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isotope workshop

Further elucidation of the “who is eating whom” question

In order to get a picture of interactions within an ecosystem, researchers need information about who is eating whom. Recently MAR-ECO researcher Dr. Tracey Sutton from Harbor Branch Oceanographic Institution led a workshop at the University of Bergen to find out more information.

Gut analyses provide valuable information about what a fish has eaten, but it is really just a “snapshot” of a particular meal (or what is left of it) on a particular day. It is hard to draw meaning inferences about trophic relationships if gelatinous plankton form significant part of the diet, because they are digested so quickly that there will normally be nothing left by the time the fish is hauled on board ship and its gut contents analysed.

Sutton explains that researchers have been wondering if this is the reason behind the high number of empty stomachs that are found in the fish collected. He is a guest of MAR-ECO researcher, Christoffer Schander, at the University of Bergen. They are leading a team of researchers in a week-long workshop. The team includes Dr. Joel Hoffman, a post-doc from the Environmental Protection Agency in Minnesota, Dr. Jennifer Putland winner of a prestigious post-doc scholarship to Harbor Branch Oceanographic Institution and two MAR-ECO Ph.D. students, Vanda Alexandra Santos Carmo from the Department of Oceanography and Fisheries (DOP) at the University of the Azores and Tom Letessier from the Pelagic Ecology Group at the University of St. Andrews.

Scientists have had to develop other techniques in order to be able to infer information about a fish’s diet over time, and thus about trophic relationships within a marine ecosystem. One of these techniques is stable isotope analysis. Many elements occur naturally in different isotopes (mass numbers).

A well known unstable example is uranium – with one of its isotopes (U-235) being the most unstable (radioactive). Biologists use the stable isotopes of elements commonly found in biological organisms, such as carbon and nitrogen, in their stable isotope work.

Dr. Joel Hoffman explains that “heavy isotope” 15N provides better information about the trophic level of an organism than 13C, because it is found at generally slightly higher concentration levels in fish tissues; the greater the proportion of heavier isotope, the higher the trophic level.

Researchers use the proportion of C-13 to complement the N-15 information and to provide information about the kind of prey items consumed.

In addition to studying tissue samples from over 500 MAR-ECO specimens, selected for their tendency to consume a specific prey group and building a “library” of the resulting signatures, the team will build another library of stable isotope signatures from the analysis of a number of potential prey species (especially gelatinous zooplankton) from MAR-ECO material as well as from material gathered summer 2007 during the ECOMAR cruise.

The results of this study will be used to learn how energy is transferred from the ocean’s surface to fish in the deep sea of the Mid-Atlantic Ridge.
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