Award: OCE-1040810

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 the food webs that support these fisheries. 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 descriptions 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 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 examined 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 that 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 of 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 fishes (Figures 3 and 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 output 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 (e.g. bluefin tuna, leatherback turtles) and allowed us to better understand the depth at which mercury enters marine food webs. Our findings from this...