Everyone who lives around Kodiak knows that the ocean and its creatures are in a constant state of change. No one needs a scientist to tell them this, but scientists like to quantify and record these changes to understand how ecosystems work. This is especially true of projects that involve researchers from different fields.  I am part of a team of archaeologists, ecologists, and paleoclimatologists investigating changes in the ecosystems where sea otters live. One aspect of our study uses the chemical signatures found in intertidal species, from kelp up to top predators like the sea otter, to construct food webs past and present. Known as stable isotope signatures, these markers can help us discover the effects of factors like climate change and hunting and gathering by humans, on sea otter ecosystems. 

Isotopes are atoms of a single element, like carbon or nitrogen, that have different atomic weights. In general carbon isotopes can tell us which plants are at the base of a food web and the productivity of an area. Carbon isotopes usually vary from place to place as well. Nitrogen isotopes most often tell us an organism’s place in the food chain.  The higher up the food chain you are, the higher your nitrogen values. Chemically speaking, you are what and where you eat.

By combining stable isotope, archaeological, and climatic data our study tracks intertidal ecosystem change on Kodiak (a place where numbers of sea otters are increasing) and then compares these results with data from Sanak Island (an area where sea otters are decreasing). We want to understand why sea otter populations are growing in the eastern Gulf of Alaska but declining in the Aleutian Islands.

Last summer we completed our first year of fieldwork, collecting intertidal samples from six areas on Kodiak. We chose these areas because they are associated with archaeological sites that hold ancient animal remains we can also test.  We collected hundreds of samples of plants and invertebrates including chitons, sea urchins, sea stars, periwinkles, and more. We bagged and tagged each one individually, recording its location, collection date, size and weight. Then we froze the samples. Later we took a subset of the samples and prepared them for carbon and nitrogen stable isotope analysis by separating the soft tissue, cleaning it, then freeze drying and powdering it. This was very smelly work!

Among its many uses, the isotope data is helping us to create a chemical view of Kodiak’s food webs, in which we can place modern day sea otter isotope ratios as well as values from sea otter bones found in Kodiak’s archaeological sites. In short, isotope data allow us to compare past and present sea otter environments.  It will also help us compare local food webs in the six different areas to see the differences in sea otter habitats around Kodiak. We will return to Kodiak this summer to collect from the same areas to gain an idea of what inter-annual variability in intertidal ecosystems looks like. This coming winter we will analyze the historic and prehistoric sea otter bones currently housed in museums from Alaska to Washington.  Stay tuned.  Results from this project will be shared at the Alutiiq Museum.