The harmful algal bloom species Alexandrium ostenfeldii has a worldwide distribution from polar to tropical habitats and from oceanic to brackish waters. Among other species of the genus Alexandrium, it is one of the causative organisms of paralytic shellfish toxins, but additionally, A. ostenfeldii has also been shown to produce another class of toxins, cyclic imines. The wide distribution of A. ostenfeldii suggests population-specific adaptations to a multitude of environmental parameters and therefore, variable responses to global change drivers, such as warming and shifts in sea surface salinity. In this study we quantified growth and toxin cell quota of two strains of A. ostenfeldii isolated from the arctic Kongsfjord and two strains from the northern European Baltic Sea at various temperature conditions, to assess the impact of global warming on locally adapted populations. Overall, growth of the arctic strains was detected at temperatures between 7.5 and 20 °C, with a maximum growth rate at 15 °C for both strains. The two strains from the Baltic Sea revealed intraspecific differences concerning their thermal tolerance. One strain showed no growth at 25 °C, while the other still had a positive growth rate at 27 °C. Furthermore, three of the strains were exposed to salinities between 10 and 40, revealing a tolerance to a broad range of salinities. Neither temperature nor salinity affected the qualitative toxin composition of any strain, but we detected novel cyclic imines in three of the four tested strains. Furthermore, different temperatures and salinities led to dynamic shifts in total toxin cell quota. Additionally, we detected novel spirolides in both arctic strains of A. ostenfeldii. These findings suggest that arctic A. ostenfeldii might significantly benefit from global warming, while populations from the Baltic Sea may not, and that the Baltic Sea might become unfavourable for western Baltic A. ostenfeldii due to climate change driven decreasing salinity in this area.