Abstract This study evaluated the effects of biogenic structures (tree roots and crab burrows) on sediment carbon (C), sulfur (S), and iron (Fe) biogeochemistry during the wet season in the Olaria mangrove forests near the city of Cananéia, São Paulo state, Brazil and the Nobrega mangrove forest approximately 2 km from the city. Anaerobic C oxidation pathways were assessed from sediment profiles and anaerobic incubations and related to the abundance of biogenic structures in the form of pneumatophores and crab burrows. Porewater depth profiles of dissolved inorganic carbon (DIC) and SO42- were less steep in the presence than absence of biogenic structures. While Fe(II) appeared unaffected by biogenic structures, Fe(III) levels were significantly higher in the upper 4 cm of the sediment in the presence than absence of vegetation and bioturbation. Surprisingly, the concentration of Fe(III) in this layer was 2-6 times higher in the Nobrega forest (6-13 µmol cm-3) than in the Olaria forest (1.5-6.5 µmol cm-3). Accordingly, depth integrated sulfate reduction (SR) tended to be highest at Olaria, while iron reduction (FeR) was highest at Nobrega. SR accounted for 54-83% of DIC production, with no diference between forested sites, while FeR accounted for 8-24% of DIC production, with a 2-3 times higher contribution in the Nobrega versus the Olaria forest. The results suggest that mangrove roots and crab burrows in mangrove sediments only promote FeR at the expense of SR in the Nobrega forest. It appears that anthropogenic discharge from Cananéia city may have overridden the impact of biogenic structures on sediment redox conditions at Olaria, thereby diminishing the role of FeR without strong stimulation of overall C oxidation rates.

Abstract Estuaries are important features for global biogeochemical understanding, due to their highly coupled interaction between sediment and water. In those places, the land-sea transect sometimes has a gradient of physical and chemical characteristics, influencing the availability of nutrients and consequently primary production. This study aimed to observe the benthic aerobic respiration and sediment-water nutrient fluxes in a freshwater to marine transect in the Cananéia-Iguape Estuarine-Lagoon Complex (CIELC) on the southeastern Brazilian Coast. This area contains the most pristine mangrove at the limit between tropical and subtropical zones, providing an ideal observatory for ecological research programs. Intact sediment cores were incubated in laboratory to determine fluxes of O2, TCO2, and dissolved nutrients. Three different sites with different salinities were sampled during the four seasons of the year. Sediment characteristics of the sites were compared, showing higher organic matter in the lowest saline (LS) site and higher phytopigments in the highest saline (HS) site, as expected. Benthic aerobic respiration, O2 and CO2 fluxes, ranged from −0.4 to −3.2 mmol m−2h−1 and 0.1 to 1.5 mmol m−2h−1, respectively, and exhibited statistically significant variations between seasons and in a salinity gradient. Dissolved inorganic nitrogen and silicate, ranging from −228.7 to 544.8 μmol m−2h−1 and −205.8 to 4,173.5 μmol m−2h−1, respectively, were generally released from the sediment, whereas phosphate ranged from −25.2 to 29.6 μmol m−2h−1 with more variation in time and space. The LS site was characterized as a nitrogen sink and a silicate and phosphate releaser, and the HS site was characterized as a nitrogen producer and a phosphorus consumer. However, seasonal and spatial variations were observed, and the interaction between space and time factors was always highly significant, indicating that the metabolic behavior of the benthic compartment depends on both trophic and physicochemical conditions.