CO2 changes ocean pH and temperature.
What was iron concentration 50 years ago, and how much has it decreased?

The following year, Sobrino et al. (2008) grew cultures of Thalassiosira pseudonana -- which they describe as "a widely distributed diatom" -- while exposing them to either photosynthetically-active radiation (PAR: 400-700 nm) or PAR plusultraviolet radiation (UVR: 280-400 nm) in 500-mL Teflon bottles maintained at 20°C, using a semi-continuous approach that employed daily dilutions with fresh growth medium (filtered seawater from the Gulf Stream that was enriched with f/2 nutrients) through which air streams of different atmospheric CO2 concentrations (380 or 1000 ppm) were continuously bubbled. In doing so, the three researchers determined that exposure of the seawater medium to air with an extra 620 ppm of CO2 increased the photosynthetic rate of the marine diatoms by approximately 45% in the presence of PAR and about 60% in the presence of both PAR and UVR, while it increased their growth rate by approximately 20% in both of the radiation environments. And in highlighting the significance of their findings, they note that "among the phytoplankton species inhabiting the [ocean's] surface layer, diatoms are responsible for almost 40% of the ocean primary productivity (Nelson et al., 1995)."