Mediterranean salt marsh sediment metal speciation and bioavailability changes induced by the spreading of non-indigenous Spartina patens

Within the Tejo Estuary, non-indigenous species (NIS) Spartina patens colonizes the upper middle marsh competing with the native Halimione portulacoides for space and resources. Due to the very different root system and metabolism between both species, this invasion can have significant biogeochemical implications, namely in terms of metal speciation and availability. In the present study, we evaluate the biogeochemical modifications in terms of metal speciation introduced by the colonization of the NIS S. patens. Total metals Cu, Zn, Pb and As within the rhizosediment varied between the two study species but was generally higher in the rhizosediment of H. portulacoides. These differences could be attributed to the higher organic content and smaller sand fraction found in the rhizosediment of H. portulacoides. Zinc was found to have highest concentration (H. portulacoides 126.22 mg g(-1), S. patens 68.35 mg g(-1)) in the rhizosediments while Cu and As were least concentrated metals. Considering the bioavailable fractions (F1 + F2) Cu, Zn and As were more readily available in the sediment beneath NIS S. patens than in H. portulacoides and Pb presented no significance (p = 0.835). Overall, H. portulacoides rhizosediments had higher total metal concentration, whilst the rhizosediments of NIS S. patens presented a higher percentage of bioavailable metals. Thus, the bioinvasion and expansion of NIS S. patens may have implications for metal biogeochemistry and the natural remediation capacity of salt marshes in estuaries along the Mediterranean and North-eastern Atlantic coasts, as well as ensuing biodiversity and potential trophic web contamination consequences.