Importance of resuspension for the sedimentary sulfide buffering capacity - a case study from the Limfjorden, Denmark

Sediment resuspension is a critical yet underexplored factor for the biogeochemical function of coastal margins. This study investigated the impact of resuspension and seasonal variations in the sulfide buffering capacity (SBC) at two eutrophic sites in the Limfjorden. The sites, Skive Fjord (SF) and Venø Sund (VS), had sediment organic matter contents of 19 and 14 wt%, respectively. Both sites showed considerable sulfide (H₂S) production during all seasons but at reduced levels during winter and consistently higher values at SF. An important removal process for free H₂S is the entrapment by oxidized iron, acting as a chemical buffer. The ratio of oxidized to reduced iron (Fe³⁺/Fe²⁺) in surface sediments varied seasonally (January, May, September) between 3, 0.2, and <0.01 at SF and between 2.9, 0.6, and 0.1 at VS, respectively. The SBC, expressed as the period of no H₂S efflux after sediment enclosure, showed that the elevated levels of Fe³⁺ in January and May prevented H₂S release for >400 h. In contrast, September sediments exhausted the SBC after only 50 h at SF and 150 h at VS. EROMES erosion devices simulated wave-generated sediment resuspension and were used to assess the impact for benthic solute exchange and the SBC. Under vigorous resuspension (4.4 N m^-2), ~6 mm of surface sediment was eroded, and sediment oxygen (O₂) uptake was enhanced by 2-, 6-, and 7-fold at VS and by 22-, 22-, and 40-fold at SF across seasons, respectively. Oxygen was used to reoxidize Fe²⁺, and the O₂ consumption rates aligned with the Fe³⁺/Fe²⁺-ratios. Similar O₂ uptake during multiple successive resuspension events in September at SF documented a large Fe²⁺ pool with negligible reoxidation occurring during each event. Full Fe²⁺ oxidation of the upper 6 mm would require ~4 h, but bottom-water O₂ would then likely be depleted after only 2 h. Thus, resuspension events must be of notable duration with sufficient supply of O₂ to enhance and maintain the SBC and mitigate the risk of H₂S emission. Such events are expected to become more frequent due to climate driven enhancement of storm surges and would benefit environmental conditions in the Limfjorden.