Award: OCE-1260055

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 Submitted by: Michael R Landry