Iron-binding ligands in the coastal waters of La Palma affected by the Tajogaite volcanic eruption

The 2021 eruption of Tajogaite volcano (La Palma, Canary Island) significantly altered the nearby coastal environment and chemistry through the formation of lava delta. Despite the importance of metal speciation for the ecosystems, our knowledge of Fe-organic speciation and the input of lava associated with the lava deltas formation in its biogeochemical cycle remains limited. This study explores the impact of the Tajogaite eruption on the Fe speciation by measuring the labile Fe-binding ligands (L_Fe) and their conditional stability constants (log K^cond_Fe’L).

Before, during and after the volcano eruption, 10 stations were monitored and analyzed by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) method, using TAC as a competitive ligand. To determine the optimal experimental conditions for comparing the different environments along the sampling years, different detection windows were employed (2, 5 and 10 µM TAC). The L_Fe concentrations ranged between 2.32 and 12.38 nM, with a maximum recorded in February 2023 near the southern lava delta (station 5, 28.616ºN, 12.38 nM). Other high concentrations were found from April to September 2024 at northern stations near to the other lava delta (28.624ºN, 11.20 nM). The minimum L_Fe concentration was observed at offshore station (station 10, 17.932ºW, 28.599ºN, 2.32 nM).

The observed log K^cond_Fe’L were between 9.33 and 10.73 under the studied conditions and correspond to weak ligands (L2-type) such as humic substances or polyphenols. The results show clear spatial and temporal variability, with significantly higher ligand concentration near lava deltas, suggesting a lasting volcanic influence on ligand production with a clear impact on the Fe speciation. Thus, the arriving of lava and the lava deltas formation act as a local source of Fe-biding ligands for several years after the eruption, keeping Fe in solution. However, the impact is locally limited, highlighting the importance of sampling site selection for accessing volcanic effects on coastal trace metal cycling.