The paper contributes to our knowledge of the impact of marine heat waves on ecosystems, a subject that is topical and that we know little of. The impact on coccolithophores is important for carbon uptake. The impact of MHW more generally on algal blooms is also important for higher trophic levels, the wider ecosystem, and for commercial interests such as aquaculture. The study gives useful confirmation that the environmental ranges defined by O’Brien et al for coccolithophore blooms do seem to apply even in these anomalous MHW conditions and in two separate regions.