Evidence increasingly demonstrates that selective removal of marine life can induce restructuring of marine food webs. Trophic structure is the central component of mass balance models, widely used tools to evaluate fisheries in an ecosystem context. Food web structure is commonly determined by stomach contents or by bulk tissue stable isotope analyses, both of which are limited in terms of resolution and versatility. The investigators will refine a tool, Amino Acid Compound-Specific Isotopic Analyses (AA-CSIA), which can be broadly applicable for quantifying the time-integrated trophic position (TP) of consumers. Differences in source and trophic nitrogen isotopic composition for specific amino acids will provide an unambiguous and integrated measure of fractional trophic TP across multiple phyla, regardless of an animal's physiological condition or of the biogeochemical cycling at the base of the food web. AA-CSIA will allow testing of the efficacy of trophic position estimates derived from ecosystem-based models and promote the evolution of these models into decision-support tools. This project has three goals: 1. To validate the application of AA-CSIA across multiple marine phyla under differing physiological conditions. 2. To compare the application of AA-CSIA across systems with contrasting biogeochemical cycling regimes. 3. To develop the use of AA-CSIA TP estimates for validating trophic models of exploited ecosystems. The investigators will test and refine the approach using a combination of laboratory feeding experiments and field studies across regions with differing biogeochemical cycling regimes. They will determine the applicability of the AA-CSIA approach in a variety of marine organisms assessed in controlled studies. Subsequently, ecosystem components will be sampled from the eastern tropical Pacific, coastal California and the subtropical Pacific gyre. They will also test the effects of sample preservation on the isotopic composition of individual AA to determine whether the approach can be used on archived samples. This tool will allow testing of the efficacy of ecosystem-based models currently used to gain insight into the ecological effects of fisheries removals and improve the reliability of future models required to manage marine resources. In addition to the goal of developing AA-CSIA for use as a TP indicator, the information obtained through this project will provide important species-specific biological data on the feeding behavior of marine organisms that could have implications for their resilience to anthropogenic pressures and climate change.
This project will have direct application to evaluating ecosystem effects of fisheries by providing an unbiased, integrated and independent approach to estimating trophic structure, and a method by which to validate existing ecosystem-based model outputs and predictions. In addition, the project will have outreach benefits through the involvement of graduate and undergraduate students, and exposure of younger students through K-12 programs. This research will contribute to the greater understanding of the biology of locally important fish species as well as globally important shrimp and endangered marine turtles.
The goal of this project was to test and refine a tool, Compound-Specific Isotopic Analyses of Amino Acids (CSIA-AA), for determining the trophic positions (TP) of marine organisms in order to improve models for managing ocean fisheries. According to the CSIA-AA method, TP estimates are determined from the difference between nitrogen isotopic compositions of individual amino acids, some that vary little from nitrogen source to primary producers (typically, phenylalanine) and some that enrich strongly in 15N with each trophic step (typically, glutamic acid). An organism’s tissue (protein) thus contains information about potential regional differences in the source of nitrogen to production, as well as the number of steps that organism is removed from primary producers in its average diet. For the project overall, a broad range of activities were undertaken, including field sampling, laboratory studies and CSIA-AA analyses of organisms ranging from microbes to zooplankton, mesopelagic, pelagic and reef fishes, and sea turtles. Most analyses relating to higher trophic level animals were done by the University of Hawaii component (P.I.s: Brian Popp and Jeff Drazen). For the SIO component, our specific attention was on organism representing the plankton food-web base (phytoplankton – protozooplankton – mesozooplankton). Field sampling was done in four regions of the Pacific Ocean with distinct differences in biogeochemistry, ecology and source nitrogen. We sampled plankton in the coastal upwelling region off of southern California, the iron-limited region of the central equatorial Pacific, the strong denitrification region of the Costa Rice Dome (eastern tropical Pacific), and an area of summertime nitrogen fixation in the subtropical north Pacific (Station ALOHA, Hawaii Ocean Time series). The key findings were that slopes in isotopic enrichment with zooplankton size class were very similar in all regions, with the variations in absolute values mainly reflecting differences in source nitrogen to the food-web base. The results therefore show an underlying similarity in zooplankton trophic structure among contrasting ecological systems, which will be useful in ocean ecosystem modeling. However, the data do not support a common assumption of models that predator and prey in marine plankton systems are related by a mean body size (length) ratio of 10:1. We also use historical samples collected in the California Current during a major El Niño/La Niña transition in 1998-1999 to assess how plankton trophic structure responded to a large environmental perturbation. Comparing samples among three years representing El Niño, La Niña and "normal" conditions, we found a significant 15N enrichment of ~2 ‰ at the base of the food web for all AAs and all zooplankton groups during the 1998 El Niño. Overall, the observed patterns of δ15N values in bulk tissues for CCE zooplankton were driven mainly by changes in source δ15N to phytoplankton, rather than by marked alteration of dietary composition (enhanced carnivory) or increased food web length to the zooplankton. However, the krill species Euphasia pacifica, an important prey for baleen whales, did show a significantly elevated TP, implying increased carnivory, more trophic steps and potentially less energy transfer to krill during 1998. This study also broadened the AA-CSIA method by developing Linear Mixed Effect models to incorporate δ15N values for all measured amino acids, providing greater statistical power for hypothesis testing than the typical approach of using only phenylalanine and glutamic acid. In laboratory experiments, we used a model two-stage chemostat system with a heterotrophic dinoflagellate feeding on a green alga to investigate whether rapid nitrogen cycling within the microbial food web might explain low TP estimates (indicating direct herbivory) for mesozooplankton in areas of the Pacific where protozoans are known to be the dominant consumers of phytoplankton. The results surprisingly showed no significant isotopic separation between consumers and prey under either of the light (N cycling) or dark (no recycling) conditions (a one trophic step difference of ~7.6 ‰ was expected). This indicates that carbon and nitrogen skeletons of ingested compounds likely remain intact during digestion and incorporation into protistan biomass, rendering the imprint of protozoan grazing in marine ecosystems isotopically invisible. This is the first direct evidence that protists may not follow the systematic 15N trophic enrichment that is well established for metazoan consumers. On average, 2/3rds of ocean primary productivity is consumed directly by protists, and the number of trophic steps between phytoplankton and mesozooplankton, one to several, is likely regionally and temporally variable. The inability to resolve the protistan contribution to food web networks using isotopic techniques represents a major uncertainty in our understanding of regional differences in elemental cycling and transfer efficiencies, and their likely responses to climate change. This project contributed to the training and academic development of one graduate student, two post-doctoral researchers, and one undergraduate who presently pursuing Ph.D. research (in chemistry) at the Univ. Illinois.
