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. 

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. 

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). 

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.

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. 

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. 

As part of the International Year of the Salmon project, I am currently investigating the trophic structure of the pelagic food webs in the eastern North Pacific. Here, I combine stable isotope analysis in bulk-tissue and in individual amino acids, fatty acid and diet data, to investigate a series of questions about the food web structure, trophic diversity, and feeding ecology of species in the open ocean winter months.  

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.  

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.