1 Diel feeding rhythms in marine microzooplankton: effects of prey concentration, 1 prey condition, and grazer nutritional history 2 Anna Arias 1,* , Enric Saiz 1 and Albert Calbet 1 3 1 Institut de Ciències del Mar (CSIC), Passeig Marítim de la Barceloneta 37-49, 08003 4 Barcelona, Spain 5 Abstract 6 In this study we aim at disentangling the causes and consequences of diel feeding 7 rhythms in marine microzooplankton. We focused on the diel feeding activity of two 8 heterotrophic dinoflagellate species, Gyrodinium dominans (one laboratory strain) and 9 Oxyrrhis marina (laboratory cultivated and wild strains). We observed higher ingestion 10 during the day in both dinoflagellate species. Feeding rhythms appeared to be 11 independent of circadian changes in prey biochemical composition. Grazers fed with 12 prey under stationary phase, with equivalent stoichiometric composition between day 13 and night, showed 5 (G. dominans) and 10 (O. marina) times higher ingestion rates 14 during the day. Previous grazer feeding history (starved vs well-fed) did not affect the 15 feeding rhythm. However, prey concentration altered the rhythm; food limiting 16 conditions reduced the amplitude of the rhythms. Our results establish a resource 17 dependence of diel periodicity in microzooplankton grazing, which can have 18 unanticipated consequences for standard field dilution grazing experiments. 19