Linking patterns of freshwater discharge and sources of organic matter within the Río de la Plata estuary and adjacent marshes

We investigated carbon isotopic ratios ($\delta^{13}$C) $v.$ carbon to nitrogen (C : N) ratios for surface sediments throughout a large estuarine system (Río de la Plata, RdlP), combined with sediment cores from adjacent marshes to infer main carbon sources. We also evaluated the influence of the El Niño–Southern Oscillation (ENSO) and associated high freshwater-discharge events on the organic-matter transport within the estuary. The isotopic pattern in surface sediments of the RdlP showed the upper reaches to be influenced by riverine particulate matter ($\delta^{13}$C range: -24 to -26‰). Similarly, in the sediment cores from marshes of the upper reaches, $\delta^{13}$C values decreased from -24‰ in ancient sediments to -28‰ in recent sediments, reflecting an increased contribution of organic matter from land, including C3 plants and freshwater phytoplankton, during the past 50 years. However, the lower reaches represent a depositional environment of marine algae ($\delta^{13}$C range: -21 to -23‰), with no influence of detritus from adjacent marshes, indicating minor erosion of the marshes in the lower reaches operating as carbon-sink habitats. Our isotopic analysis showed that the transport and deposition of terrigenous organic matter within the RdlP and adjacent marsh habitat appear to be both temporally and spatially linked to hydrology patterns.This work was partly funded by PEDECIBA–Geociencias and Agenica Nacional de Investigación en Innovación (SNI–ANII). This paper is part of the M.Sc. Thesis of A. Tudurí. This project (CP1211) was financially supported by a grant of the Cluster of Excellence 80 ‘The Future Ocean’ to Mark Schuerch. ‘The Future Ocean’ is funded within the framework of the Excellence Initiative by the ‘Deutsche Forschungsgemeinschaft’ (DFG) on behalf of the German federal and state governments. L. Bergamino thanks CSIC-Program ‘Contratación de Científicos Provenientes del Exterior’