Cultures of Cylindrotheca sp. 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. Cylindrotheca sp. cells were counted after preserving in 0.96% Glutaraldehyde and mounting a subsample in a Phycotech phytoplankton counting chamber under an inverted Olympus IX51 microscope. Cell size was measured by mounting a subsample of preserved cells on a phytoplankton counting chamber (Phycotech) and using an Evident LC35 camera and LCMicro software connected to an inverted Olympus IX51 microscope. Two measurements of Cylindrotheca sp. cells were made: both the length and the width of the chlorophyll-containing cell centre. The volume of the cell-centres was calculated based on the geometric model of a prolate sphere as described in 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.