Relationships between sediment chemical buffering capacity and H2S accumulation: comparative study in two temperate estuarine brackish lagoons

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To reveal the mechanisms of sedimental H2S accumulation, annual investigations on sedimental environments were conducted in two temperate estuarine lagoons. The lagoons, Gamo and Idoura (Japan), have similar shapes, locations, and topographical properties but different degrees of H2S accumulation. Water stagnation causes a high phytoplankton biomass (Chl. a; 26–52 μg l−1) in the inner Gamo Lagoon. Gamo Lagoon sediment was characterized by high bounded sulfides (bounded S mainly FeS) and H2S contents, and low C/N ratios (mean = 10.4) and iron (reactive Fe2+ and total Fe) contents. H2S was not detected in Idoura Lagoon where phytoplankton biomass was much lower (Chl. a; 0.6–4 μg l−1). Idoura Lagoon sediment had high C/N ratios (mean=17.9) and high iron contents. The C/N ratio difference implies that organic matter in Gamo Lagoon originates mainly from more decomposable phytoplankton, while organic matter in Idoura Lagoon derives mainly from terrestrial vascular plants with lower decomposability. The excess loading of phytoplanktonic detritus in Gamo accelerates sedimentary microbial activity, including sulfate reduction (i.e., H2S production). High Fe2+and low bounded S contents in Idoura sediment indicate a high chemical buffering capacity toward H2S. In contrast, almost all Fe2+ in Gamo Lagoon had already reacted with H2S as FeS. H2S accumulation in Gamo Lagoon is caused by low sedimentary chemical buffering capacity toward H2S, as well as higher microbial H2S production, caused by the excess loading of phytoplanktonic detritus.