Potential impact of marine heatwaves on selected phytoplankton adapted to the Gulf of Guinea during stable hydrographic periods

Reports suggest that the Gulf of Guinea (northeastern tropical Atlantic) frequently experiences marine heatwaves (MHW)—prolonged periods of anomalously warm seawater—of ≥1.5 °C above baseline. We assessed the likely impact of this anomaly on two microalgae, Thalassiosira weissflogii (diatom) and Gymnodinium sp. (dinoflagellate), adapted to the surface temperature (28 ± 1.5 °C) of the gulf during stable hydrographic periods. The algae were adapted over ∼400 generations. They were assessed for specific growth rate (μ), dry weight, and protein content after exposure to 5 or 6 days of warming (+2 °C or +4 °C above the temperature of the stock cultures), in line with the minimum duration of MHW. Under each of the investigated warming scenarios, the effect of warming on the diatom was immediate, occurring during the first day of exposure, and μ had decreased by ∼75% by the end of the warming period. In contrast, the warming effect on the dinoflagellate became significant during the second day, with μ reduced by ∼78–86%. Also, the protein content of the dinoflagellate had been reduced by ∼32% by the end of the warming period. The dry weight of T. weissflogii increased three-fold under +2 °C of warming. In contrast, the dry weight of Gymnodinium sp. decreased by ∼78% and did not recover when warming was removed. These results highlight vulnerability of these algae to MHW and their unique responses to the anomaly.