Deciphering the Seasonal Cycle of Copepod Trophic Dynamics in the Strait of Georgia, Canada, Using Stable Isotopes and Fatty Acids

Characterizing trophic flows is central to our understanding of energy transfer in marine ecosystems. The food webs of coastal systems are difficult to study because the proportion of autochthonous to allochthonous sources often varies seasonally and is often overlaid on a seasonal cycle of zooplankton composition. Here, we use a combination of fatty acids and stable isotopes to disentangle the trophic pathways in a productive coastal system (the Strait of Georgia (SoG), Canada). Over the span of a year, Metridia pacifica, a ubiquitous omnivorous copepod, can utilize a wide range of dietary items including diatoms, flagellates, bacteria, detritus, and microzooplankton. M. pacifica can switch from herbivory to carnivory in response to declining chlorophyll concentrations after the spring bloom and can occasionally utilize detrital sources. These findings are discussed in the context of previous knowledge of the SoG ecosystem, the current state of ecosystem modeling in the region, and the use of stable isotopes and fatty acids to assess trophic dynamics.