Cultures of Gephyrocapsa huxleyi were isolated from the Subantarctic water (SAW) off the coast of New Zealand in 2023 and exposed to a simulated marine heatwave. Thermal performance curve experiments were performed for G. huxleyi at two time points after exposure to the marine heatwave: T1: immediately after heatwave exposure, and T2: 5 weeks after heatwave exposure. At the end of the thermal performance curve experiments, 1mL of each replicate was preserved using 25% Glutaraldehyde to a final concentration of 0.96% and stored at 4°C. A subsample of preserved cells was mounted on a phytoplankton counting chamber (Phycotech), and cell size was measured using an Evident LC35 camera and LCMicro software connected to an inverted Olympus IX51 microscope. The diameters of G. huxleyi cells were measured, and the cell volume was calculated by modelling the cell as a sphere (Hillebrand et al., 1999). Only the cell body was measured, as the preservation process results in the coccoliths detaching from the cell. The data was collected in 2024 as part of a Master's research project investigating the impacts of marine heatwaves on subantarctic phytoplankton.