Spatial food‐web structure in the eastern tropical Pacific Ocean based on compound‐specific nitrogen isotope analysis of amino acids

Abstract

The effective evaluation of trophic interactions in pelagic food webs is essential for understanding food web ecology, conservation biology, and management. We tested the applicability of compound‐specific isotope analysis of amino acids (CSIA‐AA) for (1) characterizing trophic positions (TPs) of nine species from four trophic groups (tunas, squids, myctophids, and euphausiids) within a pelagic food web in the eastern tropical Pacific (ETP) Ocean, (2) evaluating trophic discrimination factors (TDFs) of each trophic group, and (3) detecting spatial changes in TPs and food chain length across a region with heterogeneous productivity. Although δ15N values of bulk tissues generally increased from south‐to‐north, CSIA‐AA revealed that trophic positions were uniform throughout our study area. These results suggest that variability in δ15N values were largely driven by nitrogen cycling dynamics in the ETP, which highlights the importance of these processes for the interpretation of δ15N values in food web studies. Absolute TP estimates were unrealistic for higher‐level species, and TDFs (tunas: 4.0‰, squids: 4.6‰, myctophids: 5.0‰, and euphausiids: 7.0‰) were lower than a widely used ecosystem TDF. We used remotely sensed oceanographic data to evaluate the physical oceanography and biological productivity throughout our study area and found significant relationships between δ15N values, nitrate concentrations, and SST across our study area. We did not find a gradient in phytoplankton cell size co‐occurring with an expected productivity gradient across our sampling region, which substantiated our isotope results indicating non‐significant spatial changes in TP and food chain length across the ETP.

Opportunism on the High Seas: Foraging Ecology of Olive Ridley Turtles in the Eastern Pacific Ocean

“…Similar to other organisms in the eastern Pacific (e.g., copepods, laternfishes and tuna; Popp et al, 2007;Olson et al, 2010;Hetherington et al, 2017), olive ridley bulk skin δ 15 N values showed a tendency to be more 15 N-enriched in higher latitudes. Often, differences in δ 15 N values of consumers may be driven by trophic differences among spatially discrete foraging populations (Vander Zanden and Rasmussen, 2001).…”

“…For example, TPs of tuna, adult leatherbacks, lanternfishes, dragonfishes, and zooplankton do not change across ocean basins Olson et al, 2010;Choy et al, 2012;Hetherington et al, 2017). This underscores the uniqueness of detecting a potential trophic shift on the Costa Rica Dome.…”

Stable isotopic evidence for the differential contribution of diazotrophs to the epipelagic grazing food chain in the mid‐Pacific Ocean

“…Corrected regression lines with a common slope of 1.51 based on the ANCOVA for each group are shown with those in previous studies for other epipelagic ecosystems in the Pacific Ocean (Aita et al, 2011). Mean stable isotopic signatures in muscle tissues of yellowfin tuna as an apex predator in the subtropical waters in previous studies (Graham, 2007;Olson et al, 2010;Revill, Young, & Lansdell, M., 2009) believed to reflect the variable stable isotopic signatures in primary producers where the tuna have foraged (Hetherington et al, 2017;Lorrain et al, 2015;Olson et al, 2010). Although shift and homogenization of isotopic signatures occur by migration between regions in which isotopic signatures of primary producers are different, the isotopic signature of yellowfin tuna collected in Hawaii is clearly lower than those caught in other areas of the Pacific (Figure 7), suggesting that the diazotroph-dominant community within the subtropical region contributes substantially to the apex predators.…”

“…Although yellowfin tuna is categorized as a highly migratory species, it tends to remain in a confined region when it encounters a preferable feeding site (Schaefer, Fuller, & Block, ). This explains the regional difference of the nitrogen stable isotope signatures in yellowfin tuna in the Pacific, which is believed to reflect the variable stable isotopic signatures in primary producers where the tuna have foraged (Hetherington et al, ; Lorrain et al, ; Olson et al, ). Although shift and homogenization of isotopic signatures occur by migration between regions in which isotopic signatures of primary producers are different, the isotopic signature of yellowfin tuna collected in Hawaii is clearly lower than those caught in other areas of the Pacific (Figure ), suggesting that the diazotroph‐dominant community within the subtropical region contributes substantially to the apex predators.…”

“…As shown in our analysis, the bulk δ 13 C–δ 15 N diagram is useful for estimating the trophic position of apex predators within the food chain because, for the moment, the amino acid compound‐specific analysis potentially results in a substantial underestimation of trophic levels for predators at trophic levels higher than ca . 3 (Hetherington et al, ; Lorrain et al, ). Moreover, the present study shows the existence of distinct food chains that are sustained by different nitrogen sources, nitrate below the euphotic layer and diazotrophic nitrogen, within the same ecological provinces (the SPSG and NPTG) in the subtropical Pacific Ocean (Figure ), where the standing stock of primary producers is consistently low (Longhurst, ).…”