We investigated the temporal variation of pelagic and benthic food sources in the diet of two marine polychaetes: a macrobenthic omnivore (Nephtys hombergii) and a suspension-deposit feeder (Lanice conchilega) by means of fatty acid (FA) biomarkers and compound-specific stable isotope analysis (CSIA). FA biomarkers in the suspended particulate matter roughly mirrored phytoplankton dynamics in the water column, consisting of a small diatom dominance early spring, succeeded by a mass Phaeocystis peak followed by a mixed diatom-dinoflagellate bloom. Deposition and subsequent bacterial degradation of the phytoplankton bloom were also reflected in sediment FA biomarkers. The main distinction in FA biomarker concentration within macrobenthic tissue was observed at the species level (48% of variation), the diet of L. conchilega consisting of bacteria and diatoms and that of N. hombergii also of diatoms, but including more dinoflagellates and invertebrates. Temporal variation explained 17%: the two species retained more bacterial and Phaeocystis markers before the bloom, while they accumulated more poly-unsaturated FA after the bloom. CSIA revealed increased accumulation or biosynthesis of poly-unsaturated FA from the suspended matter in L. conchilega upon bloom deposition, which is probably related to energy storage for gametogenesis. In contrast, bloomdependent accumulation or biosynthesis of FA was not detected in N. hombergii, probably because of its reliance on invertebrate prey.