Abstract
Abstract. 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 pCO2 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 (µgC 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 CO2 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 CO2 had the most clear and significant effect on particulate Fe concentrations, decreasing its concentration under high CO2. Indeed, high CO2 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 CO2. Furthermore, high CO2 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 CO2 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.
Highlights
Marine phytoplankton contribute half of the world’s total primary productivity, sustaining marine food webs and driving the biogeochemical cycles of carbon and nutrients (Field et al, 1998)
4.1 The effects of CO2 and dFe on the plankton community. In this experiment we investigated changes in particulate trace metal concentrations in response to increased CO2 and/or an addition of the siderophore desferrioxamine B (DFB) in a coastal mesocosm experiment
The results presented here show that, in the fjord where we carried out the present experiment, particulate Fe was dominated by lithogenic material and was significantly decreased in the treatments with high CO2 concentrations and/or DFB addition
Summary
Marine phytoplankton contribute half of the world’s total primary productivity, sustaining marine food webs and driving the biogeochemical cycles of carbon and nutrients (Field et al, 1998). Phytoplankton incorporate approximately 45 to 50 billion metric tons of inorganic carbon (Field et al, 1998), removing a quarter of the CO2 emitted to the atmosphere by anthropogenic activities (Canadell et al, 2007). The dissolution of anthropogenic CO2 in seawater causes shifts in the carbonate chemical speciation and leads to ocean acidification (OA) (Doney et al, 2009). The biogeochemical cycling of nutrients is predicted to be highly affected by OA (Hutchins et al, 2009), as well as the distribution and speciation of trace metals in the ocean (Millero et al, 2009)
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