Lipid-based assessments of omnivory in Arctic copepods

Stevens, C.J. (2003) Lipid-based assessments of omnivory in Arctic copepods. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The lipid compositions of three dominant arctic copepods, Calanus hyperboreus, C. glacialis and Metridia longa, were used to deduce feeding strategy (i.e., degree of omnivory) and diet in the highly productive North Water Polynya during autumn. A quick way of processing lipid samples, well suited to fieldwork in remote regions, was tested against conventional methods. The quick method, in which copepods were frozen together and sorted later, provided accurate estimates of lipid classes in Calanus hyperboreus CV and fatty acids in Metridia Longa females. However, it was inappropriate for sensitive analyses of certain biomarkers in the former species and lipid classes in the latter. -- A new omnivory index, the unsaturation coefficient (UC), was tested during a long-term incubation with Calanus glacialis CV. The UC is the ratio between polyunsaturated and total wax ester (WE). The WE fraction of three arctic copepods split according to the degree of acyl lipid unsaturation during non-polar chromatographic development. UC and diatom biomarker levels decreased significantly after copepods were fed a bacterivorous dinoflagellate for three weeks. At the same time, proportions of the bacterial biomarker 18:1 (n-7) increased significantly. When copepods were reintroduced to a diatom diet, their lipid profiles reversed to a more herbivorous signature over a period of three weeks, characterized by higher UC levels. -- In several regions of the polynya, Metridia Longa females had significantly lower UC than Calanus hyperboreus CV and C. glacialis CV. M Longa contained high proportions of triacylglycerols, polar lipid and 18: 1 (n-9), and low relative amounts of WE, polyunsaturated fatty acids, 20:1(n-9) and 22:1(n-11). UC and previously established determinants of feeding strategy were in agreement and showed that M longa fed more onmivorously than the two Calanus species. Principal components analysis revealed that all three species were feeding onmivorously, to varying degrees, in the southeastern (SE) polynya. Copepods here contained low proportions of diatom, phytoplankton and herbivory indices and had low UC, as compared to those at northwestern stations (NW). Instead, copepods in the SE generally had elevated levels of carnivory, dinoflagellate and bacterial markers. Spatial patterns in seston lipids and other data indicated that the microbial loop was more active in the SE than in the NW.

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