Temporal dynamics in the diet of two marine polychaetes as inferred from fatty acid biomarkers

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, bloom-dependent accumulation or biosynthesis of FA was not detected in N. hombergii, probably because of its reliance on invertebrate prey.

► Polychaete diet was investigated using fatty acid biomarkers and their stable isotope analysis.
► L. conchilega diet consisted of bacteria and diatoms; N. hombergii diet also of invertebrates.
► Bacteria and Phaeocystis were assimilated pre-bloom; PUFA accumulated post-bloom.
► Increased accumulation or biosynthesis of PUFA from the SPM in L. conchilega upon bloom deposition.