Complexing fluorescent dissolved organic matter (FDOM) along the Parnaíba River Delta, Northeast Brazil.

The Parnaíba River Delta (PRD) is one of the largest open sea delta of the Americas that is minimally affected by anthropogenic activities along the Equatorial zone of Brazil. Its subsurface waters were collected in December 2019 and December 2021 during the dry season. Both campaigns occurred under the spring tide condition to observe the scenario of seawater intrusion over the delta. It investigated the dynamic of dissolved organic matter (DOM) sources, characteristics, and interactions with copper in the PRD waters flowing through its mangrove forests. The fluorescence spectrophotometer indicated DOM was composed of humic substances from terrestrial sources with signatures of humic acid-like (peak C) and fulvic acid-like (peak A). The ultraviolet-visible absorbance indicated that DOM molecular weight, humification, and aromaticity degree were distinct throughout the years. This difference between the campaigns resulted from the interaction between the terrestrial input, estuarine mixing processes, and the biogeochemical transformations of DOM in the subsurface water of the PRD. Furthermore, the fluorescence quenching showed a strong interaction between terrestrial fulvic acid-like DOM with copper observed in the second collect campaign. These results demonstrate that the quality of DOM derived from terrestrial sources may modify the availability, transport, and fate of trace metal along estuarine environments of the Equatorial zones.