Habitat use and isotopic niche variability during ontogeny of the gilthead seabream Sparus aurata within a coastal lagoon

The gilthead seabream Sparus aurata is known to enter coastal lagoons and estuaries during the first stages of post-larval and juvenile development. The high habitat heterogeneity and shallowness that characterize such ecosystems offer plentiful food and shelter for this species that hence undergoes rapid growth. The juvenile stage development occurs through stepwise changes in anatomy, physiology, and behavior that is linked strongly to ontogenetic changes in habitat and resource use. We used carbon and nitrogen stable isotopes to assess: (1) the role of different habitats in supporting the trophic pathway leading to S. aurata over early stages of development within a coastal lagoon, (2) ontogenetic diet shifts and how they reflect in changes of the trophic niche of the species. Fish and organic matter sources were sampled in spring and summer 2014 and 2015, in different sites of the Venice lagoon representing salt marsh and seagrass meadow habitats located along a confinement gradient. Bayesian mixing models, standard ellipse areas and community-wide metrics were applied to the four size classes detected, according to standard length (SL): post-larvae, SL<20 mm, juveniles I 20<SL<40 mm, juveniles II, 40<SL<60 mm and sub-adults SL>60 mm. Organic matter sources considered were plankton, particulate and sedimentary organic matter (POM and SOM), seagrasses and macroalgae. We observed a clear shift in source contribution through size classes, with slight differences among habitats. The greatest contribution to the trophic pathway of S. aurata shifted from a mix of POM, SOM and plankton for the post-larval stage, when the fish is only able to prey on very low trophic levels, to mainly seagrasses for the middle stages, to a mix of macroalgae and seagrasses for the sub-adults, characterised also by a high trophic plasticity. Additionally, the shift manifest in the isotopic niche, which varied in shape, width and position across size classes, with the main differences occurring between post-larvae and juveniles/sub-adults, in accordance to their diet shift that goes respectively from zooplanctivorous to zoobenthivorous habits. This study emphasises the important trophic role that coastal lagoon habitats provide to the early stages of S. aurata life cycle and links it to the implications that may result in terms of coastal management.