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

Rising concentrations of atmospheric carbon dioxide are causing ocean acidification and will influence marine processes and trace metal biogeochemistry. The importance of the combined impacts of elevated CO2 and changes in trace metal availability on marine plankton remain largely unknown. A mesocosm experiment was performed to study changes in particulate trace metal concentrations during a bloom dominated by the coccolithophore Emiliania huxleyi. We employed a full-factorial experimental design, comprising all combinations of ambient and elevated pCO2 and dissolved iron (dFe). Particulate metal concentrations (Fe, Cu, Zn, Co, Mn, Cd, Mo, Ti and Pb) were determined by high-resolution inductively coupled plasma mass spectrometry (HR-ICPMS). We examined biogenic and lithogenic sources of particulate metals, and their evolution during the experiment. Biogenic metal concentrations were estimated from bulk particle measurements by comparing phosphorus (P)-normalised quotas with published ratios, as well as concentrations of particulate trace metals in the presence and absence of an oxalate-EDTA wash. Our results demonstrate that particulate Ti and Fe concentrations were dominated by lithogenic material in the fjord. In contrast, particulate Cu, Co, Mn, Zn, Mo and Cd concentrations correlated with P concentrations and phytoplankton biomass, indicative of their strong biogenic character. Furthermore, ocean acidification changed the relative concentrations of particulate metals; a result mainly driven by the effects of ocean acidification on the growth of different phytoplankton phyla. This study demonstrates the utility and robustness of combining trace metal analyses of particles in a controlled mesocosm experiment with manipulations of CO2 and Fe concentrations using natural assemblages of marine phytoplankton.