Vertical heterogeneity of dissolved ferrous and iron speciation in tropical seagrass beds.

Iron is an important trace metal in regulating the ocean biogeochemical cycle. Through in situ field measurements using reverse flow dual-injection analysis (rFDIA), we demonstrate a significant spatial heterogeneity in iron distribution in the water column in three Chinese seagrass beds. Fe2+ concentrations ranged from 2.38 to 10.82 nM and dFe from 10.44 to 18.35 nM across 26 sampling stations. The concentration of Fe2+ in the surface water layer was significantly higher than the bottom within seagrass beds, while opposite trend was observed in unvegetated area, revealing that the seagrass canopy presence exerts a profound influence on iron dynamics, particularly vertical stratification. Furthermore, our characterization of suspended mineral compositions provides novel insights into possible mineral-mediated iron transformation processes. This study elucidates the potential interplay between seagrass and iron biogeochemical cycling. Nevertheless, the driving mechanism behind this still needs further study, including both biological and geochemical processes on the iron cycling.