Saltmarsh plants role in metals retention and the potential of vegetation for metal removal in the long term

Estuaries along with their saltmarshes are extremely relevant areas to what ecosystems conservation is concerned. Not only do they provide unique conditions to house numerous species but can also play an important role in pollution mitigation. This study aimed to evaluate the role of saltmarsh plants in metals retention in the long term, using a previously monitored estuary as a case study (Lima river estuary). Seasonal sampling campaigns were carried out in 2022 to determine the metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) concentrations in vegetated and non-vegetated sediments, as well as in saltmarsh plants at three sites within the estuarine area, Canogem, Salinas and Srª das Areias. Results showed saltmarsh plants, despite some seasonal variability, are concentrating metals in their rhizosediments (ratio metal in vegetated sediment / metal in non-vegetated sediment >1) and in their roots, namely Cd, Cu and Zn (ratio metal in plant roots / metal in non-vegetated sediment >1). This role seems to be maintained in the long term, with plant metal retention levels similar in 2009 and 2022, indicating plants are probably contributing to remove metals from the surface water. However, this feature seems to be decreasing in one of the sites, Srª das Areias, that shows signs of degradation. Thus, saltmarsh plants have the potential to retain metals in estuarine areas, contributing to reduce metals present in the aquatic environment and preventing them from spreading through the estuarine area, from reaching coastal areas and eventually from reaching underlying aquifers. Protection of this environment is mandatory and the promotion of re-vegetation of non-vegetated estuarine areas is needed so that the saltmarsh works as a nature-based solution that prevents and/or recovers impacted environments, in order that saltmarshes can continue to deliver their multiple co-benefits.