Particulate trace metal dynamics in response to increased CO2 and iron availability in a coastal mesocosm experiment

• Published in: Biogeosciences
• Main Authors: Lorenzo, M. Rosario, Segovia, María, Cullen, Jay T., Maldonado, María T.
• Format: Text
• Language: English
• Published: 2020
• Subjects: Bergen; Norway; Ocean acidification
• Online Access: https://doi.org/10.5194/bg-17-757-2020; https://www.biogeosciences.net/17/757/2020/

Rising concentrations of atmospheric carbon dioxide are causing ocean acidification and will influence marine processes and trace metal biogeochemistry. In June 2012, in the Raunefjord (Bergen, Norway), we performed a mesocosm experiment, comprised of a fully factorial design of ambient and elevated p CO 2 and/or an addition of the siderophore desferrioxamine B (DFB). In addition, the macronutrient concentrations were manipulated to enhance a bloom of the coccolithophore Emiliania huxleyi . We report the changes in particulate trace metal concentrations during this experiment. Our results show that particulate Ti and Fe were dominated by lithogenic material, while particulate Cu, Co, Mn, Zn, Mo and Cd had a strong biogenic component. Furthermore, significant correlations were found between particulate concentrations of Cu, Co, Zn, Cd, Mn, Mo and P in seawater and phytoplankton biomass ( µ gC L −1 ), supporting a significant influence of the bloom in the distribution of these particulate elements. The concentrations of these biogenic metals in the E. huxleyi bloom were ranked as follows: Zn < Cu ≈ Mn < Mo < Co < Cd. Changes in CO 2 affected total particulate concentrations and biogenic metal ratios (Me : P) for some metals, while the addition of DFB only significantly affected the concentrations of some particulate metals (mol L −1 ). Variations in CO 2 had the most clear and significant effect on particulate Fe concentrations, decreasing its concentration under high CO 2 . Indeed, high CO 2 and/or DFB promoted the dissolution of particulate Fe, and the presence of this siderophore helped in maintaining high dissolved Fe. This shift between particulate and dissolved Fe concentrations in the presence of DFB, promoted a massive bloom of E. huxleyi in the treatments with ambient CO 2 . Furthermore, high CO 2 decreased the Me : P ratios of Co, Zn and Mn while increasing the Cu : P ratios. These findings support theoretical predictions that the molar ratios of metal to phosphorous (Me : P ratios) of metals whose seawater dissolved speciation is dominated by free ions (e.g., Co, Zn and Mn) will likely decrease or stay constant under ocean acidification. In contrast, high CO 2 is predicted to shift the speciation of dissolved metals associated with carbonates such as Cu, increasing their bioavailability and resulting in higher Me : P ratios.