The impact of Río de la Plata plume favors the small-sized copepods during summer

The Río de la Plata drains the second largest basin in South America and the fifth-largest in the world. In this study, we analyzed the influence of the estuarine plume generated by the river (Plata plume) upon the abundance, composition, and size structure of the copepod's community during summer. Zooplankton samples were collected by employing 67 and 300 μm mesh size nets, water samples were obtained to quantify bacterioplankton, protozooplankton and phytoplankton biomass, and CTD profiles were also obtained during a summer cruise. Amongst the 60 copepods categories identified, calanoid and cyclopoid nauplii were numerically dominant across the shelf, followed by small-sized copepodites of Para-Clausocalanidae and Oithonidae. The presence of adults of the small and medium-sized Centropages calaninus, Calocalanus plumulosus, Delibus sewelli, Temora turbinata and Oithona flemingeri represented a novel finding for the area, attributed mainly to the small mesh size used. Copepods community was organized into four assemblages that occupied distinct areas closely related to different water masses (Plata Plume Waters, Subantarctic Shelf Waters, and Tropical Waters). Each assemblage was characterized by species abundance, composition, and the indicator species, being salinity and temperature the main explanatory variables for these assemblage's arrangement. The Plata plume was characterized by higher numerical abundances, especially of the small-sized fraction, which was also the most diverse, denoting the strong ecological impact of the plume in summer. Offshore the plume, copepods size composition was homogeneous, with the addition of medium and large-sized taxa, which were important discriminating species between assemblages. Our results allow to infer that in the plume the energy would be channeled through a microbial food web in summer, which could be a less productive season as occur for the rest of the shelf. Given the large spatial scale of the Plata plume (ca.1000 km), our results could reflect impacts on the plankton community and trophic web at a regional scale. Taking into account that models predict an increase of precipitations in southern South America, and consequently, on river discharges, our results could aid in understanding aspects of the impact of global change in a region relevant to fisheries of Argentina and Uruguay.