyrosome, Pyrostrema spinosum, in grazing and particle export in the depth stratum below the surface mixed layer.
Project results were disseminated in 7 peer-reviewed journal articles, one Ph.D. dissertation, presentations at national and international meetings, data products at the project website (http://www.bco dmo.org/project/556514), two undergraduate courses, and public outreach activities at Palomar College Upward Bound Program and at a STEM workshop (Expanding Your Horizons) for ~400 6-10th grade girls. The project supported the professional development of a female, under-represented minority postdoc and the dissertation research of a female graduate student.
Last Modified: 07/15/2018
Modified by: Michael R Landry
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