Sea turtles are far-travelling creatures with an ecological impact left wherever they go. In this review paper, we get a summary of data from experiments that track sea turtles using stable isotope analysis. By gathering information on animal migration, foraging behavior, and habitat range, we can better protect sea turtles and understand their role in marine ecosystems.
How can stable isotopes track sea turtles?
Every atom has protons, neutrons, and electrons. Protons are the essence of the atom, and how many of them are their define what kind of atom it is. Electrons typically match the number of protons, and more or less of them give the atom an electric charge. Neutrons contribute to the isotopic identity of the atom; there are typically as many neutrons as there are protons, but more or less will impact the weight of the atom. Of course, unstable isotopes get all the attention: uranium, radium, and plutonium are radioactive because their nuclei are too heavy, and they emit radioactive material.
Here, however, we are looking at stable isotopes. Carbon can come as C13 or C12, nitrogen as N14 or N15, and oxygen as O16 or O18. These elements all feature prominently in organic materials and are not radioactive. They all have standard isotopic ratios throughout the environment, so when the ratios change, they provide clues to the past.
In the case of carbon, it’s all about working up the food chain. When you eat carbon sources, you incorporate the carbon into biosynthetic pathways, pushing glucose towards molecules needed throughout cells and tissues. As it happens, these chemical reactions favor C12 over C13. That means that as you move up trophic levels, carbon isotopes undergo fractionation, and there are higher and higher concentrations of C13.
In terrestrial ecosystems, there is typically a max of four or five trophic levels. A hawk eats a snake that eats mice that eats plants. In the oceans, food webs become complicated and you see much longer food chains. A shark eats a big fish that eats a smaller fish that eats a smaller fish . . . The result? Land-based diets result different C-isotope ratios than sea-based diets in a consumer, and archaeologists have used this for years in understanding the feeding habits of ancient – even pre-ancient – people. In sea turtles, C-isotope ratios denote the animal’s position in the trophic web.
Nitrogen works a little differently. Biosynthesis has an effect on fractionation, of course, but so do other types of chemical reactions. Different nitrogen sources – you can get it from nitrate, ammonium, or the atmospheric gas – have different starting ratios, and nitrogen has its own cycle. Nitrogen fixation fixes gaseous nitrogen into ammonia, nitrification converts ammonium to nitrate, and denitrification converts nitrite back to nitrogen gas. These reactions are all carried out by bacteria, and they all alter N-isotope ratios. Marine environments are often characterized by what kind of nitrogen cycling, which means N-isotope ratios can serve as a geographical fingerprint.
Finally, we’ll look at oxygen. Oxygen isotope ratios are closely linked to temperature and the water cycle. The idea here is that water is built out of oxygen with the same O-isotope ratios as oxygen itself. When that water evaporates, there is a preference molecules with lighter O16 isotopes. That means rain and snow is rich in O16, and heavy evaporation makes warm water rich in O18. Thus, oxygen has been used to study glacial cores to date ice ages, and calcium carbonate formations in corals capture temperature like tree rings capture drought. In animal samples, O-isotopes provide further geographical information on the latitude regions the animal inhabits.
So in sea turtles, measuring isotope ratios can tell you the basics of feeding and migration habits. By sampling the blood, epidermis, carapace, egg yolk, muscle tissue, or something else, marine biologists have collected stable isotope data for numerous sea turtle species around the world. Sea turtles have been tracked with satellites, genetics, and flipper tagging, but each have their own disadvantages, whether in expense, small sample sizes, or lack of geographical information in between tagging sites. Stable isotope analysis is cheap and precise; you can sample both the blood and the skin to get the recent menu from blood and more historical data from epidermal tissue.
Coastal habitats have high primary productivity rich in C13, while the pelagic is the desert of the ocean.
This review showcases high-resolution data about each species’ foraging grounds, diet, and how closely they stick to their habitats. Neritic loggerheads have benthic shellfish diets; pelagic leatherbacks have planktonic diets of jellies and other macrozooplankton. Green turtles eat a diet similar to loggerheads, but lower on the food chain. We also learned that leatherbacks around the world graze differently – some stick to the open ocean more than others.
Finally, it can be helpful to connect stable isotope data with satellite data. Stable isotope data assumes that the baseline isotope ratios are the same, but in reality, they can vary a little over space. Thus, using satellite data to corroborate your data can tell you when there are actual differences or when the differences are due to a geographical oddity.
But . . . why?
Why exactly do scientists want to know all this, and get better at collecting isotopic data on sea turtles? Are marine biologists just exceptional stalkers?
Isotope analysis has been used as a forensic tool in elephant ivory poaching, and while more research is needed to apply this to sea turtles, it is possible to police the state of sea turtle poaching using stable isotope analysis data to get the origin of sea turtle products.
We can use stable isotope analysis to track the recovery of sea turtles after natural or anthropogenic disasters, as we know that they tend to remain in their usual habitat even after something like an oil spill occurs. Isotope analysis also told us that turtles with the same diet have comparable levels of pollution whether they live around industrial areas or not. And it’s been used to tell us that sea turtle bycatch depends more on where you fish in their habitats, and less on what type of equipment fisheries use. Protecting foraging grounds might be a more impactful way to reduce sea turtle bycatch than switching to friendlier fishing mechanisms.
Chasing sea turtles to chemically analyze biological samples is proving to be an efficient means of collecting data on multiple species. The science behind this tool is straightforward and depends on concepts learned in a high school chemistry course – simple, yet powerful.
Haywood, J. C., Fuller, W. J., Godley, B. J., Shutler, J. D., Widdicombe, S., & Broderick, A. C. (2019). Global review and inventory: how stable isotopes are helping us understand ecology and inform conservation of marine turtles. Marine Ecology Progress Series, 613(May), 217–245. https://doi.org/10.3354/meps12889