Are you new to stable isotopes and interested in applying this tool to investigate cetaceans' foraging and movement ecology? We have recently published a paper, led by my colleague and friend, Dr. Clarissa Teixeira, that provides a useful guide for students and researchers new to this method. In this paper, we talk about everything you need to know, from planning your research, to carrying out the lab work, and analysing and interpreting your data.
A few examples of what you will find in this paper include a description of the temporal resolution that the different tissues offer, how to choose the ideal tissue, depending on the research question you want to address, how to store and preserve the different tissue types, sample treatment (e.g., lipid extraction, decalcification), and processing for stable isotope measurements in the Isotope Ratio Mass Spec. We also present the different tools to analyse isotope data, including the different packages developed for use in R.
The article is published in the journal Marine Mammal Science, and can be accessed here.
If we know what cetaceans eat, we can better understand how they affect their ecosystems, and how environmental changes may impact them. For instance, dietary information is relevant to assess how co-occurring species interact (sharing or competing for food), to estimate their energetic requirements, and to anticipate how human-related disturbances might affect their feeding success, distribution and survival.
There are different methods that we can apply to study the feeding habits of cetaceans, and these include visual observation, the analysis of gut contents of animals that are washed ashore after they die, and the analysis of natural chemical tracers (e.g., stable isotopes) in their tissues, such as skin, teeth or baleen.
Stable isotopes are chemical elements of the same type that have different atomic masses (hence, different weights!). They make up the different macromolecules that build our tissues, and we obtain them through our food (we are what we eat!).
Because it is easier to process the lighter elements, there are some natural discriminations against the heavy stable isotopes during metabolic processes, and organisms end up excreting the lighter isotopes, and accumulating the heavy ones in their tissues. This accumulation propagates through the food chain in a predictable way. Therefore, by looking at the stable isotope values in the tissues of cetaceans (or in any other consumer, for that matter), we can learn about their food sources and estimate their trophic positions within food webs.
I have been applying this method since my masters' studies to investigate the feeding habits of coastal and oceanic cetacean populations.
Several cetacean species inhabit the oceanic waters off Brazil. Because of the distance from the coast, relatively little is still known about what they eat and how they interact with one another. During my PhD, I analysed skin samples of different toothed-whales that were collected during marine-mammal dedicated research expeditions along the oceanic waters off southern Brazil (Projeto Talude).
We found that the nitrogen stable isotopes in cetacean skin followed the same patterns as those in zooplankton, which had been previously described in a paper mapping the isotopic signature of organisms at the base of the food webs in these oceanic waters. By considering these spatial differences in the stable isotopes throughout this region, we could better interpret the isotopic values in cetacean skin, and make more accurate inferences about the feeding habits of these odontocete species. These analyses helped refine our knowledge about the trophic ecology and interspecific trophic interactions among several odontocete species that occur in the oceanic waters of the western South Atlantic. Click here to read the full paper.
Nevertheless, the patterns in the nitrogen isotope values of some cetacean species remained unresolved. For instance, δ15N values indicated that the rough-toothed dolphin (Steno bredanensis) could be occupying a higher trophic position than species such as the sperm whale (Physeter macrocephalus) or the killer whale (Orcinus orca), which is unlikely.
Trying to understand these patterns that remained unexplained in those previous analyses, I decided to analyse nitrogen isotopes in the individual amino acids of cetacean skin.
A schematic of the difference in δ15N values between the source and trophic amino acids (AA) in organisms of different trophic positions. Image: Genyffer Troina.
Source amino acids: because some amino acids change very little during metabolic processes, they preserve the isotopic signature of organisms at the base of the food webs. Hence, they can be used to trace the baseline sources where the consumer is foraging.
Trophic amino acids, on the other hand, undergo great changes in the nitrogen values, and can, therefore, be used to estimate a consumer's trophic position within food webs.
These analyses allowed the characterization of the trophic structure of the cetacean community that inhabit these offshore waters, as well as how they use this habitat. We found that the nitrogen stable isotopes in cetacean skin source amino acids were correlated with those in zooplankton, following the same latitudinal patterns. This revealed some level of fidelity to these regions, and allowed us to assess their different foraging areas.
Additionally, with the analysis of nitrogen isotopes in amino acids we could see that the trophic position of the rough-toothed dolphin was in fact lower than bulk-tissue isotope data had previously suggested.
These new analyses also resolved the high isotopic overlap that had been observed between sperm whales and the other smaller odontocetes. We could see that this overlap is much smaller, as species feed at different depths. For example, the sperm whales forage in the oceanic waters of the continental slope, and occupy the highest trophic position among the cetacean species studied.
Analysis of amino acid δ15N helped reveal the trophic structure and habitat use of cetaceans from the oceanic waters of the western South Atlantic. Image: Genyffer C. Troina. Cetacean illustration by José Luis Vázquez, adapted from Bastida et al. 2018.
The common dolphin (Delphinus delphis) feeds in neritic waters, while the Atlantic spotted dolphin (Stenella frontalis) and the bottlenose dolphin (Tursiops truncatus) search for food along the outer continental shelf. These last two species were often observed together, and the high overlap in their isotopic values indicated that they forage together, but consume different prey and occupy different trophic positions.
These analyses revealed that these cetacean species rely on spatial or trophic segregation as a means to minimize or avoid competition. The complete article can be downloaded here.
Franciscanas (Pontoporia blainvillei) are small cetaceans that are endemic to coastal waters off Brazil, Uruguay and Argentina. During my Masters, I analysed carbon and nitrogen stable isotopes in teeth, to assess whether there were sexual or age-related patterns in the feeding habits of franciscanas from southern Brazil. I also used isotopic mixing models to estimate the importance of different prey types to the diet of franciscanas from distinct age groups (juveniles vs. adults). This paper is published in the journal Marine Mammal Science, and can be accessed here.
Following on the franciscana dolphin paper, and trying to improve our method, we have recently published a paper in the journal Marine Environmental Research. We showed that the isotopic signal from the nursing period that occurred during the first few months of life still affects the whole tooth stable isotope values in juvenile franciscanas (up to 3 years old). Why does it matter? Tooth tissue is frequently used for stable isotope measurements to assess the feeding ecology of marine mammals, including franciscana dolphins.
By measuring stable isotopes in different tooth portions, including the center which excludes the dentin layer deposited during the nursing period, we demonstrated the effect of the dentin deposition patterns on the whole tooth isotope values of franciscana dolphin, and how it can affect dietary assessments.
Isotope mixing models showed that dietary estimates were different when using the isotope measurements of the whole tooth in comparison to using only the center tooth portion, affecting dietary inferences for juvenile franciscanas.
With this, we call for caution when using stable isotope measurements from the whole tooth of this and other small-toothed dolphins. We recommend, when possible, to exclude the dentin layer deposited during the nursing period, remove juveniles from analysis when only whole tooth isotopic analysis is feasible, or choose another tissue for isotopic measurements.
The full paper can be accessed in the here.
The analysis of chemical tracers, such as carbon and nitrogen stable isotopes provide a useful tool to characterize the structure of food webs. I have been using these tools to estimate the trophic position of different organisms, trophic redundancy and interspecies interactions, food web length and the level of trophic diversity within communities that compose the pelagic ecosystems.
Photos: Steve Lindley - NOAA Fisheries
I am currently investigating the structure and dynamics of the pelagic food webs that sustain salmon in the high seas of the North Pacific. This is part of the International Year of the Salmon project, which involves scientists from Canada, the United States, Russia, Japan, and Republic of Korea. I am analysing carbon and nitrogen stable isotopes and fatty acids in the different components of these pelagic food webs to identify the main trophic pathways that support salmon, and the direct and indirect effects of inter-species interactions. More to come soon!
As part of my PhD research, I analysed stable isotopes in samples of different organisms that occur in the offshore waters of the outer continental shelf and slope in the Southwestern Atlantic Ocean. These samples included the particulate organic matter (POM, representing the primary producers), zooplankton, forage and large fishes and squids, as well as the top predator species such as cetaceans. These analyses allowed me to describe the trophic structure of these pelagic food webs, providing a baseline for future works to refer to. This work is currently in preparation for submission to a scientific journal for publication.
Isotopic maps, or isoscapes, are used to describe the spatial patterns in the stable isotope values of organisms, usually those at the base of the food webs such as phytoplankton and zooplankton, hereafter called baseline organisms. Isoscapes are useful to study the trophic and spatial ecology of high trophic-level animals, such as sharks, marine turtles, seabirds, whales and dolphins. The isotopic values of baseline organisms are affected by the oceanographic conditions in that ecosystem, such as temperature, nutrient concentration and productivity levels. Since different marine ecosystems are subject to different oceanographic conditions, the isotopic values of baseline organisms are expected to vary through space and time. Consequently, if we want to apply the stable isotope methodology to assess a predators' feeding habit and foraging area, it is essential that we develop isotopic maps that describe how the isotopic values of baseline organisms vary through space.
In this work, I analysed carbon and nitrogen stable isotopes in zooplankton to describe the patterns in isotopic variability at the base of the food webs along the oceanic waters of the western South Atlantic. The paper related to this work is published in the journal Deep Sea Research Part I, and can be accessed here.
Overseen by Prof. Dr. Brian Hunt and situated at the Institute for the Oceans and Fisheries, University of British Columbia.
Laboratory of Ecology and Conservation of the Marine Megafauna (Laboratório de Ecologia e Conservação da Megafauna Marinha), at Universidade Federal do Rio Grande, southern Brazil. Led by Professors Dr. Eduardo Secchi, Dr. Silvina Botta and Dr. Luciano Dalla Rosa.
