Insights into the trophic roles of eastern Pacific olive ridley sea turtles from compound-specific isotope analysis of amino acids

Background/Question/Methods:

Eastern Pacific (EP) olive ridley sea turtles (Lepidochelys olivacea) uniquely inhabit and forage in the open-ocean and are thought to be opportunistic, generalist consumers. Intentional movement patterns, possible distinct -- even if dynamic -- foraging regions, niche width, and trophic role(s) in the EP ocean (EPO) are largely unknown.  Most, if not all, other sea turtle species consistently use identifiable foraging grounds, including fellow pelagic consumer the Pacific leatherback.  Stable isotope analysis can elucidate foodweb interactions and roles of consumers, as δ¹⁵N values of turtle tissue provide information about previous location because spatially discrete food webs can be isotopically distinct.  Bulk tissue analyses alone are not sufficient to tease apart the influences of a foodweb’s baseline ¹⁵N isotope value versus the consumer’s trophic status.  However, compound-specific isotopic analyses of amino acids (CSIA-AA) provide the information needed to distinguish influences of source nitrogen and/or trophic position.  To examine large-scale δ¹⁵N spatial relationships, we performed CSIA-AA on five or more olive ridley skin (epidermis) samples in five oceanographically distinct sub-regions in the EPO: (1) Gulf of California (GC); (2) North Equatorial Current (NEC); (3) Eastern Pacific Warm Pool (EPWP); (4) Costa Rica Dome (CRD), and (5) Peruvian Current (PC).  

Results/Conclusions:

CSIA-AA were utilized to determine whether trophic position changed depending on the open-ocean region where the individual was sampled.  Following Chikaraishi et al. 2009, trophic amino acid glutamic acid and source amino acid phenylalanine isotopic values were used to calculate the Trophic Level (TL_Glu/Phe) of each individual.  Average TL_Glu/Phe in the GC, NEC, EPWP, and the PC subregions was ~3.1, suggesting that the trophic role of an individual olive ridley sea turtle does not significantly change with foraging location over a large spatial scale in the EPO.  Average TL_Glu/Phe in the CRD, a highly productive oceanographic region due to a shallow thermocline and persistent upwelling, was ~3.5, suggesting that olive ridleys may be consuming higher-nutrient prey and perhaps exhibiting individual specialization in that subregion. These results are part of a larger EP olive ridley foraging ecology study that is generating spatial knowledge of olive ridley foraging in the EPO on a seascape scale with the goal of determining if and where discrete, dynamic pelagic foraging areas exist.  These results support the use of CSIA-AA as a useful tool to identify drivers of δ¹⁵N signals in the EPO.