Trophic Enrichment Factor Research Articles

Application of stable isotopes to assess the feeding ecology of long-finned pilot whale (Globicephala melas) in the Northeast Atlantic Ocean

In order to improve our knowledge on the feeding ecology of long-finned pilot whales (Globicephala melas) in Northeast Atlantic waters, skin samples of 68 long-finned pilot whales stranded in Northwest Iberia (n=22) and Scotland (n=46) were analysed using stable isotopes of δ13C and δ15N. Isotopic mixing models were applied to obtain a quantitative estimate of the proportion of the main prey species in the diet of pilot whales. Stable isotope analysis revealed that 57.8–73.8% of the diet in Northwest Iberia consisted in curled octopus (Eledone cirrhosa), followed by European flying squid (Todarodes sagittatus), while in Scotland the predominant prey species was either Histioteuthis sp. or T. sagittatus, depending of the trophic enrichment factor applied. These results are generally in accordance with previous stomach content studies; however, the isotopic analysis may provide new information regarding key prey species and habitat use that could be missed or underestimated if only stomach contents analysis were used. Additionally, considering that the Atlantic Coast of Iberia was responsible for 95% of the landings of the main prey consumed by pilot whales in this area, between 2000 and 2010, these data provide trophic baseline information to be taken into account in fishery impact assessment studies and management decisions.

A coupled stable isotope-size spectrum approach to understanding pelagic food-web dynamics: A case study from the southwest sub-tropical Pacific

This study investigated the food web structure of the oligotrophic picophytoplankton-dominated pelagic ecosystem in the vicinity of New Caledonia, within the Archipelagic Deep Basin (ARCH) province of the southwest sub-tropical Pacific. Nitrogen stable isotope (δ15N) data were collected for mesozooplankton (0.2–2mm), macrozooplankton (2–20mm), micronekton (20–200mm) and nekton (>200mm) during 2002–2004 and 2011. Using a coupled δ15N size-spectrum approach, we estimated (1) organism trophic level (TL); (2) food chain length (FCL); (3) predator prey mass ratio (PPMR); and (4) transfer efficiency (TE). The role of phytoplankton size structure in determining these parameters was investigated. Applying a trophic enrichment factor (TEF) of 3.4, maximum TL was calculated at ~5. The number of TLs spanned by each length class was 1.97 for mesozooplankton, 2.07 for macrozooplankton, 2.75 for micronekton, and 2.21 for nekton. Estimated PPMR was 10,099:1 for mesozooplankton, 3683:1 for macrozooplankton/micronekton, and 2.44×105:1 for nekton, corresponding to TEs of 6.3%, 8.5% and 2.4%, respectively. PPMR and TE were strongly influenced by the TEF used, and TEF 3.4 likely over and underestimated PPMR and TE, respectively, for mesozooplankton and macrozooplankton/micronekton. Comparatively low PPMR for mesozooplankton and macrozooplankton/micronekton indicated longer food chains and higher connectivity within these groups than for the nekton. Conversely, the high PPMR yet high trophic niche width for the nekton indicated that they prey primarily on macrozooplankton/micronekton, with a relatively high degree of dietary specialisation. Our results are discussed in the context of other marine food webs. The ARCH food chain was found to be 1–1.5 trophic levels longer than the eutrophic micro-/nanophytoplankton-dominated Californian upwelling system, providing empirical support for the role of phytoplankton size in determining FCL. Group specific PPMR estimates demonstrated that it is changes in trophic pathways across the mesozooplankton/macrozooplankton/micronekton groups that are primarily responsible for higher FCL under oligotrophic conditions. Finally, we discuss consistently low δ15N values to the east of New Caledonia, and implications for the contribution of diazotroph nitrogen to the pelagic food web in this region.

Ontogenetic changes in the trophic position of a freshwater Unionidae mussel

Freshwater mussels have often been used as indicators of a trophic baseline (i.e., primary consumers) in the food web. To assess the utility of a large filter-feeding mussel, Cristaria plicata, as an isotopic indicator, we compared the carbon (delta C-13) and nitrogen (delta N-15) stable isotope ratios of the mussel with those of its potential food sources, particulate organic matter (POM) and sediment organic matter (SOM). The delta C-13 values of large mussels (shell length > 140 mm, -31 parts per thousand to -29.2 parts per thousand) were similar to those of POM (-30.2 parts per thousand) rather than those of SOM (-27.6 parts per thousand), indicating that mussel carbon was derived mainly from POM (mainly phytoplankton). In contrast, the mussels exhibited 6.3 parts per thousand to 9.0 parts per thousand higher delta N-15 values than did POM. Assuming a previously reported trophic enrichment factor (+3.4 parts per thousand), the trophic level estimate of the mussel ranged from 2.9 to 3.6, indicating that they functioned as secondary rather than primary consumers. Our results also revealed positive correlations between shell lengths and delta N-15 values, suggesting that the mussels changed their trophic position from primary consumer (shell length 140 mm) with growth. A significant inter-annual difference was found in the y-intercepts of the regression lines between shell length and delta N-15, indicating that mussels can reflect inter-annual changes in the isotopic baseline (i.e., primary producers).

Foraging habits in a generalist predator: Sex and age influence habitat selection and resource use among bottlenose dolphins (Tursiops truncatus)

AbstractThis study examines resource use (diet, habitat use, and trophic level) within and among demographic groups (males, females, and juveniles) of bottlenose dolphins (Tursiops truncatus). We analyzed the δ13C and δ15N values of 15 prey species constituting 84% of the species found in stomach contents. We used these data to establish a trophic enrichment factor (TEF) to inform dietary analysis using a Bayesian isotope mixing model. We document a TEF of 0‰ and 2.0‰ for δ13C and δ15N, respectively. The dietary results showed that all demographic groups relied heavily on low trophic level seagrass‐associated prey. Bayesian standard ellipse areas (SEAb) were calculated to assess diversity in resource use. The SEAb of females was nearly four times larger than that of males indicating varied resource use, likely a consequence of small home ranges and habitat specialization. Juveniles possessed an intermediate SEAb, generally feeding at a lower trophic level compared to females, potentially an effect of natal philopatry and immature foraging skills. The small SEAb of males reflects a high degree of specialization on seagrass associated prey. Patterns in resource use by the demographic groups are likely linked to differences in the relative importance of social and ecological factors.

Amino acid 15N trophic enrichment factors of four large carnivorous fishes

Ecosystem-based fisheries management strategies require knowledge of trophic relationships. Trophic position (TP) estimates from compound specific nitrogen isotopic analysis of amino acids (AA-CSIA) show promise as the method can disentangle confounding factors associated with changing δ15N values at the base of the food web, but it has yet to be tested in many organisms. This novel technique requires two empirically determined biological parameters: 1) β, the difference in δ15N values between glutamic acid (glu) and phenylalanine (phe) in primary producers and 2) trophic enrichment factor (TEF), the 15N enrichment of glu and phe at each trophic step. Values of β (3.4‰) and TEF (7.6‰) have been suggested for animals in aquatic environments; however recent observations indicate that TEF values may be variable, particularly among elasmobranchs where urea retention may alter nitrogen isotope fractionation between glu and phe. To test these uncertainties, we determined TEF values for three species of sharks, sand tiger (Carcharias taurus), lemon (Negaprion brevirostris), and leopard sharks (Triakis semifasciata), and one teleost species, opakapaka (Pristipomoides filamentosus) grown on controlled and well characterized diets for durations ranging from three (T. semifasciata) to over five years (P. filamentosus). TEF values for both elasmobranchs and opakapaka were ~2‰, significantly lower than TEFs previously reported. These results do not support the hypothesis that urea retention lowers 15N trophic enrichment between glu and phe in elasmobranchs. Rather, isotopic enrichment factors may be primarily driven by differences in dietary protein quality, leading to distinct TEFs for herbivores (~7.6‰) and carnivores (<7.6‰). We propose a method to calculate TP which integrates different TEF values for herbivores and carnivores.

Nitrogen isotope fractionation in amino acids from harbor seals: implications for compound-specific trophic position calculations

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 482:265-277 (2013) - DOI: https://doi.org/10.3354/meps10257 Nitrogen isotope fractionation in amino acids from harbor seals: implications for compound-specific trophic position calculations Leslie R. Germain1,*, Paul L. Koch2, James Harvey3, Matthew D. McCarthy1 1Department of Ocean Sciences, University of California, Santa Cruz, California 95065, USA 2Department of Earth and Planetary Sciences, University of California, Santa Cruz, California 95065, USA 3Department of Vertebrate Ecology, Moss Landing Marine Laboratories, Moss Landing, California 95039, USA *Email: leslieroland@gmail.com ABSTRACT: Compound-specific isotope analysis of individual amino acids (AA) is a rapidly growing tool in ecological studies to assess diet and trophic position (TP) in both modern and ancient foodwebs. We conducted the first controlled feeding study examining δ15N values in AAs in a marine mammal (harbor seal Phoca vitulina). The pattern of δ15N variation among AAs in seals was similar to that observed in other heterotrophs, although exceptions were found with proline and threonine. However, many δ15N changes with trophic transfer were very different than those reported for zooplankton and other lower TP marine consumers. In particular the measured trophic enrichment factor (TEF) now broadly used for TP estimation (TEFGlu-Phe) was much lower in harbor seals (~4.3‰) than the current commonly applied value (~7.5‰). Recently published data on wild marine birds (penguins) and elasmobranchs (stingrays) suggests that similar, low TEF values may also be characteristic of these taxa. Together, these data imply that marine mammals and other higher animals have different, but also diagnostic, changes in δ15N-AA with trophic transfer vs. organisms examined in previous feeding studies (e.g. zooplankton, bony fish and mollusks), possibly due to dietary protein content, trophic position, and/or form of nitrogen excretion (urea vs. ammonia). Therefore, we propose that for marine mammals, a multi-TEF calculation is required to account for variations of TEF between animals within a food web, and we demonstrate that this approach can predict accurate TP estimates for harbor seals. These results also have significant implication for the application of compound-specific isotope analysis of AAs on terrestrial ecology and trophic structure. KEY WORDS: Compound-specific isotope analysis · Trophic enrichment factors · Phoca vitulina · Trophic position · TEF · Urea · Ammonia · Protein content Full text in pdf format Supplementary material PreviousNextCite this article as: Germain LR, Koch PL, Harvey J, McCarthy MD (2013) Nitrogen isotope fractionation in amino acids from harbor seals: implications for compound-specific trophic position calculations. Mar Ecol Prog Ser 482:265-277. https://doi.org/10.3354/meps10257 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 482. Online publication date: May 22, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Evidence for benthic primary production support of an apex predator–dominated coral reef food web

Five hundred and ninety-nine primary producers and consumers in the Papahānaumokuākea Marine National Monument (PMNM) (22°N–30°N, 160°W–180°W) were sampled for carbon and nitrogen stable isotope composition to elucidate trophic relationships in a relatively unimpacted, apex predator–dominated coral reef ecosystem. A one-isotope (δ13C), two-source (phytoplankton and benthic primary production) mixing model provided evidence for an average minimum benthic primary production contribution of 65 % to consumer production. Primary producer δ15N values ranged from −1.6 to 8.0 ‰ with an average (2.1 ‰) consistent with a prevalence of N2 fixation. Consumer group δ15N means ranged from 6.6 ‰ (herbivore) to 12.1 ‰ (Galeocerdo cuvier), and differences between consumer group δ15N values suggest an average trophic enrichment factor of 1.8 ‰ Δ15N. Based on relative δ15N values, the larger G. cuvier may feed at a trophic position above other apex predators. The results provide baseline data for investigating the trophic ecology of healthy coral reef ecosystems.

Food web structure of a shallow eutrophic lake (Lake Taihu, China) assessed by stable isotope analysis

Lake Taihu is a large, shallow, and eutrophic lake in China. It has provided local communities with valuable fisheries for centuries, but little is known of the trophodynamics, or of its faunal communities. Carbon and nitrogen isotopic composition was used to assess its trophic pathways and the food web structure [food sources and trophic levels (TL)]. Basal food sources were distinguishable based on their δ13C values, ranging from −27.2 to −15.2‰. Consumers were also well separated in δ13C (−26.9 to −17.9‰ for invertebrates and −25.7 to −18.1‰ for fishes), which allowed for an effective discrimination of carbon sources between these fauna. An average trophic enrichment factor of 3.4‰ was used to calculate the TLs based on δ15N of zooplankton, with results indicating a food web having four TLs. Although δ15N values overlap and cover a large range within trophic compartments, the isotopic signatures of the species assessed revealed a general trend of 15N enrichment with increasing TL. Stable isotope signatures were also used to establish a general food web scheme in which five main trophic pathways were analyzed.

Food-web inferences of stable isotope spatial patterns in copepods and yellowfin tuna in the pelagic eastern Pacific Ocean

Evaluating the impacts of climate and fishing on oceanic ecosystems requires an improved understanding of the trophodynamics of pelagic food webs. Our approach was to examine broad-scale spatial relationships among the stable N isotope values of copepods and yellowfin tuna ( Thunnus albacares), and to quantify yellowfin tuna trophic status in the food web based on stable-isotope and stomach-contents analyses. Using a generalized additive model fitted to abundance-weighted-average δ 15N values of several omnivorous copepod species, we examined isotopic spatial relationships among yellowfin tuna and copepods. We found a broad-scale, uniform gradient in δ 15N values of copepods increasing from south to north in a region encompassing the eastern Pacific warm pool and parts of several current systems. Over the same region, a similar trend was observed for the δ 15N values in the white muscle of yellowfin tuna caught by the purse-seine fishery, implying limited movement behavior. Assuming the omnivorous copepods represent a proxy for the δ 15N values at the base of the food web, the isotopic difference between these two taxa, “ Δ YFT-COP,” was interpreted as a trophic-position offset. Yellowfin tuna trophic-position estimates based on their bulk δ 15N values were not significantly different than independent estimates based on stomach contents, but are sensitive to errors in the trophic enrichment factor and the trophic position of copepods. An apparent inshore–offshore, east to west gradient in yellowfin tuna trophic position was corroborated using compound-specific isotope analysis of amino acids conducted on a subset of samples. The gradient was not explained by the distribution of yellowfin tuna of different sizes, by seasonal variability at the base of the food web, or by known ambit distances (i.e. movements). Yellowfin tuna stomach contents did not show a regular inshore–offshore gradient in trophic position during 2003–2005, but the trophic-position estimates based on both methods had similar scales of variability. We conclude that trophic status of yellowfin tuna increased significantly from east to west over the study area based on the spatial pattern of Δ YFT-COP values and the difference between the δ 15N values of glutamic acid and glycine, “trophic” and “source” amino acids, respectively. These results provide improved depictions of trophic links and biomass flows for food-web models, effective tools to evaluate climate and fishing effects on exploited ecosystems.

An evaluation of stable nitrogen isotopes and polychlorinated biphenyls as bioenergetic tracers in aquatic systems

This study investigated the relationship between stable nitrogen isotopes (δ15N) and polychlorinated biphenyl (PCB) concentrations in rock bass (Ambloplites rupestris) and bluegill sunfish (Lepomis macrochirus) with respect to age, size, and diet. δ15N signatures in both species exhibited enrichment with increasing size and approached steady state with respect to dietary δ15N values by the second year of growth. Young-of-the-year fish, however, exhibited lower nitrogen isotope enrichment over the diet, indicating that the commonly held trophic enrichment factor of 3.4‰ is more suitable for older, slow-growing individuals. PCB accumulation in both species progressed from being dominated by uptake from water in small (&lt;100 g) fish to dietary uptake in larger individuals as a function of bioenergetic constraints such as food energy conversion and contaminant assimilation efficiencies. Significant increases in PCB accumulation were attributed to decreased specific growth rates such that ≤60% of body mass was gained on an annual basis. This effect was most noted in bluegills where higher PCB biomagnification factors were a consequence of increased foraging costs associated with an invertebrate diet. It is concluded that growth-related changes in species bioenergetics regulate both contaminant accumulation and δ15N dynamics.

Faunal trophic interaction in an oligotrophic-dystrophic lake (Shirakoma-ike, Japan)

The trophic interaction in the pelagic food web of oligotrophic-dystrophic Lake Shirakoma-ike was investigated by a stable isotope technique. Carbon and nitrogen isotope analyses revealed the trophic interaction among Piona carnea (a water mite), Chaoborus sp. (a phantom midge), Daphnia longispina (a cladoceran), and Acanthodiaptomus pacificus (a calanoida copepoda). The δ13C values for estimating the relative contributions of D. longispina and A. pacificus to carnivores were used, and they were also used for the constraint of the trophic enrichment factor of δ15N. Both Chaoborus sp. and P. carnea were suggested to consume D. longispina and A. pacificus at almost the same ratio (1 : 1). However, both carnivores preferred A. pacificus to D. longispina, because D. longispina was much more abundant in the lake than A. pacificus. Omnivory of P. carnea was also analyzed by the mixing model.