December 21, 2021
by Mary Keenan
I am a master’s student working on the Aleutian Mercury Dynamics project and a major part of this project is looking at stable isotopes in the Aleutian Islands. Stable isotopes are a pretty complicated subject, but they can be an extremely useful and important science tool. In this blog I’m going to tell you about stable isotopes and how they can be used to predict an animal’s diet and level in food web.
Stable isotopes are atoms (particles of matter) of the same element with different number of neutral particles (neutrons), but the same number of positive (protons) and negative (electrons). Neutrons are the parts of the atom that give the element mass. Stable isotopes with extra neutrons are often called “heavy” isotopes. For example, the element carbon has 2 stable isotopic forms: the “light” form is Carbon-12 (¹²C, 6 neutrons) and the “heavy” form is Carbon-13 (¹³C, 7 neutrons). Carbon is in all life forms and all life forms will contain some percentage of “heavy” and “light” carbon isotopes. Stable isotopes are different from radioactive isotopes because the nucleus is “stable” and will not release energy or decay like radioactive atoms do. This means that they leave a record of environmental changes over long time periods. Stable isotopes are also very useful because they are stored in and so can be studied in a wide range of biological samples such as hair, teeth, whiskers, fingernails, and bones. Some of the most common stable isotopes to study for ecological purposes are bulk carbon and nitrogen.
When you measure stable isotope ratios in an animal or environment you are comparing the amount of “heavy” isotope to the amount of “light” isotope. Using our example from above this would mean that you are comparing the amount of ¹³C in an animal to the amount of ¹²C in an animal. But these are not just numbers and ratios that we are looking at!
These isotopes can tell us important things about the ecosystems that we want to study! Carbon stable isotopes can tell us where/what an animal is feeding on. Many plants have different carbon isotope values based on slight changes in how they do photosynthesis. Therefore, we can determine what plants an animal is eating based on their carbon isotope values. There are also different carbon isotope values for terrestrial plants vs. marine plants and between different marine primary producers such as phytoplankton and seagrasses/macroalgae (kelp). Since seagrasses/macroalgae are often located in coastal locations and phytoplankton are often located the open ocean, it is possible to narrow down a location of the animal based on those isotope values as well. Below is an example graph of stable isotopes values in a food web including the different primary producers such as phytoplankton and macroalgae (kelp).
Figure above by Nicole Misarti.
Some level of nitrogen is found in all protein which would include marine fishes and mammals. Nitrogen stable isotope ratios can tell you what level in a food web the animal is located in. There are different nitrogen isotope ratios for herbivorous animals (animals that eat plants) and carnivorous animals (animals that eat meat) so these values can determine where an animal belongs in the food web.
Now, let’s put our knowledge into the context of this project! We will be measuring stable isotopes in bones from Pacific cod (Gadus macrocephalus), Steller sea lions (Eumetopias jubatus), and Northern fur seals (Callorhinus ursinus) to determine how the Aleutian Island environment and each of these species’ diets may have changed over time. Some studies have suggested that changes in prey abundance and prey nutrition quality have affected the growth and reproduction of these important Aleutian Island species. We can evaluate this hypothesis by using carbon and nitrogen stable isotope analysis to see what these animals were feeding on and where they were located in the food web from the past 3000 years.
Food webs often change in response to environmental conditions such as temperature, wind patterns and nutrients and biological conditions such as competition and predation. Stable isotope analysis is one tool for understanding the dynamic ecological history of our world and predicting future responses.
