Organic matter processing through an estuarine system: Evidence from stable isotopes (δ13C and δ15N) and molecular (lignin phenols) signatures

Abstract

This study unraveled the origin, fate and accumulation of organic matter (OM) of surface sediments, spread between the fresh and marine end members of Guaratuba Bay (SE- Brazil). Sedimentation rates were obtained through the 210Pb sediment dating method, biogenic element accumulation (C-organic carbon, N-nitrogen, P-phosphorus, and lignin phenols) and stable isotopes of organic carbon (δ13C) and nitrogen (δ15N) were measured along the well-defined salinity gradient (0–35). The higher C:N ratios and lower δ13C values signatures indicate major terrestrial contribution in regions dominated by fluvial inputs, while the contributions from autochthonous primary production and mangrove forests were greater in the middle and outer estuarine sectors. A stable isotope mixing model showed that the material contribution from rivers and mangroves in surface sediments ranged from 80 to 90% in the upstream sector, 50–70% in the central sector and 25–50% at the outer region, whereas this contribution decreased to 15–20% in the adjacent ocean. Ratios between phenolic groups indicated material deposited from predominantly woody angiosperms. Vertical CNP deposition, accumulation rates and lignin phenol concentrations along the sedimentary profiles indicated three phases: Phase I, the more recently deposited sediments, which contained the highest CNP sedimentary fluxes; Phase II, dated to be from 1960s to 1990s, was characterized by changes related with increasing deforestation associated to fires; Phase III from the 1920s–1960s, exhibited four-fold lower C, N and P concentrations compared the more recent rates. Along the estuarine gradient, the C accumulation was found to be related to specific sources as indicated by the lignin's, stable isotopes as well as C:N ratios. Furthermore, the vertical accumulation of OM in sediments concomitantly with geochronology showed the recent anthropogenic perturbations in the estuary. This study highlights the land-ocean sedimentary continuum of which are poorly studied with respect to the transport of C, N and P in estuarine systems, reinforcing the presence of a marked land-ocean gradient. Knowledge of isotopic fractions, as well as lignin phenols, for different areas along the estuarine gradient is important as sampling along coastal areas can represent a bias in understanding the transport and cycling of OM to the sea if gradients and specific regions are not taken into account.