Trophic interaction among organisms in a seagrass meadow ecosystem as revealed by bulk δ¹³C and amino acid δ¹⁵N analyses

Abstract

Numerous studies have used stable isotope analysis (SIA) of carbon and nitrogen within the bulk tissues of organisms to determine the trophic structure among organisms in a food web. Recently, SIA has evolved to compound‐specific stable isotope analysis (CSIA) of nitrogen within amino acids to significantly reduce the uncertainty in the estimated trophic position (TP) of organisms based on the isotopic difference between glutamic acid and phenylalanine within a single organism. However, because the initial offset (β) between glutamic acid and phenylalanine differs between aquatic algae (ca. +3.4‰) and vascular plants (ca. −8.4‰) in food webs that rely on both resources, β should be replaced by a value adapted to the admixture of primary producers for each specimen. In this study, we established a new method involving the β value (βmix) of each consumer specimen determined based on its bulk tissue δ13C value and successfully obtained realistic TPs (TPmix) for organisms in a complex seagrass meadow food web. Remarkable differences between the TPmix and traditional TPalgal values were found in deposit feeders due to the large contribution of seagrass to their basal resources. The estimated TPs of organisms increased by up to 1.5 units (from TPalgal to TPmix), in terms of trophic transfer, when their diets included substantial seagrass‐derived contributions. Thus, combinatorial analysis of the amino acid δ15N and specimen‐specific βmix values provides better understanding of the trophic interactions in food webs, even in complex seagrass meadow ecosystems.

Elucidating food web structure of the Poyang Lake ecosystem using amino acid nitrogen isotopes and Bayesian mixing model

“…However, vascular plant resources accounted for approximately 20% fraction of the dietary composition for these planktivorous fish (Table ), which may indicate the potential assimilation of small particle sizes of organic or microbial aggregates. The TP of seston (1.38 ± 0.03) in Poyang Lake is much higher than 1.0, which is a value similar to those found in a stream or marine samples (Batista et al ; Choi et al ). These higher TP values may be due to the segments of zooplankton or heterotrophic bacteria contained in the seston samples.…”

“…The β value presents relative initial differences between the δ 15 N Glu and δ 15 N Phe in primary producers (i.e., −8.4‰ for vascular C3 plants, +0.4‰ for C4 plants, +3.4‰ for algae, and + 2.6‰ for cyanobacteria) (Chikaraishi et al ; Chikaraishi et al ; Nielsen et al ). Therefore, the β value should be further evaluated for the estimation of the TP when consumers rely on an admixture of different types of dietary resources (Choi et al ; Ruiz‐Cooley et al ). For example, Vander Zanden et al () and Choi et al () suggested that the contribution of seagrass to individual species should be considered to determine the trophic structure in seagrass meadow ecosystems because estimating the TP using the β values of aquatic algae or C3 vascular plants would lead to significant errors (Vander Zanden et al ; Choi et al ).…”

“…Therefore, the β value should be further evaluated for the estimation of the TP when consumers rely on an admixture of different types of dietary resources (Choi et al ; Ruiz‐Cooley et al ). For example, Vander Zanden et al () and Choi et al () suggested that the contribution of seagrass to individual species should be considered to determine the trophic structure in seagrass meadow ecosystems because estimating the TP using the β values of aquatic algae or C3 vascular plants would lead to significant errors (Vander Zanden et al ; Choi et al ).…”

Use of compound‐specific nitrogen isotope analysis of amino acids in trophic ecology: assumptions, applications, and implications

“…The b value of animals in these systems should be corrected using animal-specific mixing proportions of vascular and non-vascular plants before calculating their TP. Without calculating mixing proportions, the TP values of sea turtles, polychaeta, and some stream macroinvertebrates are significantly underestimated (Arthur et al 2014;Ishikawa et al 2014;Choi et al 2017 (Chikaraishi et al 2007;Ohkouchi and Takano 2014;Ohkouchi et al 2015). The use of the d…”

Effects of ethanol preservation and formalin fixation on amino acid stable isotope analysis (δ13C and δ15N) and its ecological applications

“…Second, amino acid δ 15 N profiles are largely conserved across tissue types within an organism (Chikaraishi et al ), and trophic positions can be estimated with unprecedented precision because trophic fractionation values are more predictable than in bulk SIA (Chikaraishi et al ; Steffan et al ; Bowes and Thorp ). Third, estimates of trophic position based on amino acid δ 15 N profiles can be further enhanced in both precision and accuracy by incorporating information on proportional producer contributions based on essential amino acid δ 13 C profiles in the same sample (see Choi et al ()). This technique is particularly advantageous in more complex ecosystems that comprise multiple primary producer groups (e.g., freshwater environments) (Ishikawa et al ).…”