Cultures of Gephyrocapsa huxleyi isolated from the Subantarctic water (SAW) off the coast of New Zealand were exposed to a simulated marine heatwave, and thermal performance curve experiments were performed immediately after the heatwave (T1) and again five weeks later (T2). After each thermal performance curve experiment, the cells in a subsample of each replicate were counted. The number of G. huxleyi cells in 25µL of fresh culture was counted using an Accuri C6 plus (BD) flow cytometer set to a slow flow rate (14µL/min, 10µm core size). 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). Additional subsamples of each replicate at the end of the thermal performance curve experiments were collected for in vitro chlorophyll-a measurements. Cells were filtered onto GF/C filters, and chlorophyll-a was extracted using 90% acetone for 24 hours (Welschmeyer, 1994). The chlorophyll-a concentration was measured before and after acidification with a Turner 10-AU fluorometer. Final chlorophyll-a measurements were normalised per cell volume. The data was collected in 2024 as part of a Master's thesis research project investigating the effect of marine heatwaves on subantarctic phytoplankton.