Spatio-Temporal Changes in Basal Food Source Assimilation by Fish Assemblages in a Large Tropical Bay in the SW Atlantic Ocean

Understanding food pathways in estuarine communities is essential to improve our ability to cope with the increasing effects of man-made activities and climate variability on the biodiversity of coastal systems. Herein, we analyzed carbon and nitrogen isotope ratios of food web components in a large tropical bay in the southwestern Atlantic, in order to investigate spatial and temporal changes in the assimilation of basal food sources by fishes. Two hypotheses were evaluated: (i) continental-derived organic matter assimilation and isotopic niches of fish trophic guilds will increase during higher rainfall conditions and (ii) relative mangrove and macroalgae-derived nutrients assimilation will be directly proportional to their relative availability at each location. Our results did not corroborate our first hypothesis, while only partially corroborating the second one. As expected, macroalgae was one of the main assimilated basal food sources (especially by omnivore and zooplanktivore fishes) at the site where this primary producer was highly abundant. In contrast, only negligible amounts of mangrove-derived nutrients were assimilated by most trophic fish guilds at the site harboring an extensive mangrove area. Bayesian mixing models also revealed a substantial contribution of seagrass to herbivore and detritivore fishes. Some mechanisms like water divergence diminishing organic matter transport into the bay and the refractory characteristics of mangrove-derived detritus could partially explain these findings.