The Potential Role of Sediment Iron and Sulfur Speciation in Seagrass Meadow Loss and Recovery

Abstract

Oxidized iron (Fe) can reduce seagrass dieback when present in sufficient quantities in the sediment to fix sulfide as pyrite (FeS 2 ) or iron monosulfide (FeS). However, the oxidized Fe pool may become depleted over time as Fe is reduced and precipitated with sulfides. In this study, we estimated long-term variations in the speciation of solid forms of reduced and oxidized Fe along a eutrophication gradient in West Falmouth Harbor (WFH) (a temperate lagoon with substantial seagrass meadows) and conducted a 6-week microcosm study to assess the role of oxidized Fe in supporting seagrass recovery. We planted seagrass in sediments obtained from 2 WFH regions with differing Fe speciation. We found depletion of oxidized Fe over a decade following a seagrass dieback, even when the soluble sulfide levels decreased to concentrations unlikely to cause toxicity in seagrass. The continued absence of large concentrations of available oxidized Fe minerals in sediments, where most Fe was bound in FeS 2 , could impede the recovery of seagrass in formerly vegetated regions. Seagrass grown in sediments with low Fe:S ratios exhibited an increased probability of survival after 4 weeks. Field and laboratory results indicated that even when the soluble sulfide levels decrease after seagrass dieback, sediments may not be able to support seagrass recovery due to the legacy effects of eutrophication on the sediment Fe pool. However, we observed signs of reoxidation in the Fe pool within a few years of seagrass dieback, including a decrease in the total sediment S concentration, which could help spur recolonization.