Autochthonous organic carbon contributions to the sedimentary pool: A multi-analytical approach in Laguna Garzon

Characterization of the sedimentary organic matter (SOM) within an intermittently open coastal lagoon in SE Uruguay was carried out using optical properties of dissolved organic matter (DOM), stable isotope analyses (delta C-13 and delta N-15) and mineralogical properties of the sedimentary pool were assessed using Fourier transform infrared spectroscopy (FTIR). Additionally, chemical and biological variables including total and dissolved nitrogen and phosphorus, chlorophyll-a in water and sediments, and benthic diatoms were analyzed. Potential sources of SOM (e.g., marine and riverine phytoplankton, terrestrial plant material) and fauna were collected during summer, autumn, and winter from seaward (South) to landward (North) zones. In the pore water, the high contribution of fluorescent, protein-like components, the delta C-13 values of sediments (-22% to -16 parts per thousand), and the low C/N ratios of sediments (similar to 7), indicate the dominant source of SOM is marine microalgae (benthic/planktonic, tychoplanktonic). Most of the diatom species recorded in the sediments were benthic marine/brackish species including the polyhaline species Pseudopodosira echinus. Furthermore, FTIR spectra showed higher contributions of quartz, carbonates, and feldspar towards the seaward zones. Clay minerals increase towards landward zones, indicating changes in the hydrodynamic energy within the lagoon with greater overall rates of sediment deposition in landward zones. In shallow systems, active resuspension and mixing conditions may help to explain spatial similarities in the composition of the sedimentary pool. The combined use of stable isotopes, FTIR, diatoms and nutrient measurements (rather than single approaches) represents an alternative comprehensive tool to infer information on the composition and sources contributing to the sedimentary pool. (C) 2018 Elsevier Ltd. All rights reserved.