Characterizing dietary variability and trophic positions of coastal calanoid copepods: insight from stable isotopes and fatty acids
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The spring zooplankton community in the Strait of Georgia (British Columbia, Canada) is characterized by the presence of several calanoid copepod species which collectively make up ~90% of the mezozooplankton biomass. Here, we investigate interspecific, interannual, and geographic variability in the diets and trophic positions of these copepods using a combination of fatty acids and stable isotopes. To characterize geographic variability in diet, we compare our findings from the Strait of Georgia with similar data from Ocean Station P in the subarctic northeast Pacific. Both fatty acid and stable isotope signatures indicate the existence of three trophic levels, even within the limited size range of these copepods: Neocalanus plumchrus and Calanus marshallae are primarily omnivorous, while Euchaeta elongata is carnivorous and Eucalanus bungii is herbivorous. Fatty acid markers of trophic position (e.g., DHA/EPA, 18:1n-9/18:1n-7) correlate significantly with δ15N, while markers indicating the proportion of diatoms to flagellates in the diet (e.g., 16PUFA/18PUFA and DHA/EPA) correlate significantly with δ13C, after the effect of lipid concentration on δ13C is accounted for. Despite the general correlation between stable isotopes and fatty acids, the former are not sensitive enough to capture the range of interannual variability observed in the latter, and can only capture substantial shifts in the diet over geographic scales. However, regardless of variability in food quality, the relative trophic positions of these copepods do not change significantly either spatially or temporally.
KeywordsStable Isotope Fatty Acid Profile Particulate Organic Matter Dietary Quality Trophic Position
The authors would like to thank the captains and crew of the R/V John Strickland (University of Victoria) and CGGS Siyay for the assistance with sampling. Chris Parrish, Sergei Verenitch and Kelly Young provided valuable technical advice for instrumentation, extraction and fatty acid identification. This research was supported by an NSERC Discovery Grant to John F. Dower and Asit Mazumder, an NSERC Strategic Grant to Susan Allen. Rana El-Sabaawi received funding from NSERC and The University of Victoria. The authors would like to thank three anonymous reviewers whose comments greatly improved this manuscript.
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