Metals in coastal seagrass habitats: A systematic quantitative literature review

Seagrasses provide crucial ecosystem services in the coastal environment, but are under increasing threat associated with anthropogenic activities. Seagrass meadows effectively trap suspended sediment, a key vector for trace metal transport, by altering local hydrodynamic conditions around their leaves and stems. As a result, seagrass meadows often have different geochemical sediment characteristics compared to adjacent bare sediments and may accumulate higher concentrations of trace metals. The aim of this systematic review was to identify research trends and knowledge gaps in the relationships between metals and seagrasses. The systematic search of the literature identified a total of 191 relevant articles. Although seagrasses have a broad geographic distribution, the highest diversity and endemism exists in the Indo-Pacific region, yet most studies (40%) were conducted in the Mediterranean Sea. There were also taxonomic gaps with some common species groups, including Amphibolis and Thalassodendron spp., being poorly studied. The reviewed studies examined the relationship between seagrass and 39 metal elements. There were no studies examining rare-earth elements (REEs) or platinum-group elements (PGEs), both of which are critical classes of metals in emerging technologies. Furthermore, the review determined that all 191 studies focused on the impact of metals on seagrasses, while only six also considered the effect of seagrasses on metal geochemistry in sediments. We conclude that further research is necessary to address these key gaps in knowledge to better understand the role(s) of seagrasses in the distribution, immobilization, and release of trace metals in coastal ecosystems.