Development of an Ocean Acidification Simulation System and Its Preliminary Application to Trace-Metal Size Fractionation in Seawater

A closed-loop CO2-aeration system, with integrated pH feedback, multiparameter sensing, water circulation, RS485 communication, stainless-steel gas-path valves, and fluorinated vessel interiors, was developed for controlled trace-metal size-fractionation studies in seawater. Representative archived seawater records showed mean pH values of 8.042 ± 0.026, 7.528 ± 0.030, and 7.005 ± 0.025 at programmed set points of 8.0, 7.5, and 7.0, respectively. Additionally, its preliminary application to trace-metal (Zn, Cu, Fe, and Mn) size fractionation with sequential pH stages of 8.0, 7.5, and 7.0 was investigated. Zn, Cu, Fe, and Mn were measured by inductively coupled plasma mass spectrometry (ICP–MS) in the <0.45 μm, <0.2 μm, and <100 kDa fractions. Lower pH was associated with higher mean <0.45 μm concentrations of all four metals, with the largest relative increase for Mn and the smallest for Fe. Cu and Zn showed increased contributions from the <100 kDa fraction, whereas the Fe contribution in this fraction decreased relative to pH 8.0. Mn shifted from an intermediate-size-dominated distribution to a larger-size-dominated distribution and then to a <100 kDa-dominated distribution. These preliminary observations illustrate the additional information provided by operational size fractionation. The platform supports proof-of-concept studies; independent incubation replication, whole-system metal blanks, and further controller validation are required for quantitative mechanistic applications.