Although food quality is thought to play an important role in the survival of marine organisms, the extent of natural variability in food quality over long time scales remains poorly characterized. We present a 6 yr time series of fatty acid data from the calanoid copepod Neocalanus plumchrus, an important contributor to mesozooplankton biomass in the Strait of Georgia and the northeast Pacific Ocean. Fatty acid profiles indicate significant spatiotemporal differences in the diet of this

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... . Spatially, oceanic specimens display fatty acid signatures characteristic of omnivorous copepods while coastal animals display primarily herbivorous, diatom-based signatures. Temporally, the fatty acid profiles of coastal N. plumchrus shifted from an omnivorous oceanic diet to an herbivorous, diatom-based diet between 2001 and 2006. The ratio of diatom to flagellate fatty acid markers increased over time, peaking from 2005 to 2006. The composition of flagellate markers also changed from primarily dinoflagellate markers (rich in docosohexaeonic acid) to green algal markers (poor in this essential fatty acid). The diet of N. plumchrus as deduced from fatty acids correlates with phytoplankton community composition. The abundance of coastal N. plumchrus in the Strait of Georgia was strongly correlated with the ratio of docosahexaeonic acid to eicosapentaeonic acid in the lipids of these copepods. We also discuss the potential for an imbalance of essential fatty acids supplied by a diatom-rich diet to limit the growth and survival of N. plumchrus in the Strait of Georgia. KEY WORDS: Copepod · Diapause · Diet · Docosahexaeonic acid · Eicosapentaeonic acid · DHA · EPA · Fatty acids · Food quality · Neocalanus Resale or republication not permitted without written consent of the publisher Editorial responsibility: Matthias Seaman,