Re-examining trophic dead ends: stable isotope values link gelatinous zooplankton to leatherback turtles in the California Current

Abstract

Predator-prey interactions provide essential information for tracing energy flow through food webs and evaluating the structure and function of ecosystems. In pelagic environments, these interactions are often difficult to discern, which is problematic for identifying specific energy pathways that support populations of protected species. We examined the trophic ecology of an endangered population of leatherback turtles Dermochelys coriacea and their gelatinous prey in the California Current-Large Marine Ecosystem (CC-LME). We combined carbon and nitrogen bulk stable isotope analysis and compound-specific isotope analysis of amino acids (CSIA-AA) with Bayesian statistical approaches to examine the diets of leatherbacks and their prey (scyphozoans and thaliaceans) sampled in the CC-LME. Our objectives were to evaluate (1) temporal changes in leatherback trophic position, (2) the contribution of different gelatinous prey to leather back diets, and (3) trophic structure of the leatherback food web by estimating trophic positions and isotopic niches of leatherbacks and their potential prey. Leatherback trophic positions did not change over time, although carbon isotope values suggest a temporary shift in leatherback habitat in 2005, coincident with anomalous upwelling conditions. Bayesian mixing models suggest that carnivorous sea nettles Chrysaora fuscescens were the largest contributor to leatherback diet, followed by filter-feeding thaliaceans. Isotope analyses provided useful and ecologically realistic estimates of trophic structure, where trophic positions were lowest for thaliaceans, intermediate for scyphozoans, and highest for leatherbacks. Overall, our findings provide information on leatherback foraging ecology over a 13 yr period and the trophic structure of gelatinous zooplankton that support the leatherback population in the CC-LME.

Large‐scale patterns of green turtle trophic ecology in the eastern Pacific Ocean

“…The TP AA method using δ 15 N Glx , δ 15 N Phe , and β marine (Eq. 3) has also performed well for olive ridley turtles (Peavey et al 2017) and leatherback turtles (Dermochelys coriacea; Hetherington et al 2019) both of which reported TP of 3.1, which is consistent with ecological knowledge about the trophic status of these pelagic consumers, which consume a diet of exclusively marine phytoplankton-derived nutrients. This method also yielded reasonable TP estimates for green turtles in oceanic waters of the central Pacific and Peru, where Arthur et al (2014) reported TPs of 2.5 AE 0.1 and 2.3 AE 0.2, respectively.…”

“…Larger sample sizes for each site would have been preferred, but were not possible due to financial constraints. Nevertheless, sample sizes of three individuals have been used in prior CSIA-AA studies to effectively describe sea turtle TP (Seminoff et al 2012, Hetherington et al 2019.…”

“…Baseline and trophic information can therefore be obtained from consumer tissues without the need for analyses of prey items or basal food web samples (Popp et al 2007, Chikaraishi et al 2009, Ohkouchi et al 2017. Previous studies using CSIA-AA have quantified trophic levels of a variety of marine taxa (Dale et al 2011, Bradley et al 2015, Hetherington et al 2019), but rarely has TP been determined for multiple subpopulations of the same species across ocean basins (but see Vander Zanden et al 2013b, Arthur et al 2014.…”

Pelagic tunicate grazing on marine microbes revealed by integrative approaches

“…In cases when baseline isotopic values are unavailable, compound-specific stable isotope analysis of individual amino acids or fatty acids (FAs) can untangle trophic relationships (Popp et al 2007). For example, variations in δ 15 N values of amino acids from leatherback turtles and gelatinous zooplankton were recently used together with Bayesian models to establish trophic structure (Hetherington et al 2019).…”

Thaliacean community responses to distinct thermohaline and circulation patterns in the Western Tropical South Atlantic Ocean

“…Historically, due to the high water content in their tissues and low caloric value, thaliaceans and other gelatinous organisms were considered irrelevant and even dead-ends in marine trophic networks. However, this concept has changed in recent decades as it has been observed that these organism are key components in the diet of groups such as crustaceans, fish and turtles (Henschke et al, 2016;Hetherington et al, 2019). Although their energetic content is indeed lower than other zooplankton organisms, due to their slow motion, predators expend less energy in the capture process making them an efficient food source (Henschke et al, 2016;Hetherington et al, 2019).…”