Allochthonous-derived organic matter subsidizes the food sources of estuarine jellyfish

Jellyfish modulate the energy fluxes between compartments of aquatic ecosystems and the period that energy remains within each compartment. However, the contribution of different sources of organic matter (OM) to the production of most estuarine jellyfish species is unknown. Therefore, this study aimed to identify and quantify the putative sources of OM for Blackfordia virginica (Cnidaria, Hydrozoa) Mayer, 1910 in two temperate estuaries, based on the analyses of carbon and nitrogen stable isotope ratios. Zooplankton was the main source of OM assimilated by B. virginica in both Mira and Guadiana estuaries. However, particulate organic matter (POM) also contributed to B. virginica biomass, up to 26 and 37% in Mira and Guadiana estuaries, respectively. The POM pool was apparently comprised by terrestrial-derived OM, as based on stable isotope ratios and C:N-POM. Terrestrial-derived OM is an undescribed carbon source for estuarine jellyfish. Our results support a working hypothesis stating that terrestrial-derived OM might support good physiological condition of B. virginica during periods of low metazooplankton abundance, through a detritus-based microbial food web transformation. Subsequent studies on trophic pathways ought to evaluate the temporal contribution of autochthonous and allochthonous OM and its consequences, accounting for the connectivity across ecosystems (terrestrial-estuarine-marine) and the interactions within compartments (pelagic-benthic).