Stable Isotopes

In order to analyze the food web of the fishpond, I'm analyzing the stable isotopes of carbon and nitrogen present in fish, crab, and algae tissues. While I am asking an ecological question, stable isotope ecology is all about chemistry. Here's a little background.
Carbon and nitrogen, the elements I'm studying, each have more than one isotope. That means that carbon can be found, for example, as 13-C or 12-C. 13-C is the heavier isotope. Many reactions can alter the ratio of heavy to light isotopes in a sample-- this alteration is called fractionation. Stable isotopes record two types of information: process information and source information (Peterson & Fry 1987). When physical and chemical reactions fractionate isotopes, isotopic distributions reflect the reaction conditions (process information). Isotopes also record source information because sources set an isotopic baseline that is then shifted by fractionation. Examples of biological fractionation include photosynthesis, which takes CO2 that's relatively "heavy" and decreases the ratio of heavy to light isotope (CO2 in air is -7.4‰ whereas terrestrial C3 plants average -27.8‰). In a food web study like this one, this source information can tell you what an animal is eating, and sometimes even what organism is at the base of the food web in a certain location.

In order to measure the stable isotope composition of anything from the fishpond, I need dried tissue. For the algae, this requires a simple collection and cleaning. The samples are put in glass jars (above) and dried. Then I use a mortar and pestle to grind them, weigh them into tiny foil capsules, and these capsules are fed into a mass spectrometer at the University of Hawai'i Isotope Biogeochemistry Lab. For fish and invertebrates, I need muscle tissue, which I get from the claws (crabs) or abdomen (fish, alpheids, shrimp). For SI analysis of microalgae, I use bulk filtered microalgae, caught on a glass fiber filter.

Stable isotope analyses are difficult to analyze, especially in the context of food webs. My goal for this study is to identify seasonal and spatial patterns in the diet of important fish, and link this to the presence of invasive algae.