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 δ34S 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.