Characterization and transformation of dissolved organic matter in a Pampean stream.

JOAN PERE CASAS RUIZ; MARÍA LAURA MESSETTA; ANDREA BUTTURINI; CECILIA HEGOBURU; CLAUDIA FEIJOÓ

Coimbra

Congreso; XIX Conference of the Iberian Association of Limnology; 2018

Iberian Association of Limnology

Dissolved organic matter (DOM) transformations depend on itschemical composition, which is modulated by the sources that provideorganic matter. Temporal dynamics of stream DOM are usuallygoverned by the discharge regime. Understanding the chemicalcomposition of DOM is important, because chemically different DOMpools have different susceptibility to biogeochemical transformationprocesses within streams. The aim of our study was; (i) to characterizethe 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 dissolvedorganic carbon (DOC) under different hydrological conditions. Thisstudy was carried out in Las Flores stream, a third-order streamlocated in the Luján River basin in the northeast of the Buenos Airesprovince, 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 endof a reach (site A), at two upstream tributaries (B1 and B2), and atthe potential hydrological contributors to stream flow (groundwater,overland and subsurface flows and rainfall). We determined DOCconcentration and DOM optical properties in the stream and endmembers by combining absorbance-fluorescence spectroscopytechniques. At baseflow conditions, when stream flow is mainlymaintained by groundwater inflow, DOM is predominately composedby a mixture of compounds derived from microbial activity (proteinlikecomponents) and compounds derived from the terrestriallandscape, which are transported to the stream during storm events.DOC concentration and inputs of humic substances from the riparianzone increased with discharge at high flow conditions. Mass balanceestimates revealed that the study reach is as a source of DOC. Proteinlikecompounds mainly derived from groundwater contribution andin-stream production at baseflow, while humic-like fractions derivedfrom materials washed out during storm events and transported tothe stream. However, part of humic-like fluorescence could originatefrom the breakdown of highly productive macrophyte communities.In conclusion, DOM chemical characteristics in Las Flores stream aremainly modulated by a differential contribution of end members tostream water depending on hydrological conditions. Stream DOMconsists of a mixing of proteinaceous and humic compounds.