Use of Zooplankton Fatty Acids to Elucidate Trophic Pathways
ABL’s Nutritional Ecology Laboratory (NEL) is collaborating with the RACE Division on a project that examines the fatty acid compositions of various taxa of zooplankton collected from the Bering Sea in the vicinity of the Pribilof Islands. Two primary objectives of the project are to use fatty acid data to elucidate trophic relationships and to examine the influence of nutrient- and zooplankton-rich slope water on populations near the continental shelf break. First, however, we needed to determine if there were differences in composition between species and locations before we could ask the more important ecosystem questions.
The study involved collection and biochemical analysis of more than 150 zooplankton samples including copepods Metcidia pacifica and Eucalanus bungii; calanoid copepods Calanus marshallae and Neocalanus cristatus; euphausiids Thysanoessa raschii, T. inermis, and T. spinifera; as well as various species of pteropods, decapods, and mysiids. The study appears to be the first large-scale analysis of the fatty acid compositions of zooplankton in the North Pacific Ocean or the Bering Sea.
Due to the small size and fragile nature of most zooplankton, sample collection and storage, lipid extraction, and subsequent chemical analyses are particularly challenging on these samples. Handling of zooplankton samples on the vessel must be carefully performed to avoid specimen destruction, lipid oxidation, and breakdown. Furthermore, conventional laboratory processing methods must be modified to accommodate the particular nature and small size of zooplankton.
Sample weights must be carefully measured and treated to avoid surface water but not desiccate the samples. Solvent volumes used for lipid extraction must be reduced and some conventionally automated methods must be performed manually. The special handling of these samples is worth it in the end, though, as the data are very intriguing and may provide clues to understanding complex trophic interactions that are not well understood.
Not all zooplankton are created equal. Prelim-inary analysis revealed significant differences in the fatty acid compositions of different zooplankton types. This variation permits comparisons between species and locations and provides various other important diet information. For instance, copepod fatty acid profiles are easily distinguishable from those of euphausiids or pteropods, likely related to diet differences.
Significant differences can also be observed between species within a zooplankton type. For example, the euphausiid T. spinifera had a distinct fatty acid profile as compared to the euphausiid T. raschii. In addition, there were surprising differences in the fatty acid complexity of species within a zooplankton type, measured by the number of major fatty acid components in a profile. The copepod C. marshallae contained nine fatty acids with abundances greater than 5% of the total fatty acid composition, while the copepod M. pacifica contained only three fatty acids above the 5% abundance threshold.
Fatty acid analysis can also identify specific types of diet components, based on the levels of key signals such as those that indicate a diatom-based diet or one that is rich in dinoflagellates. Specific ratios of fatty acids can indicate general dietary characteristics such as carnivory and herbivory. Such fatty acid-derived diet information is useful for the characterization of plankton-based trophic webs that include planktivorous fish and mammals and can be used to evaluate specific hypotheses regarding geographical influences on production.
