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.
Over exploitation of resources continues to be a major threat to the health of marine environments. Over 50% of fisheries are currently considered fully or over exploited, which could potentially alter the structure and stability of these food webs. Ecosystem-based models are essential tools to guide management efforts aimed at restoring or maintaining robust ecosystems and fisheries. The models emphasize the flow of matter through food webs and they use predator-prey relationships based on diet data, generally obtained from analysis of the stomach contents of organisms. To date, however, there has been no reliable independent method to validate description of trophic structure in these models. We have tested and applied a new stable isotopic approach that can efficiently provide trophic position estimates of marine consumers from zooplankton to fish and sharks, and in both natural and exploited ecosystems (Figure 1). Compound-specific nitrogen isotope analysis of amino acids is a technique that avoids many of the short-comings of more traditional stable isotope analyses that are conducted on whole animals or on all the material in a tissue sample. We examine the isotopic composition of individual amino acids, the building blocks of proteins. In samples of consumer tissues, individual "source" amino acids such as phenylalanine appear to retain the isotopic composition of nitrogen sources or nutrients at the base of the food web, whereas nitrogen isotope values of "trophic" amino acids such as glutamic acid become substantially higher with each trophic transfer. Trophic position is estimated from the difference between trophic and source amino acid isotopic compositions and the average shift in isotopic composition with each change in trophic level (Figure 1). The key advantage of the amino acid isotopic technique is a predator sample alone is sufficient for estimating trophic position, making separate analysis of the isotopic composition of the phytoplankton at the base of the food web unnecessary. The overarching goal of our research was to develop amino acid compound specific isotopic analysis as a tool that can provide a rapid and unbiased estimate of trophic position for a wide variety of marine organisms (Figure 2) and use this information to validate or correct output from trophic based ecosystem models. To accomplish this goal, we performed a combination of targeted laboratory experiments with different organisms (shrimp, bluefin tuna, snappers and sharks) and field collections in contrasting marine ecosystems that support major fisheries. The laboratory studies determined turnover rates of individual amino acids and the robustness of individual amino acid isotopic values in consumer tissues compared to their prey, under varying conditions of food intake and metabolic rate. These results suggest that the new isotopic method can be applied to consumers regardless if they are young or old, well fed or starving. The field component focused on organisms from marine environments with distinctly different and varying biogeochemical conditions to examine trophic position for a range of individual species from zooplankton to large fish (Figures 3 & 4). Our results show that this new approach can be used to accurately estimate trophic position between locations despite differences in ocean biogeochemistry and in a way not possible with conventional isotope analysis. Currently, our results are being compared to ecosystem model outputs and will assist fisheries managers. In addition to our original goals, we discovered that the isotopic composition of amino acids can provide unique markers to trace the importance of different types of food through ecosystems, to understand transoceanic movements and residency in some highly migratory marine animals (bluefin tuna, leatherback turtles) and allowed us to better understand the depth at which mercury enters marine food webs. Our findings from this research project have been published in nine peer-reviewed scientific papers. Another three peer-reviewed scientific papers have been accepted and await publication. In addition, nine more manuscripts have been submitted and are currently undergoing peer-review. We have presented various aspects of this research at national and international conferences (16 published abstracts). This research directly supported the thesis research of two Ph.D. students, three Master's students and one undergraduate student. We have been a leader in the application of compound-specific nitrogen isotopic analyses of amino acids in marine organisms and we have made our facilities available to teach this important technique to others outside the University of Hawaii. To this end we have trained seven graduate students, one undergraduate and three researchers from other institutions worldwide who have visited and worked in our laboratories. We also maintained a website that described our research to the general public (http://cameo.noaa.gov/).
