Sediment capping causes metabolic stress and hydrogen sulfide intrusion in Posidonia australis: Implications for seagrass restoration

Anthropogenic pollution in developed coastal areas often causes widespread seagrass loss. In Cockburn Sound, Western Australia, industrial run-off drastically reduced cover of Posidonia spp. by 77% in the 1960s–1990s. Despite significant water quality improvements, natural recovery remains limited, potentially due to legacy pollution and phytotoxic hydrogen sulfide (H₂S) production in sediments. Using a novel multidisciplinary approach combining metabolomics, nutrient (carbon, nitrogen, phosphorus), and δ ³⁴S isotope analysis, we assessed whether capping existing sediment with clean, dredged material could support Posidonia australis restoration. Seagrass was transplanted into 15 garden beds across three treatments: i) Experimental control (no sediment capping); ii) Capped (capped sediment); iii) Capped + wrack (capped sediment mixed with dried seagrass leaf material). Within two weeks, sulfur cycle-related metabolites were up-regulated in seagrass growing in capped sediment which was likely due to elevated H₂S intrusion into the leaves. Up-regulation of tocopherols suggested that P. australis activated vitamin E -related pathways to mitigate stress. Overall, sediment capping impaired seagrass health and failed to reduce conditions promoting H₂S intrusion into plant tissue, likely because of the fine texture of the dredged material. Careful sediment assessment and modification are essential before repurposing such material for seagrass restoration.