Anaerobic carbon oxidation in sediment of two Brazilian mangrove forests: the influence of tree roots and crab burrows

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.