Characterization and transformation in Dissolved Organic Matter in a pampean stream

MESSETTA, M. L; BUTTURINI, ANDREA; CASAS RUIS, JOAN PERE; HEGOBURU, CECILIA; FEIJOO, CLAUDIA

Coimbra

Congreso; XIX Congreso de la Asociación Ibérica de Limnología; 2018

Universidad de Coimbra

Dissolved organic matter (DOM) transformations depend on its chemical composition, which is modulated by the sources that provide organic matter. Temporal dynamics of stream DOM are usually governed by the discharge regime. Understanding the chemical composition of DOM is important, because chemically different DOM pools have different susceptibility to biogeochemical transformation processes within streams. The aim of our study was; (i) to characterize the composition of DOM in stream water and their potential sources (groundwater, overland flow, subsurface flow and rain water) and (ii) to determine net in-stream retention/release efficiencies of dissolved organic carbon (DOC) under different hydrological conditions. This study was carried out in Las Flores stream, a third-order stream located in the Luján River basin in the northeast of the Buenos Aires province, Argentina. We selected a 2.2 km reach of Las Flores stream, which originates at the confluence of two tributaries (B1 and B2). On each sampling occasion, we collected water samples at the end of a reach (site A), at two upstream tributaries (B1 and B2), and at the potential hydrological contributors to stream flow (groundwater, overland and subsurface flows and rainfall). We determined DOC concentration and DOM optical properties in the stream and end members by combining absorbance-fluorescence spectroscopy techniques. At baseflow conditions, when stream flow is mainly maintained by groundwater inflow, DOM is predominately composed by a mixture of compounds derived from microbial activity (protein-like components) and compounds derived from the terrestrial landscape, which are transported to the stream during storm events. DOC concentration and inputs of humic substances from the riparian zone increased with discharge at high flow conditions. Mass balance estimates revealed that the study reach is as a source of DOC. Protein-like compounds mainly derived from groundwater contribution and in-stream production at baseflow, while humic-like fractions derived from materials washed out during storm events and transported to the stream. However, part of humic-like fluorescence could originate from the breakdown of highly productive macrophyte communities. In conclusion, DOM chemical characteristics in Las Flores stream are mainly modulated by a differential contribution of end members to stream water depending on hydrological conditions. Stream DOM consists of a mixing of proteinaceous and humic compounds.