A combined data set, combining data from field campaigns and oceanographic cruises, was used to ascertain the influence of both river discharges and upwelling processes, covering spatial and temporal variation in dissolved inorganic carbon (DIC) and aragonite saturation state. This work was conducted in one of the most productive river-influenced upwelling areas in the South Pacific coasts (36°S). Additionally, further work was also conducted to ascertain the contribution of different DIC sources, influencing the dynamics of DIC along the land-ocean range. Six sampling campaigns were conducted across seven stations at the Biobío River basin, covering approximately 200 km. Three research cruises were undertaken simultaneously, covering the adjacent continental shelf, including 12 sampling stations for hydrographic measurements. Additionally, six stations were also sampled for chemical analyses, covering summer, winter, and spring conditions over 2010 and 2011. Our results evidenced that seaward extent of the river plume was more evident during the winter field campaign, when highest riverine DIC fluxes were observed. The carbonate system along the river-ocean continuum was very heterogeneous varying over spatial and temporal scales. High DIC and pCO 2 were observed in river areas with larger anthropogenic effects. CO 2 supersaturation at the river plume was observed during all campaigns due to the influence of low pH river waters in winter/spring and high-pCO 2 upwelling waters in summer. δ 13 C DIC evidenced that main DIC sources along the river and river plume corresponded to the respiration of terrestrial organic matter. We have linked this natural process to the carbonate saturation on the adjacent river-influenced coastal area, suggesting that Ω aragonite undersaturation in surface/subsurface waters is largely modulated by the influence of both river discharge and coastal upwelling events in this productive coastal area. Conditions of low Ω aragonite might impact negatively physiological traits for marine organisms, such as bivalves, gastropods, and crustaceans. Therefore, local populations from river-influenced sites could be inherently more tolerant to ocean acidification than organisms living in regions with lower Ω aragonite variability. VARGAS ET AL.

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. However, since HNF dominated in terms of abundance, they were the dominant grazers on bacterioplankton populations. Estimates of grazing pressure for the microbial food web showed that MNF were capable of removing 1 to 51% BP d . Given the area's relatively high nutrient condition, the elevated MNF biomass in the river plume and the greater bacterivory impact from MNF in winter, it seems that light and, thus, the energy/carbon limitation could be the main trigger for mixotrophy in this river-influenced coastal upwelling area. KEY WORDS: Mixotrophy · River discharge · Upwelling area · Plankton food webResale or republication not permitted without written consent of the publisher

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